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Nobel Prize : Quantum Entanglement Unveiled

7 October 2022: We have replaced our initial one-paragraph announcement with a full-length Focus story.

The Nobel Prize in Physics this year recognizes efforts to take quantum weirdness out of philosophy discussions and to place it on experimental display for all to see. The award is shared by Alain Aspect, John Clauser, and Anton Zeilinger, all of whom showed a mastery of entanglement—a quantum relationship between two particles that can exist over long distances. Using entangled photons, Clauser and Aspect performed some of the first “Bell tests,” which confirmed quantum mechanics predictions while putting to bed certain alternative theories based on classical physics. Zeilinger used some of those Bell-test techniques to demonstrate entanglement control methods that can be applied to quantum computing, quantum cryptography, and other quantum information technologies.

Since its inception, quantum mechanics has been wildly successful at predicting the outcomes of experiments. But the theory assumes that some properties of a particle are inherently uncertain—a fact that bothered many physicists, including Albert Einstein. He and his colleagues expressed their concern in a paradox they described in 1935 [ 1 ]: Imagine creating two quantum mechanically entangled particles and distributing them between two separated researchers, characters later named Alice and Bob. If Alice measures her particle, then she learns something about Bob’s particle—as if her measurement instantaneously changed the uncertainty about the state of his particle. To avoid such “spooky action at a distance,” Einstein proposed that lying underneath the quantum framework is a set of classical “hidden variables” that determine precisely how a particle will behave, rather than providing only probabilities.

The hidden variables were unmeasurable—by definition—so most physicists deemed their existence to be a philosophical issue, not an experimental one. That changed in 1964 when John Bell of the University of Wisconsin-Madison, proposed a thought experiment that could directly test the hidden variable hypothesis [ 2 ]. As in Einstein’s paradox, Alice and Bob are each sent one particle of an entangled pair. This time, however, the two researchers measure their respective particles in different ways and compare their results. Bell showed that if hidden variables exist, the experimental results would obey a mathematical inequality. However, if quantum mechanics was correct, the inequality would be violated.

Bell’s work showed how to settle the debate between quantum and classical views, but his proposed experiment assumed detector capabilities that weren’t feasible. A revised version using photons and polarizers was proposed in 1969 by Clauser, then at Columbia University, along with his colleagues [ 3 ]. Three years later, Clauser and Stuart Freedman (both at the University of California, Berkeley) succeeded in performing that experiment [ 4 ].

The Freedman-Clauser experiment used entangled photons obtained by exciting calcium atoms. When a calcium atom de-excites, it can emit two photons whose polarizations are aligned. The researchers installed two detectors (Alice and Bob) on opposite sides of the calcium source and measured the rate of coincidences—two photons hitting the detectors simultaneously. Each detector was equipped with a polarizer that could be rotated to an arbitrary orientation.

Freedman and Clauser showed theoretically that quantum mechanics predictions diverge strongly from hidden variable predictions when Alice and Bob’s polarizers are offset from each other by 22.5° or 67.5°. The researchers collected 200 hours of data and found that the coincidence rates violated a revamped Bell’s inequality, proving that quantum mechanics is right.

The results of the first Bell test were a blow to hidden variables, but there were “loopholes” that hidden-variable supporters could claim to rescue their theory. One of the most significant loopholes was based on the idea that the setting of Alice’s polarizer could have some influence on Bob’s polarizer or on the photons that are created at the source. Such effects could allow the elements of a hidden-variable system to “conspire” together to produce measurement outcomes that mimic quantum mechanics.

To close this so-called locality loophole, Aspect and his colleagues at the Institute of Optics Graduate School in France performed an updated Bell test in 1982, using an innovative method for randomly changing the polarizer orientations [ 5 ]. The system worked like a railroad switch, rapidly diverting photons between two separate “tracks,” each with a different polarizer. The changes were made as the photons were traveling from the source to the detectors, so there was not enough time for coordination between supposed hidden variables.

Zeilinger, who is now at the University of Vienna, has also worked on removing loopholes from Bell tests (see Viewpoint: Closing the Door on Einstein and Bohr’s Quantum Debate , written by Aspect). In 2017, for example, he and his collaborators devised a way to use light from distant stars as a random input for setting polarizer orientations (see Synopsis: Cosmic Test of Quantum Mechanics ).

Zeilinger also used the techniques of entanglement control to explore practical applications, such as quantum teleportation and entanglement swapping. For the latter, he and his team showed in 1998 that they could create entanglement between two photons that were never in contact [ 6 ]. In this experiment, two sets of entangled photon pairs are generated at two separate locations. One from each pair is sent to Alice and Bob, while the other two photons are sent to a third person, Cecilia. Cecilia performs a Bell-like test on her two photons, and when she records a particular result, Alice’s photon winds up being entangled with Bob's. This swapping could be used to send entanglement over longer distances than is currently possible with optical fibers (see Research News: The Key Device Needed for a Quantum Internet ).

“Quantum entanglement is not questioned anymore,” says quantum physicist Jean Dalibard from the College of France. “It has become a tool, in particular in the emerging field of quantum information processing, and the three nominated scientists can be considered as the godfathers of this new domain.”

Quantum information specialist Jian-Wei Pan of the University of Science and Technology of China in Hefei says the winners are fully deserving of the prize. He has worked with Zeilinger on several projects, including a quantum-based satellite link (see Focus: Intercontinental, Quantum-Encrypted Messaging and Video ). “Now, in China, we are putting a lot of effort into actually turning these dreams into reality, hoping to make the quantum technologies practically useful for our society.”

–Michael Schirber

Michael Schirber is a Corresponding Editor for Physics Magazine based in Lyon, France.

• A. Einstein et al. , “Can quantum-mechanical description of physical reality be considered complete?” Phys. Rev. 47 , 777 (1935) .
• J. S. Bell, “On the Einstein Podolsky Rosen paradox,” Physics 1 , 195 (1964) .
• J. F. Clauser et al. , “Proposed experiment to test local hidden-variable theories,” Phys. Rev. Lett. 23 , 880 (1969) .
• S. J. Freedman and J. F. Clauser, “Experimental test of local hidden-variable theories,” Phys. Rev. Lett. 28 , 938 (1972) .
• A. Aspect et al. , “Experimental test of Bell’s inequalities using time-varying analyzers,” Phys. Rev. Lett. 49 , 1804 (1982) .
• J. W. Pan et al. , “Experimental entanglement swapping: Entangling photons that never interacted,” Phys. Rev. Lett. 80 , 3891 (1998) .

More Information

Research News: Hiding Secrets Using Quantum Entanglement

Research News: Diagramming Quantum Weirdness

APS press release

The Nobel Prize in Physics 2022 (Nobel Foundation)

Experimental Test of Bell's Inequalities Using Time-Varying Analyzers

Alain Aspect, Jean Dalibard, and Gérard Roger

Phys. Rev. Lett. 49 , 1804 (1982)

Published December 20, 1982

Experimental Entanglement Swapping: Entangling Photons That Never Interacted

Jian-Wei Pan, Dik Bouwmeester, Harald Weinfurter, and Anton Zeilinger

Phys. Rev. Lett. 80 , 3891 (1998)

Published May 4, 1998

Experimental Test of Local Hidden-Variable Theories

Stuart J. Freedman and John F. Clauser

Phys. Rev. Lett. 28 , 938 (1972)

Published April 3, 1972

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What is quantum entanglement? A physicist explains the science of Einstein’s ‘spooky action at a distance’

Associate Professor of Physics, University of South Florida

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The 2022 Nobel Prize in physics recognized three scientists who made groundbreaking contributions in understanding one of the most mysterious of all natural phenomena: quantum entanglement.

In the simplest terms, quantum entanglement means that aspects of one particle of an entangled pair depend on aspects of the other particle, no matter how far apart they are or what lies between them. These particles could be, for example, electrons or photons, and an aspect could be the state it is in, such as whether it is “spinning” in one direction or another.

The strange part of quantum entanglement is that when you measure something about one particle in an entangled pair, you immediately know something about the other particle, even if they are millions of light years apart. This odd connection between the two particles is instantaneous, seemingly breaking a fundamental law of the universe . Albert Einstein famously called the phenomenon “spooky action at a distance.”

Having spent the better part of two decades conducting experiments rooted in quantum mechanics , I have come to accept its strangeness. Thanks to ever more precise and reliable instruments and the work of this year’s Nobel winners, Alain Aspect , John Clauser and Anton Zeilinger , physicists now integrate quantum phenomena into their knowledge of the world with an exceptional degree of certainty.

However, even until the 1970s, researchers were still divided over whether quantum entanglement was a real phenomenon. And for good reasons – who would dare contradict the great Einstein, who himself doubted it? It took the development of new experimental technology and bold researchers to finally put this mystery to rest.

Existing in multiple states at once

To truly understand the spookiness of quantum entanglement, it is important to first understand quantum superposition . Quantum superposition is the idea that particles exist in multiple states at once. When a measurement is performed, it is as if the particle selects one of the states in the superposition.

For example, many particles have an attribute called spin that is measured either as “up” or “down” for a given orientation of the analyzer. But until you measure the spin of a particle, it simultaneously exists in a superposition of spin up and spin down.

There is a probability attached to each state, and it is possible to predict the average outcome from many measurements. The likelihood of a single measurement being up or down depends on these probabilities, but is itself unpredictable .

Though very weird, the mathematics and a vast number of experiments have shown that quantum mechanics correctly describes physical reality.

Two entangled particles

The spookiness of quantum entanglement emerges from the reality of quantum superposition, and was clear to the founding fathers of quantum mechanics who developed the theory in the 1920s and 1930s.

To create entangled particles you essentially break a system into two, where the sum of the parts is known. For example, you can split a particle with spin of zero into two particles that necessarily will have opposite spins so that their sum is zero.

In 1935, Albert Einstein, Boris Podolsky and Nathan Rosen published a paper that describes a thought experiment designed to illustrate a seeming absurdity of quantum entanglement that challenged a foundational law of the universe.

A simplified version of this thought experiment , attributed to David Bohm, considers the decay of a particle called the pi meson. When this particle decays, it produces an electron and a positron that have opposite spin and are moving away from each other. Therefore, if the electron spin is measured to be up, then the measured spin of the positron could only be down, and vice versa. This is true even if the particles are billions of miles apart.

This would be fine if the measurement of the electron spin were always up and the measured spin of the positron were always down. But because of quantum mechanics, the spin of each particle is both part up and part down until it is measured. Only when the measurement occurs does the quantum state of the spin “collapse” into either up or down – instantaneously collapsing the other particle into the opposite spin. This seems to suggest that the particles communicate with each other through some means that moves faster than the speed of light. But according to the laws of physics, nothing can travel faster than the speed of light. Surely the measured state of one particle cannot instantaneously determine the state of another particle at the far end of the universe?

Physicists, including Einstein, proposed a number of alternative interpretations of quantum entanglement in the 1930s. They theorized there was some unknown property – dubbed hidden variables – that determined the state of a particle before measurement . But at the time, physicists did not have the technology nor a definition of a clear measurement that could test whether quantum theory needed to be modified to include hidden variables.

Disproving a theory

It took until the 1960s before there were any clues to an answer. John Bell, a brilliant Irish physicist who did not live to receive the Nobel Prize, devised a scheme to test whether the notion of hidden variables made sense.

Bell produced an equation now known as Bell’s inequality that is always correct – and only correct – for hidden variable theories, and not always for quantum mechanics. Thus, if Bell’s equation was found not to be satisfied in a real-world experiment, local hidden variable theories can be ruled out as an explanation for quantum entanglement.

The experiments of the 2022 Nobel laureates, particularly those of Alain Aspect , were the first tests of the Bell inequality . The experiments used entangled photons, rather than pairs of an electron and a positron, as in many thought experiments. The results conclusively ruled out the existence of hidden variables, a mysterious attribute that would predetermine the states of entangled particles. Collectively, these and many follow-up experiments have vindicated quantum mechanics. Objects can be correlated over large distances in ways that physics before quantum mechanics can not explain.

Importantly, there is also no conflict with special relativity, which forbids faster-than-light communication . The fact that measurements over vast distances are correlated does not imply that information is transmitted between the particles. Two parties far apart performing measurements on entangled particles cannot use the phenomenon to pass along information faster than the speed of light.

Today, physicists continue to research quantum entanglement and investigate potential practical applications . Although quantum mechanics can predict the probability of a measurement with incredible accuracy, many researchers remain skeptical that it provides a complete description of reality. One thing is certain, though. Much remains to be said about the mysterious world of quantum mechanics.

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MIT scientists tune the entanglement structure in an array of qubits

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Entanglement is a form of correlation between quantum objects, such as particles at the atomic scale. This uniquely quantum phenomenon cannot be explained by the laws of classical physics, yet it is one of the properties that explains the macroscopic behavior of quantum systems.

Because entanglement is central to the way quantum systems work, understanding it better could give scientists a deeper sense of how information is stored and processed efficiently in such systems.

Qubits, or quantum bits, are the building blocks of a quantum computer. However, it is extremely difficult to make specific entangled states in many-qubit systems, let alone investigate them. There are also a variety of entangled states, and telling them apart can be challenging.

Now, MIT researchers have demonstrated a technique to efficiently generate entanglement among an array of superconducting qubits that exhibit a specific type of behavior.

Over the past years, the researchers at the Engineering Quantum Systems ( EQuS ) group have developed techniques using microwave technology to precisely control a quantum processor composed of superconducting circuits. In addition to these control techniques, the methods introduced in this work enable the processor to efficiently generate highly entangled states and shift those states from one type of entanglement to another — including between types that are more likely to support quantum speed-up and those that are not.

“Here, we are demonstrating that we can utilize the emerging quantum processors as a tool to further our understanding of physics. While everything we did in this experiment was on a scale which can still be simulated on a classical computer, we have a good roadmap for scaling this technology and methodology beyond the reach of classical computing,” says Amir H. Karamlou ’18, MEng ’18, PhD ’23, the lead author of the paper.

The senior author is William D. Oliver, the Henry Ellis Warren professor of electrical engineering and computer science and of physics, director of the Center for Quantum Engineering, leader of the EQuS group, and associate director of the Research Laboratory of Electronics. Karamlou and Oliver are joined by Research Scientist Jeff Grover, postdoc Ilan Rosen, and others in the departments of Electrical Engineering and Computer Science and of Physics at MIT, at MIT Lincoln Laboratory, and at Wellesley College and the University of Maryland. The research appears today in Nature .

Assessing entanglement

In a large quantum system comprising many interconnected qubits, one can think about entanglement as the amount of quantum information shared between a given subsystem of qubits and the rest of the larger system.

The entanglement within a quantum system can be categorized as area-law or volume-law, based on how this shared information scales with the geometry of subsystems. In volume-law entanglement, the amount of entanglement between a subsystem of qubits and the rest of the system grows proportionally with the total size of the subsystem.

On the other hand, area-law entanglement depends on how many shared connections exist between a subsystem of qubits and the larger system. As the subsystem expands, the amount of entanglement only grows along the boundary between the subsystem and the larger system.

In theory, the formation of volume-law entanglement is related to what makes quantum computing so powerful.

“While have not yet fully abstracted the role that entanglement plays in quantum algorithms, we do know that generating volume-law entanglement is a key ingredient to realizing a quantum advantage,” says Oliver.

However, volume-law entanglement is also more complex than area-law entanglement and practically prohibitive at scale to simulate using a classical computer.

“As you increase the complexity of your quantum system, it becomes increasingly difficult to simulate it with conventional computers. If I am trying to fully keep track of a system with 80 qubits, for instance, then I would need to store more information than what we have stored throughout the history of humanity,” Karamlou says.

The researchers created a quantum processor and control protocol that enable them to efficiently generate and probe both types of entanglement.

Their processor comprises superconducting circuits, which are used to engineer artificial atoms. The artificial atoms are utilized as qubits, which can be controlled and read out with high accuracy using microwave signals.

The device used for this experiment contained 16 qubits, arranged in a two-dimensional grid. The researchers carefully tuned the processor so all 16 qubits have the same transition frequency. Then, they applied an additional microwave drive to all of the qubits simultaneously.

If this microwave drive has the same frequency as the qubits, it generates quantum states that exhibit volume-law entanglement. However, as the microwave frequency increases or decreases, the qubits exhibit less volume-law entanglement, eventually crossing over to entangled states that increasingly follow an area-law scaling.

Careful control

“Our experiment is a tour de force of the capabilities of superconducting quantum processors. In one experiment, we operated the processor both as an analog simulation device, enabling us to efficiently prepare states with different entanglement structures, and as a digital computing device, needed to measure the ensuing entanglement scaling,” says Rosen.

To enable that control, the team put years of work into carefully building up the infrastructure around the quantum processor.

By demonstrating the crossover from volume-law to area-law entanglement, the researchers experimentally confirmed what theoretical studies had predicted. More importantly, this method can be used to determine whether the entanglement in a generic quantum processor is area-law or volume-law.

“The MIT experiment underscores the distinction between area-law and volume-law entanglement in two-dimensional quantum simulations using superconducting qubits. This beautifully complements our work on entanglement Hamiltonian tomography with trapped ions in a parallel publication published in Nature in 2023,” says Peter Zoller, a professor of theoretical physics at the University of Innsbruck, who was not involved with this work.

“Quantifying entanglement in large quantum systems is a challenging task for classical computers but a good example of where quantum simulation could help,” says Pedram Roushan of Google, who also was not involved in the study. “Using a 2D array of superconducting qubits, Karamlou and colleagues were able to measure entanglement entropy of various subsystems of various sizes. They measure the volume-law and area-law contributions to entropy, revealing crossover behavior as the system’s quantum state energy is tuned. It powerfully demonstrates the unique insights quantum simulators can offer.”

In the future, scientists could utilize this technique to study the thermodynamic behavior of complex quantum systems, which is too complex to be studied using current analytical methods and practically prohibitive to simulate on even the world’s most powerful supercomputers.

“The experiments we did in this work can be used to characterize or benchmark larger-scale quantum systems, and we may also learn something more about the nature of entanglement in these many-body systems,” says Karamlou.

Additional co-authors of the study are   Sarah E. Muschinske, Cora N. Barrett, Agustin Di Paolo, Leon Ding, Patrick M. Harrington, Max Hays, Rabindra Das, David K. Kim, Bethany M. Niedzielski, Meghan Schuldt, Kyle Serniak, Mollie E. Schwartz, Jonilyn L. Yoder, Simon Gustavsson, and Yariv Yanay.

This research is funded, in part, by the U.S. Department of Energy, the U.S. Defense Advanced Research Projects Agency, the U.S. Army Research Office, the National Science Foundation, the STC Center for Integrated Quantum Materials, the Wellesley College Samuel and Hilda Levitt Fellowship, NASA, and the Oak Ridge Institute for Science and Education.

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February 13, 2023

Quantum Entanglement Isn’t All That Spooky After All

The way we teach quantum theory conveys a spookiness that isn’t actually there

By Chris Ferrie

Conceptual artwork of a pair of entangled quantum particles.

Science Photo Library/Alamy Stock Photo

Quantum entanglement is a complex phenomenon in physics that is usually poorly described as an invisible link between distant quantum objects that allows one to instantly affect the other. Albert Einstein famously dismissed this idea of entanglement as “spooky action at a distance.” In reality, entanglement is better understood as information, but that’s admittedly bland. So nowadays, every news article , explainer , opinion piece and artistic interpretation of quantum entanglement equates the phenomenon with Einstein’s spookiness. The situation has only worsened with the 2022 Nobel Prize in Physics going to Alain Aspect, John F. Clauser and Anton Zeilinger for quantum entanglement experiments. But it’s time to cut this adjective loose. Calling entanglement spooky completely misrepresents how it actually works and hinders our ability to make sense of it.

In 1935, physicist Erwin Schrödinger coined the term entanglement , emphasizing that it was “not one but rather the characteristic trait of quantum mechanics, the one that enforces its entire departure from classical lines of thought.” He was writing in response to a famous paper (known simply to physicists as the EPR argument ) by Einstein, Boris Podolsky and Nathan Rosen, that claimed quantum physics was incomplete. The New York Times headline read, “Einstein attacks quantum theory,” which solidified the widespread perception that Einstein hated quantum physics.

The EPR argument concerns the everyday notion of reality as a collection of things in the world with physical properties waiting to be revealed through measurement. This is how most of us intuitively understand reality. Einstein’s theory of relativity fits into this understanding, and says reality must be local, meaning nothing can influence anything else faster than the speed of light. But EPR showed that quantum physics isn’t compatible with these ideas—that it can’t account for a theory of local reality. In other words, quantum physics was missing something. To complete quantum physics, Einstein suggested scientists should look for a “deeper” theory of local reality. Many physicists responded in defense of quantum theory, but the matter remained unresolved until 1964 when physicist John S. Bell proposed an experiment that could rule out the existence of a local reality. Clauser was the first to perform the test, which was later improved and perfected by Aspect and Zeilinger.

A typical article about entanglement tells us it arises when particles interact to create a “link,” which persists no matter how far apart those particles are. Moreover, actions taken on one particle instantly affect the other, or so we are told. But—and here’s the thing even many experts get wrong—quantum physics doesn’t say that. Quantum physics says nothing about how the world is. Instead, quantum physics only describes the experiments we do to test our theories of how the world works—it gives us probabilities for the outcomes that may happen in an experiment. The compulsion to interpret quantum physics concepts as prescriptions for physical reality derives from the unfortunate way we traditionally teach physics.

I teach quantum physics to second-year computer science students at the University of Technology Sydney. Every autumn, I give teenagers a working knowledge of quantum entanglement without telling them it is spooky by guiding them through the process of engineering quantum phenomena for themselves. A former student said they understood the 2022 Nobel Prize in Physics reporting because I have students program quantum computers to produce entanglement. Another former student told me they were having trouble figuring out where the mysterious spookiness was supposed to be. I suggested that perhaps they needn’t look for something they’re not going to find.

Typically, a physics teacher starts a lecture on entanglement with Einstein, introducing concepts like local realism and ending with necessarily invoking the free will of the experimenter. But it doesn’t need to be this way. It’s much easier to understand how quantum physics works, and how it departs from the classical world, from the perspective of information, not physics. Let’s consider an example.

Imagine two people, Alice and Bob, are implicated in a crime and are being questioned in separate rooms with no way to communicate. They are each asked one of two possible questions. They must corroborate each other’s story to be set free. But there’s a catch: the questions contain a trap such that if they are both asked the second question, they must give opposite answers. Alice and Bob know all this before heading into their rooms for questioning. So, they do the obvious thing and devise a strategy so that their answers will be correlated in just the right way. However, it quickly becomes apparent that no possible strategy can set them free since they won’t know which question the other investigator asked. The best Alice and Bob can do is answer correctly 75 percent of the time, by both giving the same answer for every question, accepting they will fail in one of the four cases.

So far, Alice and Bob have only used classical information. But by sharing quantum information, they succeed with a probability higher than 75 percent. They do this by devising a strategy using the mathematics of quantum information rather than classical information. Intuiting the solution requires some familiarity with linear algebra, so I won’t detail it here. But it is a fact that the quantum information they share requires correlations, which means it is entangled. This appears spooky to the investigators because they only reason with classical information. But it’s not spooky. In any theory of information, correlations are ubiquitous. Through the lens of quantum information, then, entanglement is not strange or rare, but rather expected. The information perspective beautifully illustrates the core problem with demanding a classical description of quantum phenomena: it’s the wrong language. The Nobel Prize–winners were the first to demonstrate this as a fact about nature. Today, you can follow in their footsteps by creating entanglement and processing the correlated quantum information on a real quantum computer .

Einstein wanted all of nature identified with a simple and compact classical description. But we now know that quantum information provides the most accurate description of nature, which is written in a language we do not speak. Accepting this liberates us from the limits of traditional physics and makes teaching it more natural by facilitating active learning. The quantum information perspective illuminates some of the most profound questions in physics. For example, quantum information is the key to understanding the mystery of black holes and perhaps the entire universe . It also leads us to new quantum technologies that quickly and automatically encode and process quantum information.

For the second half of the 20th century, computers rapidly changed every facet of society, transforming our understanding of the universe and ourselves. We thought they were the ultimate tool for this purpose, but we were wrong. Scientists now believe the ultimate machine is a quantum computer, the full potential of which we have yet to realize. Determining when quantum computers will become ubiquitous and what problems they will solve is an exercise in crystal gazing. However, we already know they can solve a small list of problems such as factoring numbers, searching databases or simulating chemical reactions. If you have such a problem, you might be in the market for a quantum computer. You'll love it; Einstein, on the other hand, would have hated it.

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First Experimental Proof That Quantum Entanglement Is Real

By California Institute of Technology October 9, 2022

Scientists, including Albert Einstein and Erwin Schrödinger, first discovered the phenomenon of entanglement in the 1930s. In 1972, John Clauser and Stuart Freedman were the first to prove experimentally that two widely separated particles can be entangled.

A Q&A with Caltech alumnus John Clauser on his first experimental proof of quantum entanglement.

When scientists, including Albert Einstein and Erwin Schrödinger, first discovered the phenomenon of entanglement in the 1930s, they were perplexed. Disturbingly, entanglement required two separated particles to remain connected without being in direct contact. In fact, Einstein famously called entanglement “spooky action at a distance,” because the particles seemed to be communicating faster than the speed of light.

Born on December 1, 1942, John Francis Clauser is an American theoretical and experimental physicist known for contributions to the foundations of quantum mechanics, in particular the Clauser–Horne–Shimony–Holt inequality. Clauser was awarded the 2022 Nobel Prize in Physics, jointly with Alain Aspect and Anton Zeilinger “for experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science.” try { window._mNHandle.queue.push(function (){ window._mNDetails.loadTag("974871025", "600x250", "974871025"); }); } catch (error) {}

To explain the bizarre implications of entanglement, Einstein, along with Boris Podolsky and Nathan Rosen (EPR), argued that “hidden variables” should be added to quantum mechanics. These could be used to explain entanglement, and to restore “locality” and “causality” to the behavior of the particles. Locality states that objects are only influenced by their immediate surroundings. Causality states that an effect cannot occur before its cause, and that causal signaling cannot propagate faster than light speed. Niels Bohr famously disputed EPR’s argument, while Schrödinger and Wendell Furry, in response to EPR, independently hypothesized that entanglement vanishes with wide-particle separation.

Unfortunately, at the time, no experimental evidence for or against quantum entanglement of widely separated particles was available. Experiments have since proven that entanglement is very real and fundamental to nature. Furthermore, quantum mechanics has now been proven to work, not only at very short distances but also at very great distances. Indeed, China’s quantum-encrypted communications satellite, Micius, (part of the Quantum Experiments at Space Scale (QUESS) research project) relies on quantum entanglement between photons that are separated by thousands of kilometers.

John Clauser standing with his second quantum entanglement experiment at UC Berkeley in 1976. Credit: University of California Graphic Arts / Lawrence Berkeley Laboratory

The very first of these experiments was proposed and executed by Caltech alumnus John Clauser (BS ’64) in 1969 and 1972, respectively. His findings are based on Bell’s theorem, devised by CERN theorist John Bell. In 1964, Bell ironically proved that EPR’s argument actually led to the opposite conclusion from what EPR had originally intended to show. Bell demonstrated that quantum entanglement is, in fact, incompatible with EPR’s notion of locality and causality.

In 1969 , while still a graduate student at Columbia University , Clauser, along with Michael Horne, Abner Shimony, and Richard Holt, transformed Bell’s 1964 mathematical theorem into a very specific experimental prediction via what is now called the Clauser–Horne–Shimony–Holt (CHSH) inequality ( Their paper has been cited more than 8,500 times on Google Scholar .) In 1972, when he was a postdoctoral researcher at the University of California Berkeley and Lawrence Berkeley National Laboratory , Clauser and graduate student Stuart Freedman were the first to prove experimentally that two widely separated particles (about 10 feet apart) can be entangled.

Clauser went on to perform three more experiments testing the foundations of quantum mechanics and entanglement, with each new experiment confirming and extending his results. The Freedman–Clauser experiment was the first test of the CHSH inequality. It has now been tested experimentally hundreds of times at laboratories around the world to confirm that quantum entanglement is real.

Clauser’s work earned him the 2010 Wolf Prize in physics. He shared it with Alain Aspect of the Institut d’ Optique and Ecole Polytechnique and Anton Zeilinger of the University of Vienna and the Austrian Academy of Sciences “for an increasingly sophisticated series of tests of Bell’s inequalities, or extensions thereof, using entangled quantum states,” according to the award citation.

John Clauser at a yacht club. Clauser enjoys sailboat racing in his spare time. Credit: John Dukat

Here, John Clauser answers questions about his historical experiments.

We hear that your idea of testing the principles of entanglement was unappealing to other physicists. Can you tell us more about that?

In the 1960s and 70s, experimental testing of quantum mechanics was unpopular at Caltech, Columbia, UC Berkeley, and elsewhere. My faculty at Columbia told me that testing quantum physics was going to destroy my career. While I was performing the 1972 Freedman–Clauser experiment at UC Berkeley, Caltech’s Richard Feynman was highly offended by my impertinent effort and told me that it was tantamount to professing a disbelief in quantum physics. He arrogantly insisted that quantum mechanics is obviously correct and needs no further testing! My reception at UC Berkeley was lukewarm at best and was only possible through the kindness and tolerance of Professors Charlie Townes [PhD ’39, Nobel Laureate ’64] and Howard Shugart [BS ’53], who allowed me to continue my experiments there.

In my correspondence with John Bell , he expressed exactly the opposite sentiment and strongly encouraged me to do an experiment. John Bell’s 1964 seminal work on Bell’s theorem was originally published in the terminal issue of an obscure journal, Physics , and in an underground physics newspaper, Epistemological Letters . It was not until after the 1969 CHSH paper and the 1972 Freedman–Clauser results were published in the Physical Review Letters that John Bell finally openly discussed his work. He was aware of the taboo on questioning quantum mechanics’ foundations and had never discussed it with his CERN co-workers.

What made you want to carry through with the experiments anyway?

Part of the reason that I wanted to test the ideas was because I was still trying to understand them. I found the predictions for entanglement to be sufficiently bizarre that I could not accept them without seeing experimental proof. I also recognized the fundamental importance of the experiments and simply ignored the career advice of my faculty. Moreover, I was having a lot of fun doing some very challenging experimental physics with apparatuses that I built mostly using leftover physics department scrap. Before Stu Freedman and I did the first experiment, I also personally thought that Einstein’s hidden-variable physics might actually be right, and if it is, then I wanted to discover it. I found Einstein’s ideas to be very clear. I found Bohr’s rather muddy and difficult to understand.

What did you expect to find when you did the experiments?

In truth, I really didn’t know what to expect except that I would finally determine who was right—Bohr or Einstein. I admittedly was betting in favor of Einstein but did not actually know who was going to win. It’s like going to the racetrack. You might hope that a certain horse will win, but you don’t really know until the results are in. In this case, it turned out that Einstein was wrong. In the tradition of Caltech’s Richard Feynman and Kip Thorne [BS ’62], who would place scientific bets, I had a bet with quantum physicist Yakir Aharonov on the outcome of the Freedman–Clauser experiment. Curiously, he put up only one dollar to my two. I lost the bet and enclosed a two-dollar bill and congratulations when I mailed him a preprint with our results.

I was very sad to see that my own experiment had proven Einstein wrong. But the experiment gave a 6.3-sigma result against him [a five-sigma result or higher is considered the gold standard for significance in physics]. But then Dick Holt and Frank Pipkin’s competing experiment at Harvard (never published) got the opposite result. I wondered if perhaps I had overlooked some important detail. I went on alone at UC Berkeley to perform three more experimental tests of quantum mechanics. All yielded the same conclusions. Bohr was right, and Einstein was wrong. The Harvard result did not repeat and was faulty. When I reconnected with my Columbia faculty, they all said, “We told you so! Now stop wasting money and go do some real physics.” At that point in my career, the only value in my work was that it demonstrated that I was a reasonably talented experimental physicist. That fact alone got me a job at Lawrence Livermore National Lab doing controlled-fusion plasma physics research.

Can you help us understand exactly what your experiments showed?

In order to clarify what the experiments showed, Mike Horne and I formulated what is now known as Clauser–Horne Local Realism [ 1974 ]. Additional contributions to it were subsequently offered by John Bell and Abner Shimony , so perhaps it is more properly called Bell–Clauser–Horne–Shimony Local Realism . Local Realism was very short-lived as a viable theory. Indeed, it was experimentally refuted even before it was fully formulated. Nonetheless, Local Realism is heuristically important because it shows in detail what quantum mechanics is not .

Local Realism assumes that nature consists of stuff, of objectively real objects, i. e., stuff you can put inside a box. (A box here is an imaginary closed surface defining separated inside and outside volumes.) It further assumes that objects exist whether or not we observe them. Similarly, definite experimental results are assumed to obtain, whether or not we look at them. We may not know what the stuff is, but we assume that it exists and that it is distributed throughout space. Stuff may evolve either deterministically or stochastically. Local Realism assumes that the stuff within a box has intrinsic properties, and that when someone performs an experiment within the box, the probability of any result that obtains is somehow influenced by the properties of the stuff within that box. If one performs say a different experiment with different experimental parameters, then presumably a different result obtains. Now suppose one has two widely separated boxes, each containing stuff. Local Realism further assumes that the experimental parameter choice made in one box cannot affect the experimental outcome in the distant box. Local Realism thereby prohibits spooky action-at-a-distance. It enforces Einstein’s causality that prohibits any such nonlocal cause and effect. Surprisingly, those simple and very reasonable assumptions are sufficient on their own to allow derivation of a second important experimental prediction limiting the correlation between experimental results obtained in the separated boxes. That prediction is the 1974 Clauser–Horne (CH) inequality.

The 1969 CHSH inequality’s derivation had required several minor supplementary assumptions, sometimes called “loopholes.” The CH inequality’s derivation eliminates those supplementary assumptions and is thus more general. Quantum entangled systems exist that disagree with the CH prediction, whereby Local Realism is amenable to experimental disproof. The CHSH and CH inequalities are both violated, not only by the first 1972 Freedman–Clauser experiment and my second 1976 experiment but now by literally hundreds of confirming independent experiments. Various labs have now entangled and violated the CHSH inequality with photon pairs, beryllium ion pairs, ytterbium ion pairs, rubidium atom pairs, whole rubidium-atom cloud pairs, nitrogen vacancies in diamonds, and Josephson phase qubits.

Testing Local Realism and the CH inequality was considered by many researchers to be important to eliminate the CHSH loopholes. Considerable effort was thus marshaled, as quantum optics technology improved and permitted. Testing the CH inequality had become a holy grail challenge for experimentalists. Violation of the CH inequality was finally achieved first in 2013 and again in 2015 at two competing laboratories: Anton Zeilinger’s group at the University of Vienna, and Paul Kwiat’s group at the University of Illinois at Urbana–Champaign. The 2015 experiments involved 56 researchers! Local Realism is now soundly refuted! The agreement between the experiments and quantum mechanics now firmly proves that nonlocal quantum entanglement is real.

What are some of the important technological applications of your work?

One application of my work is to the simplest possible object defined by Local Realism—a single bit of information. Local Realism shows that a single quantum mechanical bit of information, a “qubit,” cannot always be localized in a space-time box. This fact provides the fundamental basis of quantum information theory and quantum cryptography. Caltech’s quantum science and technology program, the 2019 $1.28-billion U.S. National Quantum Initiative, and the 2019 $400 million Israeli National Quantum Initiative all rely on the reality of entanglement. The Chinese Micius quantum-encrypted communications satellite system’s configuration is almost identical to that of the Freedman–Clauser experiment. It uses the CHSH inequality to verify entanglement’s persistence through outer space.

Can you tell us more about your family’s strong connection with Caltech?

My dad, Francis H. Clauser [BS ’34, MS ’35, PhD ’37, Distinguished Alumni Award ’66] and his brother Milton U. Clauser [BS ’34, MS ’35, PhD ’37] were PhD students at Caltech under Theodore von Kármán . Francis Clauser was Clark Blanchard Millikan Professor of Engineering at Caltech (Distinguished Faculty Award ’80) and chair of Caltech’s Division of Engineering and Applied Science. Milton U. Clauser’s son, Milton J. Clauser [PhD ’66], and grandson, Karl Clauser [BS ’86] both went to Caltech. My mom, Catharine McMillan Clauser was Caltech’s humanities librarian, where she met my dad. Her brother, Edwin McMillan [BS ’28, MS ’29], is a Caltech alum and ’51 Nobel Laureate. The family now maintains Caltech’s “Milton and Francis Doctoral Prize” awarded at Caltech commencements.

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21 comments on "first experimental proof that quantum entanglement is real".

The interactions and balances of topological vortex fields cover all short-distance and long-distance contributions, and are the basis of the formation and evolution of cosmic matter. 1.According to the topological vortex field theory, not only light, almost all rays and particles have electric effects. 2.The nature of electricity is perfect fluid.It has no shear stresses,viscosity,or heat conduction.Electric current generates heat because it interacts with vortex current. 3.Entanglement is one of the forms of interaction between vortexes. 4.If you are interested, please see https://zhuanlan.zhihu.com/p/463666584 . Good luck to your team.

The physical characteristics of the fluid vortex center are suitable to be described by energy rather than mass. According to the topological vortex field theory, there are two types of vortex centers: one is constant temperature and the other is variable temperature.

The expansion of space is due to star dynamics or relativity is a part of this.So,entanglement is a natural property present uniformly in universe for quantum particle after adding,saý GR(but not essential beside an arbitrary rational standard taken for measurement in the experiment);hence speed of light has no relation for entanglment of quantum particles at any two points in the same galaxy,or else.These facts have well proven before by many experimentations and are in due course of appĺied field,thus nothing to state about noble prize of current year in physics,but taken as secoded phenomena of the principle of entanglement.Congratulations to the physicists for their good presentations and wise works,granted late.

Fascinating!!!

Quantum Entanglement is perhaps the strangest phenomenon in physics, when some small particles may communicate with each other instantly and over vast distances. How can that happen without violating the maximum speed in the universe, the speed of light?

As this article says, the two ways physicists have used to answer this, that the particles contain hidden variables of unknown natures and that the universe is completely deterministic with all results predefined, have been shown to be incorrect.

Perhaps concepts in String Theory can help. There are 11 possible dimensions in String Theory and I suggest one of them leads a way around, what Einstein called this “spooky action at a distance”. Specifics on this can be found by searching YouTube for “Quantum Entanglement – A String Theory Way”

Bùt credìt of research on particle physics goes for quark-gluon to the America,charm quark to the CERN particĺe physicists.Thus,spin or magnetism required for entanglement has been done in parallel is an established work.

GR in connection to star dynamics is well proven concept taken in all kinds of measurements.

Metaphysics in Quantum Computation field is usual natural part has also been proven and established by experimentation.

All these are distinct works present ofcourse in fractional forms,but commonly adopted jointly in Quantum Computation.

So,alĺ these discoveries with their applications express happiness on behalf of this year’s Physics Nobel Prize with gratefulness to community and all with thanks.

It’s so fascinating contemplating the theme and variations of line of sight communication being moot through the newer mechanical developments

Energy can not be created or destroyed. Our thoughts are forms of energy. And scripture says “as a man think so is he”. Negative thoughts create depression, lack and poverty. Positive thoughts create abundance, wealth and prosperity.

FTL effects and hidden variable are not clearly ruled out and failure of localism could arise from FTL effects, it seems. “Non-local” with “hidden variables” still point to invisible FTL gravity effects, I believe.

Just to clarify, I wanted to note that it still seems “non-localism” and “hidden variables” can fit FTL gravity effects.

Refraction of fire and chief fields to contain high density gravity using quantum Magnetic codings will intensify the field of gravity to project

Quantum energy and its distant entanglement might be a breakthrough for holistic medical science. So therefore mysteries of working of homeopathic remedies on living organisms including humans could be explained and placebo effects of homeopathic remedies can further be explored. Diversity of conventional medical treatment can be boxed into single holistic approach. Thumbs up to marvelous discovery.

I have found a name for what goes on in my mind

ha ha ha… It’s the bizarre world where those embarrassments attempt to qualify as an authority by making word salad… to use their deleterious language once reserved for those sacrifices for the greater good, fire pits, abattoirs, and bomb vests. China still kills them, an economic champion, at what sacrifice? But you may be talking of mirror neurons, that not critical part of physical motion that allows instant … ok. entanglement for such as line dancers, but don’t confuse that with your critical thinking. Remember mirror neurons don’t really care, it’s a temporary allowing of one’s trust to be like another, not forebrain activity.

There is, of course, an information ‘matrix’ associated with the isotropic energy substrate underlying all measurable phenomena. ‘Particles’, therefore, isuue from this substrate and have, ipso facto, access to the information at any point of manifestation. It seems to me. So, no problem really with ‘Spooky Action’.

‘Particles’ isuue from this substrate put this notion well. The interactions and balances of topological vortex fields cover all short-distance and long-distance contributions, wich are the substrate of the formation and evolution of cosmic matter.

There are many here who are eminently more qualified than myself but it seems “apparent” that particles simultaneously exist in a different dimension and in that different dimension are essentially quite local.

you guys are just figuring this whole thing out now, this whole thing had been figured out a long time ago by ancient spiritualism, probably over 10 000 years ago. ancient spirituality had been trying to tell humanity that there is another dimension( “invisible reality”) which is the source of all things happening in this universe and outside the universe, they call it “the all”, some spiritual traditions call it the infinite consciousness, non-duality, the timeless dimension, the formless dimension and more. What’s happening in this universe of relativity is ultimately an illusion because people perceive reality as separate entities and the dimension I’m talking about is beyond forms, time, and space which all the dualistic categories of this universe and mental principles ceases to exist and what left is pure energy, the existence of this present moment(now).thats what science is trying to figure out and spirituality had already figure this whole thing out very long time ago. if someone wants to figure out what’s going on in quantum entanglement, I highly recommend you to access spirituality and non-dual teachings. it is not surprising that science is shocked about this because this whole had been figured out a long time ago, it’s just that science is catching up with spirituality. whatever is happening in the phenomenon of quantum entanglement that seems spooky is governed by that invisible reality called infinite conscioussness, which you cannot understand conceptually but realize as the oness.

Pure drivel, to start with Einstien who this fraudulent author misquotes, said quantum entanglement DOESN’T occur and there was no spooky action at a distance… completely misquoting others and besmirching their names by such slanders is common among such complete frauds as By CALIFORNIA INSTITUTE OF TECHNOLOGY or Not?

Just saying, anyone else ever seen a supposedly academic publication without a long list of authors, and co-authors all wanting credit for the publication as well as a long list of citations? No? Also the long list of word salad comments, anything false spawns false, proof of contraction is abundant, no such thing as quantum entanglement.

i suspect the quantum entanglement experiments are flawed but I have not found the details of these experiments. My skepticism arises from theories surrounding the origin of the universe. Black holes are gravitationally sorted spheres with the densest particles in the center. In order to have a big bang black holes (remnants of adjacent universes would have to collide. The resulting explosion propelled particles into space while preserving some of the more dense particles from the core which formed the early galaxies. The bulk of the mass shot into space the gavitational force decreasing with greater volume and distance from the center dense particles resulting in acceleration. No dark matter required. I am also skeptical of the atomic clock experiment which showed time slows with speed. all the experiment shows is that atomic radius is not constant. As an atom approaches the dense matter from the big bang at the center of the earth its radius deceases. I also suspect the current pole rotation we are in is tied to pre big bang dense matter at the center of the earth, and does not involve liquid iron suddenly changing direction. If quantum entanglement is real the experimental proceedures should be published and available to the layman.

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What Is Entanglement and Why Is It Important?

This article was reviewed by a member of Caltech's Faculty .

Entanglement is at the heart of quantum physics and future quantum technologies. Like other aspects of quantum science, the phenomenon of entanglement reveals itself at very tiny, subatomic scales. When two particles, such as a pair of photons or electrons, become entangled, they remain connected even when separated by vast distances. In the same way that a ballet or tango emerges from individual dancers, entanglement arises from the connection between particles. It is what scientists call an emergent property.

How do scientists explain quantum entanglement?

In the video below, Caltech faculty members take a stab at explaining entanglement. Featured: Rana Adhikari, professor of physics; Xie Chen, professor of theoretical physics; Manuel Endres, professor of physics and Rosenberg Scholar; and John Preskill, Richard P. Feynman Professor of Theoretical Physics, Allen V. C. Davis and Lenabelle Davis Leadership Chair, and director of the Institute for Quantum Information and Matter.

Unbreakable Correlation

When researchers study entanglement , they often use a special kind of crystal to generate two entangled particles from one. The entangled particles are then sent off to different locations. For this example, let's say the researchers want to measure the direction the particles are spinning, which can be either up or down along a given axis. Before the particles are measured, each will be in a state of superposition , or both "spin up" and "spin down" at the same time.

If the researcher measures the direction of one particle's spin and then repeats the measurement on its distant, entangled partner, that researcher will always find that the pair are correlated: if one particle's spin is up, the other's will be down (the spins may instead both be up or both be down, depending on how the experiment is designed, but there will always be a correlation). Returning to our dancer metaphor, this would be like observing one dancer and finding them in a pirouette, and then automatically knowing the other dancer must also be performing a pirouette. The beauty of entanglement is that just knowing the state of one particle automatically tells you something about its companion, even when they are far apart.

Are particles really connected across space?

But are the particles really somehow tethered to each other across space, or is something else going on? Some scientists, including Albert Einstein in the 1930s, pointed out that the entangled particles might have always been spin up or spin down, but that this information was hidden from us until the measurements were made. Such "local hidden variable theories" argued against the mind-boggling aspect of entanglement, instead proposing that something more mundane, yet unseen, is going on.

Thanks to theoretical work by John Stewart Bell in the 1960s, and experimental work done by Caltech alumnus John Clauser (BS '64) and others beginning in the 1970s, scientists have ruled out these local hidden-variable theories. A key to the researchers' success involved observing entangled particles from different angles. In the experiment mentioned above, this means that a researcher would measure their first particle as spin up, but then use a different viewing angle (or a different spin axis direction) to measure the second particle. Rather than the two particles matching up as before, the second particle would have gone back into a state of superposition and, once observed, could be either spin up or down. The choice of the viewing angle changed the outcome of the experiment, which means that there cannot be any hidden information buried inside a particle that determines its spin before it is observed. The dance of entanglement materializes not from any one particle but from the connections between them.

Relativity Remains Intact

A common misconception about entanglement is that the particles are communicating with each other faster than the speed of light, which would go against Einstein's special theory of relativity. Experiments have shown that this is not true, nor can quantum physics be used to send faster-than-light communications. Though scientists still debate how the seemingly bizarre phenomenon of entanglement arises, they know it is a real principle that passes test after test. In fact, while Einstein famously described entanglement as "spooky action at a distance," today's quantum scientists say there is nothing spooky about it.

"It may be tempting to think that the particles are somehow communicating with each other across these great distances, but that is not the case," says Thomas Vidick , a professor of computing and mathematical sciences at Caltech. "There can be correlation without communication," and the particles "can be thought of as one object."

Networks of Entanglement

Entanglement can also occur among hundreds, millions, and even more particles. The phenomenon is thought to take place throughout nature, among the atoms and molecules in living species and within metals and other materials. When hundreds of particles become entangled, they still act as one unified object. Like a flock of birds, the particles become a whole entity unto itself without being in direct contact with one another. Caltech scientists focus on the study of these so-called many-body entangled systems, both to understand the fundamental physics and to create and develop new quantum technologies. As John Preskill, Caltech's Richard P. Feynman Professor of Theoretical Physics, Allen V. C. Davis and Lenabelle Davis Leadership Chair, and director of the Institute for Quantum Information and Matter, says, "We are making investments in and betting on entanglement being one of the most important themes of 21st-century science."

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Quantum Entanglement and Information

Quantum entanglement is a physical resource, like energy, associated with the peculiar nonclassical correlations that are possible between separated quantum systems. Entanglement can be measured, transformed, and purified. A pair of quantum systems in an entangled state can be used as a quantum information channel to perform computational and cryptographic tasks that are impossible for classical systems. The general study of the information-processing capabilities of quantum systems is the subject of quantum information theory.

1. Quantum Entanglement

2. exploiting entanglement: quantum teleportation, 3. quantum information, 4. quantum cryptography, 5. quantum computation, 6. interpretative remarks, other internet resources, related entries.

In 1935 and 1936, Schrödinger published a two-part article in the Proceedings of the Cambridge Philosophical Society in which he discussed and extended an argument by Einstein, Podolsky, and Rosen. The Einstein-Podolsky-Rosen (EPR) argument was, in many ways, the culmination of Einstein’s critique of the orthodox Copenhagen interpretation of quantum mechanics and was designed to show that the theory is incomplete. (See the entries on the Einstein-Podolsky-Rosen argument in quantum theory and the Copenhagen interpretation of quantum mechanics .) In classical mechanics the state of a system is essentially a list of the system’s properties — more precisely, it is the specification of a set of parameters from which the list of properties can be reconstructed: the positions and momenta of all the particles comprising the system (or similar parameters in the case of fields). The dynamics of the theory specifies how properties change in terms of a law of evolution for the state. In a letter to Max Born, Wolfgang Pauli characterized this mode of description of physical systems as a ‘detached observer’ idealization (see The Born-Einstein Letters , Born, 1992; p. 218). On the Copenhagen interpretation, such a description is not possible for quantum systems. Instead, the quantum state of a system should be understood as a catalogue of what an observer has done to the system and what has been observed, and the import of the state then lies in the probabilities that can be inferred (in terms of the theory) for the outcomes of possible future observations on the system. Einstein rejected this view and proposed a series of arguments to show that the quantum state is simply an incomplete characterization of a quantum system. The missing parameters are sometimes referred to as ‘hidden parameters’ or ‘hidden variables.’

It should not be supposed that Einstein’s notion of a complete theory included the requirement that the theory should be deterministic. Rather, he required certain conditions of separability and locality for composite systems consisting of separated component systems: each component system separately should be characterized by its own properties (its own ‘being-thus,’ as Einstein put it — ‘So-sein’ in German), and it should be impossible to alter the properties of a distant system instantaneously (or the probabilities of these properties) by acting on a local system. In later analyses, notably in Bell’s argument for the nonlocality of quantum correlations, it became apparent that these conditions, suitably formulated as probability constraints, are equivalent to the requirement that statistical correlations between separated systems should be reducible to probability distributions over common causes (deterministic or stochastic) in the sense of Reichenbach. (See the entries on Bell’s theorem and Reichenbach’s common cause principle .)

In the original EPR article, two particles are prepared from a source in a certain ‘pure’ quantum state of the composite system (a state that cannot be expressed as a mixture or probability distribution of other pure quantum states, and cannot be reduced to a pure quantum state of each particle separately). After the particles move apart, there are ‘matching’ correlations between both the positions of the two particles and their momenta: a measurement of either position or momentum on a particular particle will allow the prediction, with certainty, of the outcome of a position measurement or momentum measurement, respectively, on the other particle. These measurements are mutually exclusive: either a position measurement can be performed, or a momentum measurement, but not both simultaneously. The subsequent measurement of momentum, say, after establishing a position correlation, will no longer yield any correlation in the momenta of the two particles. It is as if the position measurement disturbs the correlation between the momentum values, and conversely. Apart from this peculiarity that either correlation can be observed, but not both for the same pair of quantum particles, the position and momentum correlations for the quantum particles are exactly like the classical correlations between two billiard balls after a collision. Classical correlations can be explained by a common cause, or correlated ‘elements of reality.’ The EPR argument is that quantum mechanics is incomplete because these common causes or elements of reality are not included in the quantum state description.

Here is how Schrödinger put the puzzle in the first part of his two-part article (Schrödinger, 1935; p. 559):

Yet since I can predict either \(x_1\) or \(p_1\) without interfering with the system No. 1 and since system No. 1, like a scholar in an examination, cannot possibly know which of the two questions I am going to ask first: it so seems that our scholar is prepared to give the right answer to the first question he is asked, anyhow . Therefore he must know both answers; which is an amazing knowledge; quite irrespective of the fact that after having given his first answer our scholar is invariably so disconcerted or tired out, that all the following answers are ‘wrong.’

What Schrödinger showed was that if two particles are prepared in an EPR quantum state, where there are matching correlations for two ‘canonically conjugate’ dynamical quantities (quantities like position and momentum whose values suffice to specify all the properties of a classical system), then there are infinitely many dynamical quantities of the two particles for which there exist similar matching correlations: every function of the canonically conjugate pair of the first particle matches with the same function of the canonically conjugate pair of the second particle. So (Schrödinger, p. 559) system No. 1 ‘does not only know these two answers but a vast number of others, and that with no mnemotechnical help whatsoever, at least with none that we know of.’

Schrödinger coined the term ‘entanglement’ to describe this peculiar connection between quantum systems (Schrödinger, 1935; p. 555):

When two systems, of which we know the states by their respective representatives, enter into temporary physical interaction due to known forces between them, and when after a time of mutual influence the systems separate again, then they can no longer be described in the same way as before, viz. by endowing each of them with a representative of its own. I would not call that one but rather the characteristic trait of quantum mechanics, the one that enforces its entire departure from classical lines of thought. By the interaction the two representatives [the quantum states] have become entangled.

He added (Schrödinger, 1935; p. 555):

Another way of expressing the peculiar situation is: the best possible knowledge of a whole does not necessarily include the best possible knowledge of all its parts, even though they may be entirely separate and therefore virtually capable of being ‘best possibly known,’ i.e., of possessing, each of them, a representative of its own. The lack of knowledge is by no means due to the interaction being insufficiently known — at least not in the way that it could possibly be known more completely — it is due to the interaction itself. Attention has recently been called to the obvious but very disconcerting fact that even though we restrict the disentangling measurements to one system, the representative obtained for the other system is by no means independent of the particular choice of observations which we select for that purpose and which by the way are entirely arbitrary. It is rather discomforting that the theory should allow a system to be steered or piloted into one or the other type of state at the experimenter’s mercy in spite of his having no access to it.

In the second part of the paper, Schrödinger showed that an experimenter, by a suitable choice of operations carried out on one member of an entangled pair, possibly using additional ‘ancilla’ or helper particles, can ‘steer’ the second system into a chosen mixture of quantum states, with a probability distribution that depends on the entangled state. The second system cannot be steered into a particular quantum state at the whim of the experimenter, but for many copies of the entangled pair, the experimenter can constrain the quantum state of the second system to lie in a chosen set of quantum states, where these states are correlated with the possible outcomes of measurements carried out on the entangled paired systems, or the paired systems plus ancillas. He found this conclusion sufficiently unsettling to suggest that the entanglement between two separating systems would persist only for distances small enough that the time taken by light to travel from one system to the other could be neglected, compared with the characteristic time periods associated with other changes in the composite system. He speculated that for longer distances the two systems might in fact be in a correlated mixture of quantum states determined by the entangled state.

Most physicists attributed the puzzling features of entangled quantum states to Einstein’s inappropriate ‘detached observer’ view of physical theory and regarded Bohr’s reply to the EPR argument (Bohr, 1935) as vindicating the Copenhagen interpretation. This was unfortunate, because the study of entanglement was ignored for thirty years until John Bell’s reconsideration of the EPR argument (Bell, 1964). Bell looked at entanglement in simpler systems than the EPR example: matching correlations between two-valued dynamical quantities, such as polarization in a particular direction or spin in a particular direction, of two separated systems in an entangled state. What Bell showed was that the statistical correlations between the measurement outcomes of suitably chosen different quantities on the two systems are inconsistent with an inequality derivable from Einstein’s separability and locality assumptions — in effect from the assumption that the correlations have a common cause. This inequality is now known as Bell’s inequality, and various related inequalities can be derived as a necessary condition for classical or common cause correlations.

Bell’s investigation generated an ongoing debate on the foundations of quantum mechanics. One important feature of this debate was confirmation that entanglement can persist over long distances, thus falsifying Schrödinger’s supposition of the spontaneous decay of entanglement as two entangled particles separate. (Free space entanglement of photons has been demonstrated over a distance of 143 km and, using satellites to distribute entanglement, between locations more than 1200 km apart on earth. See Herbst et al 2014 and Yin et al 2017.) But it was not until the 1980s that physicists, computer scientists, and cryptologists began to regard the non-local correlations of entangled quantum states as a new kind of non-classical physical resource that could be exploited, rather than an embarrassment for quantum mechanics to be explained away. For a discussion of entanglement — what it is, why it is conceptually puzzling, and what you can do with it, including a simple proof of Bell’s theorem — see the graphic novel Totally Random: Why Nobody Understands Quantum Mechanics (A Serious Comic on Entanglement) , Bub and Bub 2018. For further discussion of entanglement as a physical resource, including measuring entanglement, and the manipulation and purification of entanglement by local operations, see “The Joy of Entanglement” by Popescu and Rohrlich in Lo, Popescu, and Spiller 1998, Bub 2016, and the classic Nielsen and Chuang 2011. Alain Aspect, John F. Clauser, and Anton Zeilinger were awared the 2022 Nobel Prize in physics for their pioneering experiments on entangled photons, validating Bell’s insight and initiating the field of quantum information.

Consider again Schrödinger’s realization that an entangled state could be used to steer a distant particle into one of a set of states, with a certain probability. In fact, this possibility of ‘remote steering’ is even more dramatic than Schrödinger demonstrated. Suppose Alice and Bob share an entangled pure state of the sort considered by Bell, say two photons in an entangled state of polarization, where Alice has in her possession one of the entangled photons, and Bob has the second paired photon. Suppose that Alice receives an additional photon in an unknown state of polarization \(\ket{u}\), where the notation ‘\(\ket{\ }\)’ denotes a quantum state. It is possible for Alice to perform an operation on the two photons in her possession that will transform Bob’s photon into one of four states, depending on the four possible (random) outcomes of Alice’s operation: either the state \(\ket{u}\), or a state that is related to \(\ket{u}\) in a definite way. Alice’s operation entangles the two photons in her possession, and disentangles Bob’s photon, steering it into a state \(\ket{u^*}\). After Alice communicates the outcome of her operation to Bob, Bob knows either that \(\ket{u^*}\) = \(\ket{u}\), or how to transform \(\ket{u^*}\) to \(\ket{u}\) by a local operation. This phenomenon is known as ‘quantum teleportation.’ After the teleportation procedure the state \(\ket{u}\) remains unknown to both Alice and Bob.

What is extraordinary about this phenomenon is that Alice and Bob have managed to use their shared entangled state as a quantum communication channel to destroy the state \(\ket{u}\) of a photon in Alice’s part of the universe and recreate it in Bob’s part of the universe. Since the linear polarization state of a photon requires specifying a direction in space (the value of an angle that can vary continuously), without a shared entangled state Alice would have to convey an infinite amount of classical information to Bob for Bob to be able to reconstruct the state \(\ket{u}\) precisely. The amount of classical information associated with a binary alternative, represented as 0 or 1, where each alternative has equal probability, is one binary digit or ‘bit.’ To specify an arbitrary angle as a decimal requires an infinite sequence of digits between 0 and 9, or an infinite sequence of 0s and 1s in binary notation. The outcome of Alice’s operation, which has four possible outcomes with equal probability of 1/4, can be specified by two bits of classical information. Remarkably, Bob can reconstruct the state \(\ket{u}\) on the basis of just two bits of classical information communicated by Alice, apparently by exploiting the entangled state as a quantum communication channel to transfer the remaining information. The idea was first proposed by Bennett et al in 1993. For further discussion of quantum teleportation, see Nielsen and Chuang 2011, or Richard Josza’s article “Quantum Information and its Properties” in Lo, Popescu, and Spiller 1998. Quantum teleportation has been achieved over a distance of around 100 km on earth, and 1400 km between earth and a satellite (Zeilinger, 2018). The development of a quantum internet using satellites to transmit entangled photons will rely on quantum teleportation.

Formally, the amount of classical information we gain, on average, when we learn the value of a random variable (or, equivalently, the amount of uncertainty in the value of a random variable before we learn its value) is represented by a quantity called the Shannon entropy, measured in bits (Shannon and Weaver, 1949). A random variable is defined by a probability distribution over a set of values. In the case of a binary random variable, with equal probability for each of the two possibilities, the Shannon entropy is one bit, representing maximal uncertainty. For all other probabilities — intuitively, representing some information about which alternative is more likely — the Shannon entropy is less than one. For the case of maximal knowledge or zero uncertainty about the alternatives, where the probabilities are 0 and 1, the Shannon entropy is zero. (Note that the term ‘bit’ is used to refer to the basic unit of classical information in terms of Shannon entropy, and to an elementary two-state classical system considered as representing the possible outputs of an elementary classical information source.)

Since information is always embodied in the state of a physical system, we can also think of the Shannon entropy as quantifying the physical resources required to store classical information. Suppose Alice wishes to communicate some classical information to Bob over a classical communication channel such as a telephone line. A relevant question concerns the extent to which the message can be compressed without loss of information, so that Bob can reconstruct the original message accurately from the compressed version. According to Shannon’s source coding theorem or noiseless coding theorem (assuming a noiseless telephone line with no loss of information), the minimal physical resource required to represent the message (effectively, a lower bound on the possibility of compression) is given by the Shannon entropy of the source.

What happens if we use the quantum states of physical systems to store information, rather than classical states? It turns out that quantum information is radically different from classical information. The unit of quantum information is the ‘qubit’, representing the amount of quantum information that can be stored in the state of the simplest quantum system, for example, the polarization state of a photon. The term is due to Schumacher (1995), who proved a quantum analogue of Shannon’s noiseless coding theorem. (By analogy with the term ‘bit,’ the term ‘qubit’ refers to the basic unit of quantum information in terms of the von Neumann entropy, and to an elementary two-state quantum system considered as representing the possible outputs of an elementary quantum information source.) An arbitrarily large amount of classical information can be encoded in a qubit. This information can be processed and communicated but, because of the peculiarities of quantum measurement, at most one bit can be accessed. According to a theorem by Holevo, the accessible information in a probability distribution over a set of alternative qubit states is limited by the von Neumann entropy, which is equal to the Shannon entropy only when the states are orthogonal in the space of quantum states, and is otherwise less than the Shannon entropy.

While classical information can be copied or cloned, the quantum ‘no cloning’ theorem (Dieks, 1982; Wootters and Zurek, 1982) asserts the impossibility of cloning an unknown quantum state. To see why, consider how we might construct a classical copying device. A NOT gate is a device that takes a bit as input and produces as output either a 1 if the input is 0, or a 0 if the input is 1. In other words, a NOT gate is a 1-bit gate that flips the input bit. A controlled-NOT gate, or CNOT gate, takes two bits as inputs, a control bit and a target bit, and flips the target bit if and only if the control bit is 1, while reproducing the control bit. So there are two inputs, the control and target, and two outputs: the control, and either the target or the flipped target, depending on the value of the control. A CNOT gate functions as a copying device for the control bit if the target bit is set to 0, because the output of the target bit is then a copy of the control bit: the input 00 produces output 00, and the input 10 produces output 11 (here the first bit is the control and the second bit is the target). Insofar as we can think of a measurement as simply a copying operation, a CNOT gate is the paradigm of a classical measuring device. Imagine Alice equipped with such a device, with input and output control and target wires, measuring the properties of an unknown classical world. The input control wire is a probe for the presence or absence of a property, represented by a 1 or a 0. The target wire functions as the pointer, which is initially set to 0. The output of the target is a 1 or a 0, depending on the presence or absence of the property.

Suppose we attempt to use a CNOT gate to copy an unknown qubit state. Since we are now proposing to regard the CNOT gate as a device for processing quantum states, the evolution from input states to output states must be effected by a physical quantum transformation. Quantum transformations are linear on the linear state space of qubits. Linearity of the state space means that any sum or superposition with coefficients \(c_0, c_1\) of two qubit states in the state space is also a qubit state in the state space. Linearity of the transformation requires that the transformation should take a qubit state represented by the sum of two qubit states to a new qubit state that is the sum of the transformed qubit states. If the CNOT gate succeeds in copying two orthogonal qubit states, represented as \(\ket{0},\ket{1}\), it cannot succeed in copying a general linear superposition of these qubits. Since the gate functions linearly, it must instead produce a state that is a linear superposition of the outputs obtained for the two orthogonal qubit states. That is to say, the output of the gate will be represented by a quantum state that is a sum of two terms, where the first term represents the output of the control and target for the first qubit state, and the second term represents the output of the control and target for the second orthogonal qubit state. This could be expressed as \(c_0 \ket{0} \ket{0}\) + \(c_1 \ket{1} \ket{1}\), which is an entangled state (unless \(c_0\) or \( c_1\) is zero) rather than the output that would be required by a successful copying operation (where the control and target each outputs the superposition qubit state \(c_0 \ket{0}\) + \(c_1 \ket{1}\)).

Suppose Alice and Bob are separated and want to communicate a secret message, without revealing any information to Eve, an eavesdropper. They can do this in a classical world if they share a ‘one-time pad,’ a cryptographic key represented by a sequence of random bits at least as long as the number of bits required to communicate the message. In fact, this is the only secure way to achieve perfect security in a classical world. To send a message to Bob, Alice communicates which bits in the key Bob should flip. The resulting sequence of bits is the message. In addition, they would need to have some way of encoding messages as sequences of bits, by representing letters of the alphabet and spaces and punctuation symbols as binary numbers, which could be done by some standard, publicly available scheme.

The problem is that messages communicated in this way are only secret if Alice and Bob use a different one-time pad for each message. If they use the same one-time pad for several messages, Eve could gain some information about the correspondence between letters of the alphabet and subsequences of bits in the key by relating statistical features of the messages to the way words are composed of letters. To share a new key they would have to rely on trusted couriers or some similar method to distribute the key. There is no way to guarantee the security of the key distribution procedure in a classical world.

Copying the key without revealing that it has been copied is also a problem for the shared key that Alice and Bob each store in some supposedly secure way. But the laws of physics in a classical world cannot guarantee that a storage procedure is completely secure, and they cannot guarantee that breaching the security and copying the key will always be detected. So apart from the key distribution problem, there is a key storage problem.

Quantum entanglement provides a way of solving these problems through the ‘monogamy’ of entangled state correlations: no third party can share entanglement correlations between Alice and Bob. Moreover, any attempt by Eve to measure the quantum systems in the entangled state shared by Alice and Bob will destroy the entangled state. Alice and Bob can detect this by checking a Bell inequality.

One way to do this is by a protocol originally proposed by Artur Ekert. Suppose Alice has a collection of photons, one for each entangled pair in the state \(\ket{0}\ket{0} + \ket{1}\ket{1}\) (ignoring the equal coefficients, for simplicity), and Bob has the collection of paired photons. Alice measures the polarization of her photons randomly in directions, \(0, \pi/8, 2\pi/8\) with respect to some direction \(z\) they agree on in advance, and Bob measures the polarizations of his photons randomly in directions \(\pi/8, 2\pi/8, 3\pi/8\). They communicate the directions of their polarization measurements publicly, but not the outcomes, and they divide the measurements into two sets: one set when they both measured polarization in the direction \(\pi/8\), or when they both measured polarization in the direction \(2\pi/8\), and one set when Alice measured polarization in directions \(0\) or \(2\pi/8\) and Bob measured polarization in directions \(\pi/8\) or \(3\pi/8\). For the first set, when they measured the polarization in the same direction, the outcomes are random but perfectly correlated in the entangled state so they share these random bits as a cryptographic key. They use the second set to check a Bell inequality, which reveals whether or not the entangled state has been altered by the measurements of an eavesdropper. (See Ekert, 1991.)

While the difference between classical and quantum information can be exploited to achieve successful key distribution, there are other cryptographic protocols that are thwarted by quantum entanglement. Bit commitment is a key cryptographic protocol that can be used as a subroutine in a variety of important cryptographic tasks. In a bit commitment protocol, Alice supplies an encoded bit to Bob. The information available in the encoding should be insufficient for Bob to ascertain the value of the bit, but sufficient, together with further information (supplied by Alice at a subsequent stage when she is supposed to reveal the value of the bit), for Bob to be convinced that the protocol does not allow Alice to cheat by encoding the bit in a way that leaves her free to reveal either 0 or 1 at will.

To illustrate the idea, suppose Alice claims the ability to predict advances or declines in the stock market on a daily basis. To substantiate her claim without revealing valuable information (perhaps to a potential employer, Bob) she suggests the following demonstration: She proposes to record her prediction, before the market opens, by writing a 0 (for ‘decline’) or a 1 (for ‘advance’) on a piece of paper, which she will lock in a safe. The safe will be handed to Bob, but Alice will keep the key. At the end of the day’s trading, she will announce the bit she chose and prove that she in fact made the commitment at the earlier time by handing Bob the key. Of course, the key-and-safe protocol is not provably secure from cheating by Bob, because there is no principle of classical physics that prevents Bob from opening the safe and closing it again without leaving any trace. The question is whether there exists a quantum analogue of this procedure that is unconditionally secure: provably secure by the laws of physics against cheating by either Alice or Bob. Bob can cheat if he can obtain some information about Alice’s commitment before she reveals it (which would give him an advantage in repetitions of the protocol with Alice). Alice can cheat if she can delay actually making a commitment until the final stage when she is required to reveal her commitment, or if she can change her commitment at the final stage with a very low probability of detection.

It turns out that unconditionally secure two-party bit commitment, based solely on the principles of quantum or classical mechanics (without exploiting special relativistic signaling constraints, or principles of general relativity or thermodynamics) is impossible. See Mayers 1997, Lo and Chau 1997 and Lo’s article “Quantum Cryptology” in Lo, Popescu, and Spiller 1998 for further discussion. (Kent 1999 has shown that one can implement a secure classical bit commitment protocol by exploiting relativistic signaling constraints in a timed sequence of communications between verifiably separated sites for both Alice and Bob.) Roughly, the impossibility arises because at any step in the protocol where either Alice or Bob is required to make a determinate choice (perform a measurement on a particle in the quantum channel, choose randomly and perhaps conditionally between a set of alternative actions to be implemented on the particle in the quantum channel, etc.), the choice can delayed by entangling one or more ancilla particles with the channel particle in an appropriate way. By suitable operations on the ancillas, the channel particle can be ‘steered’ so that this cheating strategy is undetectable. In effect, if Bob can obtain no information about the committed bit, then entanglement will allow Alice to ‘steer’ the bit to either 0 or 1 at will.

In 2022 the US National Institute of Standards and Technology (NIST) announced a suite of encryption protocols designed to be secure against attacks by a quantum computer: NIST Announces First Four Quantum-Resistant Cryptographic Algorithms .

Quantum information can be processed, but the accessibility of this information is limited by the Holevo bound (mentioned in Section 3). David Deutsch (1985) first showed how to exploit quantum entanglement to perform a computational task that is impossible for a classical computer. Suppose we have a black box or oracle that evaluates a Boolean function \(f\), where the arguments or inputs of \(f\) are either 0 or 1, and the values or outputs of \(f\) are also 0 or 1. The outputs are either the same for both inputs (in which case \(f\) is said to be constant), or different for the two inputs (in which case \(f\) is said to be balanced). Suppose we are interested in determining whether \(f\) is constant or balanced. Classically, the only way to do this is to run the black box or query the oracle twice, for both arguments 0 and 1, and to pass the values (outputs of \(f\)) to a circuit that determines whether they are the same (for ‘constant’) or different (for ‘balanced’). Deutsch showed that if we use quantum states and quantum gates to store and process information, then we can determine whether \(f\) is constant or balanced in one evaluation of the function \(f\). The trick is to design the circuit (the sequence of gates) to produce the answer to a global question about the function in an output qubit register that can then be read out or measured.

Consider again the quantum CNOT gate, with two orthogonal qubits \(\ket{0}\) and \(\ket{1}\) as possible inputs for the control, and \(\ket{0}\) as the input for the target. One can think of the input control and output target qubits, respectively, as the argument and associated value of a function. This CNOT function associates the value 0 with the argument 0 and the value 1 with the argument 1. For a linear superposition of the orthogonal qubits with equal coefficients as input to the control, and the qubit \(\ket{0}\) as the input to the target, the output is the entangled state \(\ket{0} \ket{0}\) + \(\ket{1} \ket{1}\) (ignoring the coefficients, for simplicity). This is a linear superposition in which the first term represents the argument 0 and associated value 0 of the CNOT function, and the second term represents the argument 1 and associated value 1 of the CNOT function. The entangled state represents all possible arguments and corresponding values of the function as a linear superposition, but this information is not accessible. What can be shown to be accessible, by a suitable choice of quantum gates, is information about whether or not the function has certain global properties. This information is obtainable without reading out the evaluation of any individual arguments and values. (Indeed, accessing information in the entangled state about a global property of the function will typically require losing access to all information about individual arguments and values.)

The situation is analogous for Deutsch’s function \(f\). Here the output of \(f\) can be represented as either \(\ket{0} \ket{0} + \ket{1} \ket{0}\) or \(\ket{0} \ket{1} + \ket{1} \ket{1}\) in the ‘constant’ case, or \(\ket{0} \ket{0} + \ket{1} \ket{1}\) or \(\ket{0} \ket{1} + \ket{1} \ket{0}\) in the ‘balanced’ case. The two entangled states in the ‘constant’ case are orthogonal in the 4-dimensional two-qubit state space and span a plane. Call this the ‘constant’ plane. Similarly, the two entangled states in the ‘balanced’ case span a plane, the ‘balanced’ plane. These two planes, representing two alternative quantum disjunctions, are orthogonal except for an intersection or overlap in a line, representing a product (non-entangled) state, where each qubit separately is in the state \(\ket{0} + \ket{1}\). It is therefore possible to design a measurement to distinguish the two alternative disjunctive or global properties of \(f\), ‘constant’ or ‘balanced,’ with a certain probability (actually, 1/2) of failure, when the measurement yields an outcome corresponding to the overlap state, which is common to the two cases. Nevertheless, only one query of the function is required when the measurement succeeds in identifying the global property. With a judicious choice of quantum gates, it is even possible to design a quantum circuit that always succeeds in distinguishing the two cases in one run.

Deutsch’s example shows how quantum information and quantum entanglement can be exploited to compute a disjunctive or global property of a function in one step that would take two steps classically. While Deutsch’s problem is rather trivial, there now exist several quantum algorithms with interesting applications, notably Shor’s factorization algorithm for factoring large composite integers in polynomial time (with direct application to ‘public key’ cryptography, a widely used classical cryptographic scheme) and Grover’s database search algorithm. Shor’s algorithm achieves an exponential speed-up over any known classical algorithm. For algorithms that are allowed access to oracles (whose internal structure is not considered), the speed-up can be shown to be exponential over any classical algorithm in some cases, e.g., Simon’s algorithm. For more on quantum computing and quantum algorithms, see Nielsen and Chuang 2011, Barenco’s article “Quantum Computation: An Introduction” in Lo, Popescu, and Spiller 1998, Bub 2006 (Section 6), Abhijith et al 2022, as well as the entry on quantum computing .

Note that there is currently no proof that a quantum algorithm can solve an NP-complete problem in polynomial time (see the entry on computational complexity theory for the concept of an NP-complete problem), so the efficiency of quantum computers relative to classical computers could turn out to be illusory. If there is indeed a speed-up, it would seem to be due to the phenomenon of entanglement. The amount of information required to describe a general entangled state of \(n\) qubits grows exponentially with \(n\). The state space (Hilbert space) has \(2^n\) dimensions, and a general entangled state is a superposition of \(2^n\) \(n\)-qubit states. In classical mechanics there are no entangled states: a general \(n\)-bit composite system can be described with just \(n\) times the amount of information required to describe a single bit system. So the classical simulation of a quantum process would involve an exponential increase in the classical informational resource required to represent the quantum state, as the number of qubits that become entangled in the evolution grows linearly, and there would be a corresponding exponential slowdown in calculating the evolution, compared to the actual quantum computation performed by the system.

Deutsch (1997) has argued that the exponential speed-up in quantum computation, and in general the way a quantum system processes information, can only be properly understood within the framework of Everett’s ‘many-worlds’ interpretation (see the entries on Everett’s relative-state formulation of quantum mechanics and the many-worlds interpretation of quantum mechanics ). The idea, roughly, is that an entangled state of the sort that arises in the quantum computation of a function, which represents a linear superposition over all possible arguments and corresponding values of the function, should be understood as something like a massively parallel classical computation, for all possible values of the function, in parallel worlds. For an insightful critique of this idea of ‘quantum parallelism’ as explanatory, see Steane 2003.

An alternative view emphasizes the non-Boolean structure of properties of quantum systems. The properties of a classical system form a Boolean algebra, essentially the abstract characterization of a set-theoretic structure. This is reflected in the Boolean character of classical logic, and the Boolean gates in a classical computer. From this perspective, the picture is entirely different. Rather than ‘computing all values of a function at once,’ a quantum algorithm achieves an exponential speed-up over a classical algorithm by computing the answer to a disjunctive or global question about a function (e.g., whether a Boolean function is constant or balanced) without computing redundant information (e.g., the output values for different inputs to the function). A crucial difference between quantum and classical information is the possibility of selecting an exclusive disjunction, representing a global property of a function, among alternative possible disjunctions — for example, the ‘constant’ disjunction asserting that the value of the function (for both arguments) is 0 or 1, or the ‘balanced’ disjunction asserting that the value of the function (for both arguments) is the same as the argument or different from the argument — without determining the truth values of the disjuncts.

Classically, an exclusive disjunction is true if and only if one of the disjuncts is true. Deutsch’s quantum circuit achieves its speed-up by exploiting the non-Boolean structure of quantum properties to efficiently distinguish between two disjunctive properties, without determining the truth values of the relevant disjuncts (representing the association of individual inputs to the function with corresponding outputs). The point of the procedure is to avoid the evaluation of the function for specific inputs in the determination of the global property, and it is this feature — impossible in the Boolean logic of classical computation — that leads to the speed-up relative to classical algorithms. (For quantum logic not specifically in relation to quantum computation, see the entry on quantum logic and quantum probability ).

Some researchers in quantum information and quantum computation have argued for an information-theoretic interpretation of quantum mechanics. In his review article on quantum computation, Andrew Steane (1998, p. 119) makes the following remark:

Historically, much of fundamental physics has been concerned with discovering the fundamental particles of nature and the equations which describe their motions and interactions. It now appears that a different programme may be equally important: to discover the ways that nature allows, and prevents, information to be expressed and manipulated, rather than particles to move.

Steane concludes his review with the following radical proposal (1998, p. 171):

To conclude with, I would like to propose a more wide-ranging theoretical task: to arrive at a set of principles like energy and momentum conservation, but which apply to information, and from which much of quantum mechanics could be derived. Two tests of such ideas would be whether the EPR-Bell correlations thus became transparent, and whether they rendered obvious the proper use of terms such as ‘measurement’ and ‘knowledge’.

There has been considerable research in the framework of so-called ‘generalized probability theories’ or ‘Boxworld’ on the problem of what information-theoretic constraints in the class of ‘no signaling’ theories would characterize quantum theories. See Brassard 2005, van Dam 2005, Skrzypczyk, Brunner, and Popescu 2009, Pawlowski et al . 2009, Allcock et al . 2009, Navascues and Wunderlich 2009, Al–Safi and Short 2013, Ramanathan et al 2010, and Ringbauer et al (2014) for interesting results along these lines. Chiribella and Spekkens 2016 is a collection of articles based on a conference at the Perimeter institute of Theoretical Physics in Waterloo, Canada on new research at the interface of quantum foundations and quantum information. See Fuchs 2014 and the entry on quantum-Bayesian and pragmatist views of quantum theory for a discussion of QBism, a radically subjectivist information-theoretic perspective.

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How to cite this entry . Preview the PDF version of this entry at the Friends of the SEP Society . Look up topics and thinkers related to this entry at the Internet Philosophy Ontology Project (InPhO). Enhanced bibliography for this entry at PhilPapers , with links to its database.

• arXiv E-print Archive for Quantum Physics .
• Todd Brun’s Lecture Notes on Quantum Information Processing .
• John Preskill’s Course on Quantum Information and Computation .
• Oxford Quantum , Oxford University.
• Institute for Quantum Optics and Quantum Information , Austrian Academy of Sciences.
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• Joint Quantum Institute , University of Maryland.
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• Spooky Actions at a Distance? , David Mermin’s Oppenheimer Lecture.

Bell’s Theorem | quantum mechanics: Copenhagen interpretation of | quantum mechanics: Everettian | quantum mechanics: many-worlds interpretation of | quantum theory: Bayesian and pragmatist views | quantum theory: quantum computing | quantum theory: quantum logic and probability theory | quantum theory: the Einstein-Podolsky-Rosen argument in | Reichenbach, Hans: common cause principle

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Researchers Investigate Quantum Entanglement as Next-Gen Computing Fuel

Matt swayne.

• June 4, 2024

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Insider Brief

• Researchers in China have demonstrated how entanglement might potentially power future generations of computers.
• The team reports that quantum engines could use their own entangled states as a form of fuel.
• The researchers report this study represents the first experimental realization of a quantum engine with entangled characteristics.
• Image: South China Morning Post/handout

Researchers in China have demonstrated how entanglement might potentially power future generations of computers, according to a story in the South China Morning Post . This advance, achieved by scientists from the Chinese Academy of Sciences’ Innovation Academy of Precision Measurement Science and Technology, points toward how quantum engines can use their own entangled states as a form of fuel.

Entanglement is a quantum phenomenon where a pair of separated photons seem to be intimately linked, regardless of the distance between them. Scientists  have long theorized that this characteristic, once robustly managed, could hold vast potential for quantum computing, and this study adds further evidence to its viability in practical applications, the researchers suggest.

“Our study’s highlight is the first experimental realization of a quantum engine with entangled characteristics. [It] quantitatively verified that entanglement can serve as a type of ‘fuel’,” said Zhou Fei, one of the corresponding authors, as reported in the SCMP.

Unlike traditional engines that rely on thermal combustion, a quantum engine can rely on lasers to transition particles between quantum states, converting light into kinetic energy. This innovative approach opens the door to efficiencies far beyond the capabilities of classical engines.

Zhou, along with fellow corresponding author Feng Mang and their team, demonstrated that the entanglement phenomenon significantly boosts the output efficiency of quantum engines. Their study, published in the journal Physical Review Letters , provides empirical evidence of this enhanced efficiency.

Theoretical and Practical Implications

Quantum engines, leveraging entangled states, could theoretically surpass the limits imposed by classical thermodynamics. They have the potential to achieve energy conversion efficiencies of over 25%, which is a substantial improvement capable of powering large-scale quantum computers and circuits.

Using ultra-cold 40Ca+ 40 Ca + ions confined in an ion trap as the working substance for the quantum engine, the researchers designed a thermodynamic cycle that converts external laser energy into the vibrational energy of the ions. The scheme requires extremely precise handling, the researchers stated.

“We chose the entangled states of two spinning ions as the working substance, with [their]vibrational modes acting as the load. Through precise adjustments of laser frequency, amplitude and duration, the ions were transitioned from their initial pure states to highly entangled states,” Zhou told the SCMP.

The team measured the engine’s performance by evaluating two key metrics: conversion efficiency and mechanical efficiency. Conversion efficiency refers to the number of vibrations (phonons) produced for each bit of light (photons) used, while mechanical efficiency assesses how much of the energy output can be practically utilized.

More than 10,000 experiments indicated that higher degrees of ion entanglement resulted in greater mechanical efficiency, although the conversion efficiency remained relatively unaffected by the level of entanglement. “

This indicates that quantum entanglement, despite its mysterious mechanism to physicists, acts as a ‘fuel’ in quantum engines,” Zhou said, as reported by the SCMP.

Future Prospects

Quantum engines are currently a very active field of research, with numerous theoretical analyses but relatively few experimental results. This study’s conclusions pave the way for the development of micro-energy devices such as quantum motors and batteries. The findings suggest that the entanglement properties of the working material can enhance the maximum extractable energy.

Zhou emphasized that while quantum batteries might not store as much energy as conventional batteries used in electric vehicles, their true benefit lies in their potential to power large-scale quantum computers and circuits.

“The future challenge lies in increasing the number of working materials without compromising fidelity of the entanglement state, thereby enhancing output,” he told the SCMP.

As with most of the studies in the emerging field of applied quantum mechanics, this work represents the first step in developing any technology to tap entanglement as fuel. One of the challenges would likely be the scalability of any design to produce quantum batteries, or leverage entanglement as a form of energy. Moving from theory to experiment to actual practical devices is a long scientific process with numerous obstacles.

The paper offers a more detailed and technical examination of the work with details that may have not been covered in this article.

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A New Study Says Quantum Entanglement May Be Reversible

So ... what does that mean for the laws of physics?

Gear-obsessed editors choose every product we review. We may earn commission if you buy from a link. Why Trust Us?

• Time might kind of flow backward for quantum systems, due to present but lower “entropy.”
• In a new paper, scientists model quantum “entropy” using a generous probabilistic model.
• This means their holdings could be true for weaker, or less generous, models of these effects.

Bartosz Regula from the RIKEN Center for Quantum Computing and Ludovico Lami from the University of Amsterdam collaborated on new, peer-reviewed research published in the journal Nature Communications . In their paper , they summarize one of the thornier questions of quantum mechanics: Can everything in a quantum entanglement be meaningfully reversed, pointing “time’s arrow” in the opposite direction? This would mean a concept or system that had no entropy , or tendency towards disorder. Instead of a spilled glass of water or squeezed tube of toothpaste, a quantum system would be more like a neat seesaw you can move back to starting position.

If you have two classical (not quantum) systems with equal entropy, the scientists explain, then you can compare them directly with specific kinds of calculations. That’s because entropy itself is “a unique resource measure.”. Many people may think of entropy purely as a concept, but for physicists, it’s also a variable and a key descriptor that smooths the working equations describing our universe. In this case, “two comparable states of equal entropy can always be connected by a reversible adiabatic transformation,” they explain—an exchange of energy that can be reversed.

When physicists began to see signs of reversibility in quantum systems, they started to debate whether or not quantum entropy existed. A clean, calculable version of quantum entropy would become a key variable the same way it has for classical systems, giving scientists a toehold to compare different states and draw conclusions. Today, the growing field of quantum computing is hindered somewhat by how difficult it is to compare systems and make simple, generalized measurements for talking about those systems.

Regula and Lami have built a mathematical model that uses probabilities rather than pure mathematical certainties in the form of equations in order to probe these questions. And it’s not just a fuzzier version of an existing equation—rather, its a true alternative that goes for the same goals. Staking out some concept of entropy and a suggestion for how to measure it is a great start for others to recreate and further in their own research.

Probabilities are all around us, and in pop culture, they’re often flattened upward or downward to 100% or 0%. In reality, very few things are so stark. On TV shows, experts consider 10 possible sites for a crime, do some handwaving probabilities , then send all their resources to one place that turns out to be right. This can make it seem less impressive that real-life investigators can use careful observation and likelihoods to cut the number from 10 down to 6 or 7.

Indeed, that’s where the researchers conclude their paper. “This may not be enough to ensure the existence of a repeatable, reversible transformation cycle in practice. Nevertheless, we regard our results as evidence that reversibility could indeed be recovered also in the deterministic setting,” they explain. “ We hope that our results stimulate further research in this direction, leading to an eventual resolution of the open questions.”

In the long pursuit of scientific research goals, probabilities have a lot of value. Even papers describing a tried and failed new model can be really useful, or even essential to unlocking the next step to understanding. But probabilities in particular are thriving today, because computing power lets scientists describe and graph out more and more possibilities.

Caroline Delbert is a writer, avid reader, and contributing editor at Pop Mech. She's also an enthusiast of just about everything. Her favorite topics include nuclear energy, cosmology, math of everyday things, and the philosophy of it all. 

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Toward testing the quantum behavior of gravity: A photonic quantum simulation

In a development at the intersection of quantum mechanics and general relativity, researchers have made significant strides toward unraveling the mysteries of quantum gravity. This work sheds new light on future experiments that hold promise for resolving one of the most fundamental enigmas in modern physics: the reconciliation of Einstein's theory of gravity with the principles of quantum mechanics.

The longstanding challenge of unifying these two pillars of physics has tantalized scientists for decades, spawning various theoretical frameworks such as string theory and loop quantum gravity . However, without experimental verification , these theories remain speculative.

How to test the quantum nature of gravity? Tangible means to probe the quantum behavior of the gravitational field were proposed within the last decade, based on the concept of "gravity-mediated entanglement."

In a study published in Advanced Photonics Nexus , an international team of researchers achieved a significant goal in preparation for future experiments in the quest to unify quantum mechanics and general relativity . Their work leverages cutting-edge tools and techniques from quantum information theory and quantum optics to demonstrate the principles of gravity-mediated entanglement using particles of light, i.e., photons.

The experiment involves the interaction between photons to mimic the gravitational field's effect on quantum particles. Remarkably, some properties of the photons, despite never directly interacting, become entangled, showcasing a quintessential quantum phenomenon: nonlocality. This entanglement is mediated by another independent photonic property and mirrors the hypothesized behavior of gravity-mediated entanglement, providing crucial insights into the quantum nature of gravity.

Importantly, the study also addresses the challenge of detecting the entanglement generated in these experiments. By elucidating the constraints and noise sources inherent in such experiments, the researchers pave the way for clarifying concepts and tools to be used for future experiments aimed at directly observing gravity-mediated entanglement.

Experimental tests of gravity-mediated entanglement could herald a new era in our understanding of the fundamental nature of the universe. According to author Emanuele Polino, who worked as a postdoc in the Quantum Lab of Sapienza University at the time of the research, supported by the QISS consortium, "The implications of this research are profound. It offers an experimental validation for the principles behind future quantum gravity experiments that will serve as litmus tests for competing theoretical frameworks."

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In collaboration with Microsoft, Photonic demonstrates quantum entanglement at telecom wavelengths

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General Manager of Azure Quantum

Photonic executed a teleported CNOT gate between physically separated silicon spin qubits, thus satisfying the first requirement of long-distance quantum communication .

In November 2023, Microsoft and Photonic initiated their collaborative effort to advance quantum networking and computing. Today, Photonic announced the capability to successfully transfer quantum information between two physically separated qubits in a point-to-point connection using photons at telecom wavelengths. In a span of only six months, Photonic was able to achieve this significant scientific milestone on the path to a quantum internet, thereby accomplishing the first of our three collaborative goals and putting theory into practice. Notably, this accomplishment demonstrates that existing telecommunication networks have the potential to enable long-distance quantum communications—the foundation for a quantum internet and distributed quantum computing. 

“ This milestone extends the boundaries of quantum computing beyond isolated systems. Effective execution of large-scale quantum algorithms across multiple quantum computers relies heavily on vast amounts of distributed entanglement. Our work with Microsoft and these recent demonstrations emphasize the promise of our unique architectural strategy in addressing the challenge of scaling beyond individual nodes. Despite the significant work that remains, recognizing the critical role of entanglement distribution in the development of scalable quantum technologies is essential.”  —Dr. Stephanie Simmons, Founder and Chief Quantum Officer of Photonic, and the Co-Chair of Canada’s National Quantum Strategy Advisory Council 

Azure Quantum

Accelerate the pace of science

Photonic’s spin-photon architecture 

Quantum computing uses qubits, or quantum bits, to store and process information. There are multiple types of qubits, one of which is a silicon spin qubit. Photonic’s architecture combines the information-storage and information-processing capabilities of silicon spin qubits with the information-transmission capabilities of photons in a spin-photon interface that can be used for quantum networking and quantum computing. This novel architecture supports quantum communication by operating natively in the O-band of telecom wavelengths, giving it the potential to scale globally by using existing telecom fibers.  

Quantum logic gates 

Both classical and quantum computers perform operations with logic gates that convert input data into outputs. One type of quantum logic gate is a controlled NOT (CNOT) gate, which operates on two qubits—a control qubit and a target qubit. If the state of the control qubit is 0, then the state of the target qubit remains unchanged. However, when the control qubit’s state is 1, the state of the target qubit is flipped, so that 0 becomes 1, or 1 becomes 0. To perform quantum computation on a large system, logic gates like the CNOT must be implemented within and between modules. As a prerequisite to scalable, long-distance quantum computation, the distribution of entanglement to physically separated quantum systems—known as distributed entanglement—must be achieved. 

Distributed quantum entanglement 

Through a collaboration with Microsoft, Photonic achieved distributed entanglement between silicon spin qubits housed in separate cryostats, connected by a 40-meter fiber-optic cable. In a sequence of three demonstrations, each building upon the success of the last, the Photonic team: 

• Verified that the photons transmitting the quantum information through the fiber were indistinguishable from one another.
• Successfully entangled the qubits with these photons.
• Executed a remote quantum logic gate sequence—for a teleported CNOT gate—between physically separated qubits.   

This accomplishment showcases the capability to operate a quantum computer in an industrial setting by using teleportation to execute logic gates between qubits in different locations . Entanglement between qubits that are not connected physically, or even located in the same cryostat, paves the way for long-distance communication between quantum computers and is one means to accomplish scaled quantum computing. Potential applications of this technology include securely distributing keys for encrypted data communication and enabling reliable, long-distance quantum networks. This animation demonstrates how the team at Photonic achieved distributed quantum entanglement:

Photonic's achievement

Distributed quantum entanglement

Quantum networking is not intended to replace classical networks—rather, it will expand their capabilities so that quantum information can be transmitted between quantum or classical endpoints. Now that we have entered Stage 1 of quantum networking , defined as achieving entanglement between two separate quantum devices in a point-to-point connection, the next step is to improve the quality of the entanglement distribution. After doing so, we will work toward entangling additional quantum devices, the achievement of which will mark entry into Stage 2. Ultimately, we aim to achieve Stage 3, which is when long-distance quantum communication will enable a quantum internet.  

Integrating Photonic’s architecture into Microsoft Azure 

Microsoft and Photonic will continue their collaboration and work toward integrating quantum-networking capabilities into everyday operating environments through the global infrastructure of the Microsoft Azure cloud. In addition to having applications in quantum networking, Photonic’s architecture is equally applicable to distributed quantum computing. We intend to provide customers of Azure Quantum Elements with an opportunity to access Photonic’s hardware when available, unlocking the potential to solve complex scientific problems.  

By working together, Microsoft and Photonic are bringing their shared vision—creating and scaling systems that can help solve issues affecting all of humanity—closer to reality. At Microsoft, we are incorporating quantum technologies, as they arise, into our existing cloud high-performance computers to create hybrid systems that—along with the power of AI—have the potential to help scientists create more sustainable products, discover new therapeutics, and more. 

Advances in AI and quantum computing have the potential to help researchers solve global scientific challenges. To advance the safe use of these technologies, we will ensure that they are developed and deployed responsibly. We will continue to adopt thoughtful safeguards, building on our commitments to responsible AI and embracing responsible computing practices as these capabilities grow.

Learn more about Photonic’s achievement and how Microsoft intends to apply it 

• Register for the upcoming expert-led panel from the Economist Insights Hour: Commercializing Quantum .
• Read Photonic’s announcement on achieving distributed quantum entanglement.
• Read the technical paper that provides more details on this achievement.
• Sign up to learn more about the private preview  of Azure Quantum Elements.
• Register for upcoming webinars as part of the Future of Cloud series .

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Entanglement used as fuel for quantum engines in new Chinese study

C hinese scientists have achieved a milestone by harnessing the power of quantum entanglement to fuel a quantum engine, potentially revolutionizing energy efficiency and powering future quantum technologies.

The entanglement phenomenon keeps two separated photons closely connected, seemingly communicating faster than light, regardless of the distance between them.

Researchers from the Chinese Academy of Sciences’ Innovation Academy of Precision Measurement Science and Technology highlighted that the breakthrough demonstrates the potential for quantum engines to utilize their entangled states as a source of fuel, South China Morning Post reported .

According to Zhou Fei, one of the corresponding authors, the study’s highlight is “the first experimental realization of a quantum engine with entangled characteristics, which quantitatively verified that entanglement can serve as a type of ‘fuel’.”

In the study, published on April 30 in the journal Physical Review Letters, Zhou, along with co-author Feng Mang and their team, demonstrated that the entanglement phenomenon enhances the output efficiency of quantum engines.

Unlocking potential for breakthrough energy efficiency

In contrast to conventional engines relying on thermal combustion, a quantum engine utilizes lasers to shift particles between quantum states, transforming light into kinetic energy.

Moreover, quantum engines theoretically possess the ability to surpass classical thermodynamic limitations, potentially achieving energy conversion efficiencies exceeding 25 percent, enough to power large-scale quantum computers and circuits.

Thus, Zhou’s team utilized ultra-cold 40Ca+ ions held in an ion trap as the working material for the quantum engine. They devised a thermodynamic process converting external laser energy into the ions’ vibrational energy.

“We chose the entangled states of two spinning ions as the working substance, with [their] vibrational modes acting as the load. Through precise adjustments of laser frequency, amplitude, and duration, the ions were transitioned from their initial pure states to highly entangled states,” Zhou clarified. 

As he further points out, those measurements give the team insight into how effectively the engine operates and how efficiently it utilizes the energy it produces.

Ion entanglement boosts mechanical efficiency

Analysis of over 10,000 experiments indicated that increased levels of ion entanglement correlated with enhanced mechanical efficiency, while the conversion efficiency remained relatively unaffected by the degree of entanglement. This suggests quantum entanglement acts as a “fuel” in quantum engines despite its mysterious mechanism to physicists.

“Quantum engines are currently a very active research field, with many theoretical analyses and studies, but very few experimental results are provided,” Zhou noted.

According to Zhou, the study’s conclusions open new perspectives for the development of micro-energy devices such as quantum motors and batteries. They suggest that the entanglement properties of the working material can enhance the maximum extractable energy.

Although quantum batteries may not store as much energy as those in electric vehicles, their true advantage lies in their capacity to energize extensive quantum computers and circuits. Thus, the future challenge is to increase the variety of materials while preserving the quality of entanglement, leading to higher output.

Generating stationary entanglement and one-way steering in a hybrid cavity electro-optomechanical system via a squeezed vacuum field

• Published: 29 May 2024
• Volume 23 , article number  214 , ( 2024 )

Cite this article

• Song-Lin Yang 1 ,
• Xin Wang 1 ,
• Jian-Song Zhang   ORCID: orcid.org/0000-0002-3162-0021 1 ,
• Guang-Lin Chen 1 &
• Wen-Xue Zhong 1  

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We propose a scheme to generate robust bipartite entanglement, genuine tripartite entanglement, and one-way steering in a hybrid cavity electro-optomechanical system with the help of a squeezed vacuum field. The system consists of an optical cavity, a mechanical resonator formed by a thin silicon nitride membrane, and two superconducting microwave circuits. The mechanical resonator is coupled to the optical cavity and two superconducting circuits simultaneously. We find there is steady-state entanglement between different modes and genuine tripartite entanglement among the cavity mode and two microwave modes which are robust against the thermal fluctuations of the mechanical mode. In addition, the robust one-way steering between two microwave modes can be generated by selecting appropriate squeezing parameter. Our scheme may have potential applications in quantum information processing and communication.

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Entanglement transfer from two-mode squeezed vacuum light to spatially separated mechanical oscillators via dissipative optomechanical coupling

Bipartite Entanglement in Optomechanical Cavities Driven by Squeezed Light

Pulsed entanglement and quantum steering in a three-mode electro-optomechanical system

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Acknowledgements

We thank Pro. Wen-Jie Nie for helpful discussions. This project was supported by the National Natural Science Foundation of China (11905064, 12165007); Natural Science Foundation of Jiangxi Province (20232ACB201013).

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Yang, SL., Wang, X., Li, A. et al. Generating stationary entanglement and one-way steering in a hybrid cavity electro-optomechanical system via a squeezed vacuum field. Quantum Inf Process 23 , 214 (2024). https://doi.org/10.1007/s11128-024-04408-8

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In his Essay on historical debates about the nature of quantum reality (see Nature 629 , 29–32; 2024 ), Jim Baggott asks whether entangled particles might “somehow remain in contact … at speeds faster than light, in conflict with Einstein’s special theory of relativity”. Such off-the-cuff interpretations of special relativity are common in the literature, but are they valid?

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Quantum physics may help lasers see through fog, aid in communications

JT Shen to pioneer two-color quantum photonic laser with DARPA grant

Communications and other laser-based technologies can be hampered by adverse conditions, such as fog, extreme temperatures or long distances. An engineer in the McKelvey School of Engineering at Washington University in St. Louis is implementing quantum technology to develop ways that lasers can operate effectively in these challenging environments.

Jung-Tsung Shen , associate professor in the Preston M. Green Department of Electrical & Systems Engineering, is developing a prototype of a quantum photonic-dimer laser with a two-year, $1 million grant from the Defense Advanced Research Projects Agency (DARPA) of the U.S. Department of Defense. With the funding, Shen will implement his lab’s two-color photonic dimer laser technology, in which carefully controlled pairs of light particles, or photonic dimers, are used to generate a powerful and concentrated beam of light, or laser. Quantum photonic-dimer lasers take advantage of quantum effects to bind two photons together, increasing their energy and efficiency.

Photons, or particles that represent a quantum of light, travel very quickly and don't carry a charge, so it is difficult to get them to interact with each other and to manipulate them. Shen’s lab found that when he “glued” two photons of different colors together to form a photonic dimer using the power of quantum mechanics, they took on the behavior of a blue photon. The entanglement between the two photons within the dimer may offer unprecedented capabilities applications in communication and imaging, Shen said.

“Photons encode information when they travel, but the travel through the atmosphere is very damaging to them,” Shen said. “When two photons are bound together, they still suffer the effects of the atmosphere, but they can protect each other so that some phase information can still be preserved.”

These two-color dimers can be tailored to the atmosphere or to the fog through a unique property of quantum mechanics known as quantum entanglement, Shen said. 

“Quantum entanglement is a correlation between photons,” he said. “We are trying to exploit the property of entanglement to do something innovative. The entanglement can do many things that we can only dream of — this is just the tip of the iceberg.” 

Shen previously received funding from the Chan Zuckerberg Initiative to develop the technology for deep brain imaging. Researchers can implant fluorescent molecules in the brain and use photons to excite them, which allows the photons to collect information about the brain’s structure.

Now, Shen is exploring more of that vast iceberg to move toward the realization of applications in telecommunications, quantum computing and more. 

Shen’s team, which includes graduate student Qihang Liu and collaborators from Texas A&M University’s Institute for Quantum Science & Engineering , will introduce the quantum photonic-dimer laser methods that will allow them to create different states of two-color dimers at a rate of 1 million pairs per second – a rate that has never been seen before.

“The unique thing about this project is its dual focus on generating these novel strongly correlated quantum photonic states and developing the theoretical framework and advanced algorithms for their efficient detection, potentially revolutionizing quantum imaging and communication,” Shen said. 

Shawn Ballard contributed to this story.

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