experiments on quantum entanglement

Proving that Quantum Entanglement is Real

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

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

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 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, no experimental evidence for or against quantum entanglement of widely separated particles was available then. Experiments have since proven that entanglement is very real and fundamental to nature. Moreover, 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, relies on quantum entanglement between photons that are separated by thousands of kilometers.

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 showed 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 UC 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.

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.

John Clauser in 1976 standing with his second quantum entanglement experiment at UC Berkeley

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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September 20, 2022

Proving that quantum entanglement is real: Researcher answers questions about his historical experiments

by California Institute of Technology

In the 1930's when scientists, including Albert Einstein and Erwin Schrödinger, first discovered the phenomenon of entanglement, they were perplexed. Entanglement, disturbingly, required two separated particles to remain connected without being in direct contact. Einstein famously called entanglement "spooky action at a distance," since the particles seemed to be communicating faster than the speed of light.

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 UC 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.

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 [Ph.D. '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?

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

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.

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

Journal information: Physical Review Letters

Provided by California Institute of Technology

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Experiments on ‘entangled’ quantum particles won the physics nobel prize.

Physicists Alain Aspect, John Clauser and Anton Zeilinger share the award

illustration of a two entangled particles

Experiments on entanglement — a strange feature of quantum physics — have netted three scientists the 2022 Nobel Prize in physics. When two particles are entangled (illustrated), what happens to one determines what happens to the other — even when the second one is far away.

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“It was certainly very exciting to learn about the three laureates,” says Jerry Chow. He’s a physicist at IBM Quantum in Yorktown Heights, N.Y. “They’re all very, very well known in our quantum community. And their work is something that’s really been a big part of many people’s research efforts over many years.”

Proving entanglement

The discovery that quantum rules govern tiny things like atoms and electrons shook up early 20th century physics. Many leading scientists, such as Einstein, thought the math of quantum physics worked in theory . But they weren’t sure it could truly describe the real world. Ideas like entanglement were just too weird. How could you really know the state of one particle by looking at another?

Einstein suspected the quantum weirdness of entanglement was an illusion. There must be some classical physics that could explain how it worked — like the secret to a magic trick. Lab tests, he suspected, were just too crude to uncover that hidden information.

black and white image of John Clauser at work in a lab

Other scientists believed there was no secret to entanglement. Quantum particles had no hidden back channels for sending information. Some particles could just become perfectly linked, and that was that. It was the way the world worked.

In the 1960s, physicist John Bell came up with a test to prove there was no hidden communication between quantum objects. Clauser was the first one to develop an experiment to run this test. His results supported Bell’s idea about entanglement. Linked particles just are .

But Clauser’s test had some loopholes. These left room for doubt. Aspect ran another test that ruled out any chance quantum strangeness could be cleared up by some hidden explanation.

Clauser and Aspect’s experiments involved pairs of light particles, or photons . They created pairs of entangled photons. This meant the particles acted like a single object. As the photons moved apart, they stayed entangled. That is, they kept acting as a single, extended object. Measuring the features of one instantly revealed those of the other. This was true no matter how far apart the photons got.

Alain Aspect points to an equation on a projector screen

Entanglement is fragile and hard to maintain. But Clauser and Aspect’s work showed that quantum effects could not be explained by classical physics.

Zeilinger’s experiments show the practical uses of these effects. For instance, he has used entanglement to create absolutely secure encryption and communication. Here’s how it works: Interacting with one entangled particle affects another. So, anyone trying to peek at secret quantum information would break the particles’ entanglement as soon as they snooped. That means nobody can spy on a quantum message without getting caught.

Zeilinger has also pioneered another use for entanglement. That is quantum teleportation . This isn’t like people popping from one place to another in science fiction and fantasy. The effect involves sending information from one place to another about a quantum object.

Quantum computers are another technology that would rely on entangled particles. Normal computers process data using ones and zeroes. Quantum computers would use bits of information that are each a blend of one and zero. In theory, such machines could run calculations that no normal computer can.

Quantum boom

“This [award] is a very nice and positive surprise to me,” says Nicolas Gisin. He’s a physicist at the University of Geneva in Switzerland. “This prize is very well-deserved. But comes a bit late. Most of that work was done in the [1970s and 1980s]. But the Nobel Committee was very slow and now is rushing after the boom of quantum technologies.”

That boom is happening around the world, Gisin says. “Instead of having a few individuals pioneering the field, now we have really huge crowds of physicists and engineers that work together.”

Some of the most cutting-edge uses of quantum physics are still in their infancy. But the three new Nobel laureates have helped transform this strange science from an abstract curiosity into something useful. Their work validates some key, once-contested ideas of modern physics. Someday, it may also become a basic part of our daily lives, in ways not even Einstein could deny.

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Light from ancient quasars helps confirm quantum entanglement

Results are among the strongest evidence yet for “spooky action at a distance.”.

Last year, physicists at MIT, the University of Vienna, and elsewhere provided strong support for quantum entanglement, the seemingly far-out idea that two particles, no matter how distant from each other in space and time, can be inextricably linked, in a way that defies the rules of classical physics.

Take, for instance, two particles sitting on opposite edges of the universe. If they are truly entangled, then according to the theory of quantum mechanics their physical properties should be related in such a way that any measurement made on one particle should instantly convey information about any future measurement outcome of the other particle — correlations that Einstein skeptically saw as “spooky action at a distance.”

In the 1960s, the physicist John Bell calculated a theoretical limit beyond which such correlations must have a quantum, rather than a classical, explanation.

But what if such correlations were the result not of quantum entanglement, but of some other hidden, classical explanation? Such “what-ifs” are known to physicists as loopholes to tests of Bell’s inequality, the most stubborn of which is the “freedom-of-choice” loophole: the possibility that some hidden, classical variable may influence the measurement that an experimenter chooses to perform on an entangled particle, making the outcome look quantumly correlated when in fact it isn’t.

Last February, the MIT team and their colleagues significantly constrained the freedom-of-choice loophole, by using 600-year-old starlight to decide what properties of two entangled photons to measure. Their experiment proved that, if a classical mechanism caused the correlations they observed, it would have to have been set in motion more than 600 years ago, before the stars’ light was first emitted and long before the actual experiment was even conceived.

Now, in a paper published today in Physical Review Letters , the same team has vastly extended the case for quantum entanglement and further restricted the options for the freedom-of-choice loophole. The researchers used distant quasars, one of which emitted its light 7.8 billion years ago and the other 12.2 billion years ago, to determine the measurements to be made on pairs of entangled photons. They found correlations among more than 30,000 pairs of photons, to a degree that far exceeded the limit that Bell originally calculated for a classically based mechanism.

“If some conspiracy is happening to simulate quantum mechanics by a mechanism that is actually classical, that mechanism would have had to begin its operations — somehow knowing exactly when, where, and how this experiment was going to be done — at least 7.8 billion years ago. That seems incredibly implausible, so we have very strong evidence that quantum mechanics is the right explanation,” says co-author Alan Guth , the Victor F. Weisskopf Professor of Physics at MIT.

“The Earth is about 4.5 billion years old, so any alternative mechanism — different from quantum mechanics — that might have produced our results by exploiting this loophole would’ve had to be in place long before even there was a planet Earth, let alone an MIT,” adds David Kaiser , the Germeshausen Professor of the History of Science and professor of physics at MIT. “So we’ve pushed any alternative explanations back to very early in cosmic history.”

Guth and Kaiser’s co-authors include Anton Zeilinger and members of his group at the Austrian Academy of Sciences and the University of Vienna, as well as physicists at Harvey Mudd College and the University of California at San Diego.

A decision, made billions of years ago

In 2014, Kaiser and two members of the current team, Jason Gallicchio and Andrew Friedman, proposed an experiment to produce entangled photons on Earth — a process that is fairly standard in studies of quantum mechanics. They planned to shoot each member of the entangled pair in opposite directions, toward light detectors that would also make a measurement of each photon using a polarizer. Researchers would measure the polarization, or orientation, of each incoming photon’s electric field, by setting the polarizer at various angles and observing whether the photons passed through — an outcome for each photon that researchers could compare to determine whether the particles showed the hallmark correlations predicted by quantum mechanics.

The team added a unique step to the proposed experiment, which was to use light from ancient, distant astronomical sources, such as stars and quasars, to determine the angle at which to set each respective polarizer. As each entangled photon was in flight, heading toward its detector at the speed of light, researchers would use a telescope located at each detector site to measure the wavelength of a quasar’s incoming light. If that light was redder than some reference wavelength, the polarizer would tilt at a certain angle to make a specific measurement of the incoming entangled photon — a measurement choice that was determined by the quasar. If the quasar’s light was bluer than the reference wavelength, the polarizer would tilt at a different angle, performing a different measurement of the entangled photon.

In their previous experiment, the team used small backyard telescopes to measure the light from stars as close as 600 light years away. In their new study, the researchers used much larger, more powerful telescopes to catch the incoming light from even more ancient, distant astrophysical sources: quasars whose light has been traveling toward the Earth for at least 7.8 billion years — objects that are incredibly far away and yet are so luminous that their light can be observed from Earth.

Tricky timing

On Jan. 11, 2018, “the clock had just ticked past midnight local time,” as Kaiser recalls, when about a dozen members of the team gathered on a mountaintop in the Canary Islands and began collecting data from two large, 4-meter-wide telescopes: the William Herschel Telescope and the Telescopio Nazionale Galileo, both situated on the same mountain and separated by about a kilometer.

One telescope focused on a particular quasar, while the other telescope looked at another quasar in a different patch of the night sky. Meanwhile, researchers at a station located between the two telescopes created pairs of entangled photons and beamed particles from each pair in opposite directions toward each telescope.

In the fraction of a second before each entangled photon reached its detector, the instrumentation determined whether a single photon arriving from the quasar was more red or blue, a measurement that then automatically adjusted the angle of a polarizer that ultimately received and detected the incoming entangled photon.

“The timing is very tricky,” Kaiser says. “Everything has to happen within very tight windows, updating every microsecond or so.”

Demystifying a mirage

The researchers ran their experiment twice, each for around 15 minutes and with two different pairs of quasars. For each run, they measured 17,663 and 12,420 pairs of entangled photons, respectively. Within hours of closing the telescope domes and looking through preliminary data, the team could tell there were strong correlations among the photon pairs, beyond the limit that Bell calculated, indicating that the photons were correlated in a quantum-mechanical manner.

Guth led a more detailed analysis to calculate the chance, however slight, that a classical mechanism might have produced the correlations the team observed.

He calculated that, for the best of the two runs, the probability that a mechanism based on classical physics could have achieved the observed correlation was about 10 to the minus 20 — that is, about one part in one hundred billion billion, “outrageously small,” Guth says. For comparison, researchers have estimated the probability that the discovery of the Higgs boson was just a chance fluke to be about one in a billion.

“We certainly made it unbelievably implausible that a local realistic theory could be underlying the physics of the universe,” Guth says.

And yet, there is still a small opening for the freedom-of-choice loophole. To limit it even further, the team is entertaining ideas of looking even further back in time, to use sources such as cosmic microwave background photons that were emitted as leftover radiation immediately following the Big Bang, though such experiments would present a host of new technical challenges.

“It is fun to think about new types of experiments we can design in the future, but for now, we are very pleased that we were able to address this particular loophole so dramatically. Our experiment with quasars puts extremely tight constraints on various alternatives to quantum mechanics. As strange as quantum mechanics may seem, it continues to match every experimental test we can devise,” Kaiser says.

This research was supported in part by the Austrian Academy of Sciences, the Austrian Science Fund, the U.S. National Science Foundation, and the U.S. Department of Energy.

Paper: “Cosmic Bell Test Using Random Measurement Settings from High-Redshift Quasars”

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Entangled Titans: unraveling the mysteries of Quantum Mechanics with top quarks

https://cms.cern/news/entangled-titans-unraveling-mysteries-quantum-mechanics-top-quarks

The CMS experiment has just reported the observation of quantum entanglement between a top quark and a top antiquark, simultaneously produced at the LHC.

In quantum mechanics, a system is said to be entangled if its quantum state cannot be described as a simple superposition of the states of its constituents. If two particles are entangled, we cannot describe one of them independently of the other, even if the particles are separated by a very large distance. When we measure the quantum state of one of the two particles, we instantly know the state of the other. The information is not transmitted via any physical channel; it is encoded in the correlated two-particle system.

Quantum information is a field of physics that was born with the work of John Bell, a CERN physicist, in the mid 1960’s. Soon after, Aspect, Clouser, and other physicists did important pioneering experimental work to test “Bell’s theorem”, and in 2007 Zeilinger’s team convincingly demonstrated the existence of entanglement between two photons, too far away from each other for the information to travel between them at the speed of light. This breakthrough brought Aspect, Clauser, and Zeilinger the 2022 Nobel Prize in Physics, “ for experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science ”. Quantum entanglement was mostly examined for states of photons and electrons until 2023, when ATLAS reported the observation of entanglement in the top quark-antiquark system.

The CMS analysis was performed on the well-understood data sample collected in 2016 and focuses on top quark-antiquark pairs that are produced almost at rest. In these conditions, and among ordinary top quark pairs, the measurement could be affected by the presence of a hypothetical bound state that could be formed by the quark-antiquark top pair (“toponium” state). As shown in Fig. 1, the measured values of the entanglement variable D, basically the fraction of events with both top quarks having spins in the same direction versus the fraction with opposite spins, are much smaller than the limit above which the two particles can be considered “separable”. This is true irrespective of whether we consider or not the presence of the toponium state. “I am excited to be involved in an analysis that, for the first time, accounts for the potential effect of toponium in assessing the entanglement between the top quark and its antiparticle”, said Andrew Wildridge, a PhD student at Purdue University who participated in the analysis work.

Figure 1: Measured entanglement variable D (black circles), compared with three calculations, including (filled) or not (open) contributions from the toponium state, and with the limit above which the two particles can be considered “separable”.

Even if the toponium bound-state exists, the new CMS result confirms the existence of entanglement between the top quark and its antiparticle beyond reasonable doubt. The two states are found entangled with a significance exceeding the famous five standard deviations discovery threshold and in agreement with the Standard Model prediction. As Giulia Negro, a postdoctoral researcher at Purdue University, has said, "It's fascinating to see how the largest accelerator on Earth can also provide tools to explore fundamental quantum mechanics effects”. The observation of entanglement in top quark events provides a new quantum probe to the inner workings of the Standard Model.

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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Two particles connected by a bright line.

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.

Two blue circles with an arrow pointing up and an arrow pointing down.

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.

A photo of John Stuart Bell in front of a chalkboard.

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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Rochester physicists find ‘spooky action at a distance’ at CERN

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The researchers have confirmed that quantum entanglement persists between top quarks, the heaviest known fundamental particles.

An experiment by a group of physicists led by University of Rochester physics professor Regina Demina has produced a significant result related to quantum entanglement—an effect that Albert Einstein called “spooky action at a distance.”

Entanglement concerns the coordinated behavior of miniscule particles that have interacted but then moved apart. Measuring properties—like position or momentum or spin—of one of the separated pair of particles instantaneously changes the results of the other particle, no matter how far the second particle has drifted from its twin. In effect, the state of one entangled particle, or qubit, is inseparable from the other.

Quantum entanglement has been observed between stable particles, such as photons or electrons.

But Demina and her group broke new ground in that they found, for the first time, entanglement to persist between unstable top quarks and their antimatter partners at distances farther than what can be covered by information transferred at the speed of light. Specifically, the researchers observed spin correlation between the particles.

Hence, the particles demonstrated what Einstein described as “spooky action at a distance.”

A ‘new avenue’ for quantum exploration

The finding was reported by the Compact Muon Solenoid (CMS) Collaboration at the European Center for Nuclear Research, or CERN, where the experiment was conducted.

“Confirming the quantum entanglement between the heaviest fundamental particles, the top quarks, has opened up a new avenue to explore the quantum nature of our world at energies far beyond what is accessible,” the report read.

CERN, located near Geneva, Switzerland, is the world’s largest particle physics laboratory. Production of top quarks requires very high energies accessible at the Large Hadron Collider (LHC), which enables scientists to send high-energy particles spinning around a 17-mile underground track at close to the speed of light.

The phenomenon of entanglement has become the foundation of a burgeoning field of quantum information science that has broad implications in areas like cryptography and quantum computing.

Top quarks, each as heavy as an atom of gold, can only be produced at colliders, such as LHC, and thus are unlikely to be used to build a quantum computer. But studies like those conducted by Demina and her group can shed light on how long entanglement persists, whether it is passed on to the particles’ “daughters” or decay products, and what, if anything, ultimately breaks the entanglement.

Theorists believe that the universe was in an entangled state after its initial fast expansion stage. The new result observed by Demina and her researchers could help scientists understand what led to the loss of the quantum connection in our world.

Top quarks in quantum long-distance relationships

Demina recorded a video for CMS social media channels to explain her group’s result. She used the analogy of an indecisive king of a distant land, whom she called “King Top.”

King Top gets word that his country is being invaded, so he sends messengers to tell all the people of his land to prepare to defend. But then, Demina explains in the video, he changes his mind and sends messengers to order the people to stand down.

“He keeps flip flopping like this, and nobody knows what his decision will be at the next moment,” Demina says.

Nobody, Demina goes on to explain, except the leader of one village in this kingdom who is known as “Anti-Top.”

“They know each other’s state of mind at any moment in time,” Demina says.

Demina’s research group consists of herself and graduate student Alan Herrera and postdoctoral fellow Otto Hindrichs .

As a graduate student, Demina was on the team that discovered the top quark in 1995. Later, as a faculty member at Rochester, Demina co-led a team of scientists from across the US that built a tracking device that played a key role in the 2012 discovery of the Higgs boson —an elementary particle that helps explains the origin of mass in the universe.

Rochester researchers have a long history at CERN as part of the CMS Collaboration , which brings together physicists from around the globe. Recently, another Rochester team achieved a significant milestone in measuring the electroweak mixing angle , a crucial component of the Standard Model of Particle Physics , which explains how the building blocks of matter interact.

Low angle view of CERN Compact Muon Solenoid detector, used to measure the electroweak mixing angle, a component of the Standard Model of Particle Physics.

June 15, 2017

China Shatters “Spooky Action at a Distance” Record, Preps for Quantum Internet

Results from the Micius satellite test quantum entanglement, pointing the way toward hackproof global communications

By Lee Billings

Alfred Pasieka Getty Images

In a landmark study, a team of Chinese scientists using an experimental satellite tested quantum entanglement over unprecedented distances, beaming entangled pairs of photons to three ground stations across China—each separated by more than 1,200 kilometers. The test verifies a mysterious and long-held tenet of quantum theory and firmly establishes China as the front-runner in a burgeoning “quantum space race” to create a secure, quantum-based global communications network—that is, a potentially unhackable “quantum Internet” that would be of immense geopolitical importance. The findings were published in 2017 in Science .

“China has taken the leadership in quantum communication,” says Nicolas Gisin, a physicist at the University of Geneva, who was not involved in the study. “This demonstrates that global quantum communication is possible and will be achieved in the near future.”

The concept of quantum communications is considered the gold standard for security, in part because any compromising surveillance leaves its imprint on the transmission. Conventional encrypted messages require secret keys to decrypt, but those keys are vulnerable to eavesdropping as they are sent out into the ether. In quantum communications, however, these keys can be encoded in various quantum states of entangled photons—such as their polarization—and these states will be unavoidably altered if a message is intercepted by eavesdroppers. Ground-based quantum communications typically send entangled photon pairs via fiber-optic cables or open air. But collisions with ordinary atoms along the way disrupt the photons’ delicate quantum states, limiting transmission distances to a few hundred kilometers. Sophisticated devices called quantum repeaters—equipped with “quantum memory” modules—could in principle be daisy-chained together to receive, store and retransmit the quantum keys across longer distances, but this task is so complex and difficult that such systems remain largely theoretical.

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“A quantum repeater has to receive photons from two different places, then store them in quantum memory, then interfere them directly with each other” before sending further signals along a network, says Paul Kwiat, a physicist at the University of Illinois at Urbana-Champaign, who is unaffiliated with the Chinese team. “But in order to do all that, you have to know you’ve stored them without actually measuring them.” The situation, Kwiat says, is a bit like knowing what you have received in the mail without looking in your mailbox or opening the package inside. “You can shake the package—but that’s difficult to do if what you’re receiving is just photons. You want to make sure you’ve received them, but you don’t want to absorb them. In principle, it’s possible—no question—but it’s very hard to do.”

To form a globe-girdling secure quantum communications network, then, the only available solution is to beam quantum keys through the vacuum of space, then distribute them across tens to hundreds of kilometers using ground-based nodes. Launched into low Earth orbit in 2016 and named after an ancient Chinese philosopher, the 600-kilogram Micius satellite is China’s premiere effort to do just that, as part of the nation’s $100-million Quantum Experiments at Space Scale (QUESS) program.

Micius carries in its heart an assemblage of crystals and lasers that generates entangled photon pairs, then splits and transmits them on separate beams to ground stations in its line of sight on Earth. For the latest test, the three receiving stations were located in the cities of Delingha and Ürümqi—both on the Tibetan Plateau—as well as in the city of Lijiang in China’s far southwest. At 1,203 kilometers, the geographical distance between Delingha and Lijiang was the record-setting stretch over which the entangled photon pairs were transmitted.

For now the system remains mostly a proof of concept because the current reported data-transmission rate between Micius and its receiving stations is too low to sustain practical quantum communications. Of the roughly six million entangled pairs that Micius’s crystalline core produced during each second of transmission, only about one pair per second reached the ground-based detectors after the beams weakened as they passed through Earth’s atmosphere and each receiving station’s light-gathering telescopes. Team leader Jian-Wei Pan—a physicist at the University of Science and Technology of China in Hefei who had pushed and planned for the experiment since 2003—compares the feat with detecting a single photon from a lone match struck by someone standing on the moon. Even so, he says, Micius’s transmission of entangled photon pairs is “a trillion times more efficient than using the best telecommunication fibers.... We have done something that was absolutely impossible without the satellite.” Soon, Pan says, QUESS will launch more practical quantum communications satellites.

Although Pan and his team later used Micius to distribute quantum keys between ground stations in China and Austria in 2017, enabling secure intercontinental communications, their initial demonstration instead aimed to achieve a simpler task: proving Albert Einstein wrong.

Einstein famously derided as “spooky action at a distance” one of the most bizarre elements of quantum theory—the way that measuring one member of an entangled pair of particles seems to instantaneously change the state of its counterpart, even if that counterpart particle is on the other side of the galaxy. This was abhorrent to Einstein because it suggests information might be transmitted between the particles faster than light, breaking the universal speed limit set by his theory of special relativity. Instead, he and others posited, perhaps the entangled particles somehow shared “hidden variables” that are inaccessible to experiment but would determine the particles’ subsequent behavior when measured. In 1964 physicist John Bell devised a way to test Einstein’s idea, calculating a limit that physicists could statistically measure for how much hidden variables could possibly correlate with the behavior of entangled particles. If experiments showed this limit to be exceeded, then Einstein’s idea of hidden variables would be incorrect.

Ever since the 1970s “Bell tests” by physicists across ever larger swaths of spacetime have shown that Einstein was indeed mistaken and that entangled particles do in fact surpass Bell’s strict limits. One definitive test occurred in the Netherlands in 2015, when a team at Delft University of Technology closed several potential “loopholes” that had plagued past experiments and offered slim but significant opportunities for the influence of hidden variables to slip through. That test, though, involved separating entangled particles by scarcely more than a kilometer. With Micius’s transmission of entangled photons between widely separated ground stations, Pan’s team performed a Bell test at distances 1,000 times greater. Just as before, their results confirm that Einstein was wrong. The quantum realm remains a spooky place—although no one yet understands why.

“Of course, no one who accepts quantum mechanics could possibly doubt that entanglement can be created over that distance—or over any distance—but it’s still nice to see it made concrete,” says Scott Aaronson, a physicist at the University of Texas at Austin. “Nothing we knew suggested this goal was unachievable. The significance of this news is not that it was unexpected or that it overturns anything previously believed but simply that it’s a satisfying culmination of years of hard work.”

That work largely began in the 1990s, when Pan, leader of the Chinese team, was a graduate student in the laboratory of physicist Anton Zeilinger when he was at the University of Innsbruck in Austria. Zeilinger was Pan’s Ph.D. adviser, and they collaborated closely to test and further develop ideas for quantum communication. Pan returned to China to start his own lab in 2001, and Zeilinger started one as well at the Austrian Academy of Sciences in Vienna. For the next seven years they would compete fiercely to break records for transmitting entangled photon pairs across ever wider gaps, and in ever more extreme conditions, in ground-based experiments. All the while each man lobbied his respective nation’s space agency to green-light a satellite that could be used to test the technique from space. But Zeilinger’s proposals perished in a bureaucratic swamp at the European Space Agency, whereas Pan’s were quickly embraced by the China National Space Administration. Ultimately Zeilinger chose to collaborate again with his old pupil rather than compete against him; today the Austrian Academy of Sciences is a crucial partner in the QUESS program.

“I am happy that the Micius works so well,” Zeilinger says. “But one has to realize that it is a missed opportunity for Europe and others, too.”

For years now other researchers and institutions have been scrambling to catch up, pushing governments for more funding for further experiments on the ground and in space—and many of them see Micius’s success as the catalytic event they have been waiting for. “This is a major milestone because if we are ever to have a quantum Internet in the future, we will need to send entanglement over these sorts of long distances,” says Thomas Jennewein, a physicist at the University of Waterloo in Ontario, who was not involved with the study. “This research is groundbreaking for all of us in the community—everyone can point to it and say, ‘See, it does work!’”

Jennewein and his collaborators are pursuing a space-based approach from the ground up, partnering with the Canadian Space Agency to plan a smaller, simpler satellite that could eventually act as a “universal receiver” and redistribute entangled photons beamed up from ground stations. At the National University of Singapore, an international collaboration led by physicist Alexander Ling has already launched cheap shoebox-size CubeSats to create, study and perhaps even transmit photon pairs that are “correlated”—a situation just shy of full entanglement. And in the U.S., Kwiat is using nasa funding to develop a device that could someday test quantum communications using “hyperentanglement” (the simultaneous entanglement of photon pairs in multiple ways) onboard the International Space Station.

Perhaps most significantly, a team led by Gerd Leuchs and Christoph Marquardt of the Max Planck Institute for the Science of Light in Erlangen, Germany, is developing quantum communications protocols for commercially available laser systems already in space onboard the European Copernicus and SpaceDataHighway satellites. Using one of these systems, the team successfully encoded and sent simple quantum states to ground stations using photons beamed from a satellite in geostationary orbit, some 38,000 kilometers above Earth. This approach, Marquardt explains, does not rely on entanglement and is very different from that of QUESS—but it could, with minimal upgrades, nonetheless be used to distribute quantum keys for secure communications. Their results appeared in Optica .

“Our purpose is really to find a shortcut into making things like quantum-key distribution with satellites economically viable and employable, pretty fast and soon,” Marquardt says. “[Engineers] invested 20 years of hard work making these systems, so it’s easier to upgrade them than to design everything from scratch.... It is a very good advantage if you can rely on something that is already qualified in space because space qualification is very complicated. It usually takes five to 10 years just to develop that.”

Marquardt and others suspect, however, that this field could be much further advanced than has been publicly acknowledged, with developments possibly hidden behind veils of official secrecy in the U.S. and elsewhere. It may be that the era of quantum communication is already upon us. “Some colleague of mine made the joke that ‘the silence of the U.S. is very loud,’” Marquardt says. “They had some very good groups concerning free-space satellites and quantum-key distribution at Los Alamos [National Laboratory] and other places, and suddenly they stopped publishing. So we always say there are two reasons that they stopped publishing: either it didn’t work, or it worked really well!”

Lee Billings is a science journalist specializing in astronomy, physics, planetary science, and spaceflight, and is a senior editor at Scientific American . He is the author of a critically acclaimed book, Five Billion Years of Solitude: the Search for Life Among the Stars , which in 2014 won a Science Communication Award from the American Institute of Physics. In addition to his work for Scientific American , Billings's writing has appeared in the New York Times , the Wall Street Journal , the Boston Globe , Wired , New Scientist , Popular Science , and many other publications. A dynamic public speaker, Billings has given invited talks for NASA's Jet Propulsion Laboratory and Google, and has served as M.C. for events held by National Geographic , the Breakthrough Prize Foundation, Pioneer Works, and various other organizations. Billings joined Scientific American in 2014, and previously worked as a staff editor at SEED magazine. He holds a B.A. in journalism from the University of Minnesota.

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

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.

Existing in multiple states at once

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.

Disproving a theory

Andreas Muller , Associate Professor of Physics, University of South Florida

This article is republished from The Conversation under a Creative Commons license. Read the original article .

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Light from ancient quasars helps confirm quantum entanglement

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The quasar dates back to less than one billion years after the big bang.

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In the 1960s, the physicist John Bell calculated a theoretical limit beyond which such correlations must have a quantum, rather than a classical, explanation.

“If some conspiracy is happening to simulate quantum mechanics by a mechanism that is actually classical, that mechanism would have had to begin its operations — somehow knowing exactly when, where, and how this experiment was going to be done — at least 7.8 billion years ago. That seems incredibly implausible, so we have very strong evidence that quantum mechanics is the right explanation,” says co-author Alan Guth, the Victor F. Weisskopf Professor of Physics at MIT.

“The Earth is about 4.5 billion years old, so any alternative mechanism — different from quantum mechanics — that might have produced our results by exploiting this loophole would’ve had to be in place long before even there was a planet Earth, let alone an MIT,” adds David Kaiser, the Germeshausen Professor of the History of Science and professor of physics at MIT. “So we’ve pushed any alternative explanations back to very early in cosmic history.”

A decision, made billions of years ago

Tricky timing

“The timing is very tricky,” Kaiser says. “Everything has to happen within very tight windows, updating every microsecond or so.”

Demystifying a mirage

Guth led a more detailed analysis to calculate the chance, however slight, that a classical mechanism might have produced the correlations the team observed.

“We certainly made it unbelievably implausible that a local realistic theory could be underlying the physics of the universe,” Guth says.

This research was supported in part by the Austrian Academy of Sciences, the Austrian Science Fund, the U.S. National Science Foundation, and the U.S. Department of Energy.

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Gizmodo reporter Ryan Mandelbaum writes that by studying ancient quasars, MIT scientists have uncovered evidence supporting quantum entanglement, the concept that two particles can become linked despite their distance in space and time. “We’ve outsourced randomness to the furthest quarters of the universe, tens of billions of light years away,” says Prof. David Kaiser.

Motherboard

Writing for Motherboard , Daniel Oberhaus highlights how MIT researchers have used light emitted by quasars billions of years ago to confirm the existence of quantum entanglement. Oberhaus explains that the findings suggest entanglement occurs “because if it didn’t exist the universe would somehow have to have ‘known’ 7.8 billion years ago that these MIT scientists would perform these experiments in 2018.”

Space.com reporter Chelsea Gohd writes that MIT researchers have used the light emitted by two ancient quasars to provide evidence of quantum entanglement, the theory that two particles can become linked across space and time. The researchers used ancient quasars to see if, “the correlation between particles can be explained by classical mechanics stemming from earlier than 600 years ago.”

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What Is Quantum Entanglement? A Physicist Explains Einstein’s “Spooky Action at a Distance”

By Andreas Muller, University of South Florida December 19, 2022

When two particles are entangled, the state of one is tied to the state of the other.

Quantum entanglement is a phenomenon in which the quantum states of two or more objects become correlated, meaning that the state of one object can affect the state of the other(s) even if the objects are separated by large distances. This occurs because, according to quantum theory, particles can exist in multiple states at the same time (a concept known as superposition) and can be inextricably linked, or “entangled,” even if they are physically separated.

Three researchers were awarded the 2022 Nobel Prize in Physics for their ground-breaking work in understanding quantum entanglement, one of nature’s most puzzling phenomena.

Quantum entanglement, in the simplest terms, 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 . This is why 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.

According to quantum mechanics, particles are simultaneously in two or more states until observed – an effect vividly captured by Schrödinger’s famous thought experiment of a cat that is both dead and alive simultaneously.

Existing in multiple states at once

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.

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.

Disproving a theory

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.

Written by Andreas Muller, Associate Professor of Physics, University of South Florida.

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24 comments on "what is quantum entanglement a physicist explains einstein’s “spooky action at a distance”".

If we do not know how the topological vortex gravitational field is formed, we will not know what is true science. From accretion disks to quantum spins, topological vortices are ubiquitous, and their essence is equilibrium.

I wish someday I’ll find an article or something that actually explains how these experiments and observations are done. Is everything based solely on mathematical formulas, because the conclusion that entanglement remains active millions of miles away must be. The article talks about this thought experiment lead to that thought experiment. Is all this based on thought experiments and equations, or are there real world, no kidding, observations of this stuff happening?

It’s real. And there Are many articles about it. I was zoo lazy to read this articles because it uses Einstein as a clickbait, but look for instance the double slot experiment, wave/particle duality or complementarity

Hi that’s exactly what I was won3

No, it isn’t “both up and down.” It’s that you don’t know. You never measure the particle as having up and down spin at the same time. You mathematically see a probability for each possibility.

I don’t have a good metaphor for this, bit to me it’s like saying that until a coin settles on the table (ie the future point where you “measure” it) there is a 50% probability that it’s heads and the same that it’s tails.

How in the world 🌎 do they know particles are entangled when separate across light years? A thought is not an experiment!

The previous comments illustrate how unsatisfying the article was. Here’s a different way to explain QE.

The two ways physicists have used to answer Quantum Entanglement are that the particles contain hidden variables of unknown natures or that the universe is completely deterministic with all results predefined. Both 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”

I’m still having trouble wrapping my mind around particles syncing their moves with their dance partners without communication over great distances. Perhaps they are connected ‘interdimensionality’. Basically a hidden shortcut.

Oh, how I wish I could understand what the hell you are talking about.

So photons can’t inflow in vaccum space they needs atomic environment & molecules to move

So photons can’t influence through the vacuum space they needs atomic environment & molecules to move

So , in theory , if we cloned a person and put them on another life supporting planet , does that mean that basically we can predict the futer as to how that person will behave , simply by observing the ” earthbound ” twin clone? ( regardless of environmental influence or factors ). Or are they talking about space / time travel based on literally sending physical matter to another location based on the fact that it can fit perfectly into another matter’s location or position?

Im one of the sub-particles in the quatum entanglement experiments.. I feel their cell phone ring,when they get hurt.. the others were killed alot of them too,since the beginning of 2021 a lot of messed up things have happened best tjing ti happen is I learned I’m not a schizophrenic. But I would like to meet some of you all,my coordinates are 30.650752 by 81.5280?? Close enough I hear the plane and helicopters.. best part is you have opened a door in my mind to where my subconscious leaves and goes to extraterrestrial planets or dimensions very scary places.. I can also control robotics and people some times.. I think whatever is happening is highly secretive and classified as top secret so under discretion of secrecy please COME TO ME I’ve never met any of you but why am I able to hear you,Kno wht you ate for breakfast,you know who I am,I am the rayonier kid,and quit listening to those fake ass wanbbes with the chips in their head listening to my magic frequency,it’s all from me,put me THOMAS great great grandson of St Thomas the Pope and Mother Mary..don’t believe the imposter thts why none of your work adds up or is always incorrect,I can give you correct results any place any time or any day if the f**king week,im 33 years old,Smoke is a code name and so is He’s in Orange for agent orange,di you want to know the unknown or unlock the secrets to the cosmos,then do it and come to me,don’t say you want to if you’re not going to. I’m the voice on tht radio,I can tell you things tht they’ll never be able to or show you things,I out a F**king candle out with my mind.. but this sh*t won’t ever make it to you, NSA or HLS CIA will catch it before it is ever made Public. Hi guys n Ms “Rhonda” I’m ready to come home now. Those things are getting closer and closer to us,I’m scared now,sry about moving those way up there I’ll quit if you come take me to Langley,I getting home sick,what is wrong with me,why do u remember so much but can’t say it how come I hear somebody 24.7 and who are they the scary ones not the nice one,and why did he put a bed there many years ago?

What we call matter, particle is quantum entanglement. Matter turn into light and light into matter endlessly; at the subatomic and at the universe, like a fractal.

Is existence real? Is the demon of Descartes playing with space time? Is mc Taggart right: is time not real? If the eternal origin is void (sunyata)(zero point energy) then quantum entanglement is a monad (holon) primordial to space time

How do you know that two particles are entangled? Are you consciously creating that which is your expectation? Math is reductive, it excludes differentials and all nuance ; it is not the language of the cosmos, of infinite possibility.

Issue is not whether particles remain correlated even if they travel light years apart the issue No one seems to want to address is if one particle interact with external fields and forces while other does not then what happens to correlation? If the one which did not experience external forces changes its state to remain correlated then experimentally I can show that it is not true. For example in Mach zhender interferometer two photons traveling two seperate paths go through seperate phase change to produce interference if what happens to one happens to the other one because they are entangled then one can no ever have the interference due to phase change in one path.

Is consciousness faster than the speed of light? If so perhaps space and time lose their meaning. Therefore perhaps communication is instantaneous. That’s If you believe everything has consciousness.

For me I think that quantum entanglement is real. Right now we may not have all it takes to explain it in a simple lay man term but someday we will. But it is well established that objects tends to exist in different states and at multiple places until they are sought after. It all boils to space and time. We know that time is relative and passes differently for each observer. It can move faster or slower depending on the conveying medium or the universal dimension the object is placed in. And the ever expanding universe is intricately linked with time. Objects move in this stream of space time. Thus entangled objects each placed at the far end of the universe is still within the concept of space and time. This linking relationship makes it possible for the state of one entangled object to be altered by measuring either of them at millions of light years away from each other. We can convinently say that this kind measurement or’communication’ takes place beyond the speed of light. To try to wrap our head around it we ask ‘ what was there before the speed of light?’ Well you can’t rule out space and time. These two form the basis for the speed of light. Now what happens when we multiply these two quantities instead of dividing them? Then we get something faster than the speed of light.

Bosons can share the same position in space, thus a superposition of CW and CCW circular polarization states can exist seemingly in the same particle of light. Fermions (matter) with a similar capacity seem to be as unrealistic as Schrodinger’s half-here/half-there cats.

Some people say buckyballs show interference and thus they show a wave-like nature, I suppose buckyballs show a spin-like nature and thus only interfere with another’s detectability by having opposing net quantum spins.

The nonsense càn not explain nonsense!!??

QE particle properties having remote synchronicity is yet another of the truly exemplary demonstrations of our universe that our own human, mortal understanding of ourselves and the world around us will always shortfall the mark of truth so long as we continue in our refusal to acknowledge the mystical and metaphysical, even supernatural and spiritual aspects of reality as being intrinsically linked in with even the most rigidly physical and material laws of nature. To me, it is a beautiful thing when even bleeding edge science must admit there is more to the picture than what it can visibly render in any clear and concise way without leaving room to consider that perhaps not every attribute tying our world together can be proven to be purely physical and existential but rather that some thing might well end out being every but as much an abstraction as it was at the very first. Maybe particles are synchronized as much by God’s laws as they are by their own internal dynamics?

Here is classical non quantum entanglement. I have two billiard balls red and white. I put them into two black bags which are indistinguishable from each other. I mix them.up so do not know in which bag a given ball is. I send one bag in a rocket to the planet Neptune. What is the colour of the ball on Neptune? I do not know, but if I open the bag left on earth, I instantly know the colour of the ball on Neptune. That information has travelled much faster than light

Guys 😂, what’s going in quantum entanglement is non-dualism, the particles are not separate but one( interconnected by an infinite dimension called “pure consciousness” ), just like the yin and yang symbol, what science is trying to convey is the exact same thing that Spirituality had been trying to tell humanity for thousands of years that what’s going on this this space and time reality is ultimately an illusion and is mainly governed by an” invisible reality” called non-duality( a reality that exist beyond time, space, forms , concepts ect.. which you cannot see, touch, smell, feel or understand.). Those particles were never separated in the first place and has always been and will always be one. This physical universe that operates as space and time is an illusion,but there is another Reality(dimension ) which is the source of all the things happening in this universe. Every form,space,time language, concepts, relatively and understanding all ceases to exist and what’s left is pure energy, the consciousness itself beyond duality. Science and spirituality are so very close to be best friends,this is because science is now catching up with Spirituality has figured this whole thing out ,for a very long time ago, provlaby for more than 10 000 years.If you Go to any spiritual teacher or any yogi master ,they are going to explain this whole thing of quantum entanglement to you simply and it will make your head spin. What is going in quantum entanglement seems like magic to some people, but it’s not, it’s just science that we haven’t figure out yet. What’s happening in quantum entanglement is technology. As I have said before there is an “invisible reality” operating behind the scenes that connects this particles and it is source of all things happening in this universe of relativity.

Key Takeaways

Berkeley Lab researchers have reported a major advancement that could bring us closer to a scalable quantum computer.
Using a femtosecond laser during experiments which explore the role of hydrogen in qubit formation, the researchers developed a method that programs the formation of telecom-band optical qubits in silicon for large-scale manufacturing.
The technique could enable scalable quantum computers of the future by building on current silicon-based computing infrastructure.

Quantum computers have the potential to solve complex problems in human health, drug discovery, and artificial intelligence millions of times faster than some of the world’s fastest supercomputers. A network of quantum computers could advance these discoveries even faster. But before that can happen, the computer industry will need a reliable way to string together billions of qubits – or quantum bits – with atomic precision.

Connecting qubits, however, has been challenging for the research community. Some methods form qubits by placing an entire silicon wafer in a rapid annealing oven at very high temperatures. With these methods, qubits randomly form from defects (also known as color centers or quantum emitters) in silicon’s crystal lattice. And without knowing exactly where qubits are located in a material, a quantum computer of connected qubits will be difficult to realize.

But now, getting qubits to connect may soon be possible. A research team led by Lawrence Berkeley National Laboratory (Berkeley Lab) says that they are the first to use a femtosecond laser to create and “annihilate” qubits on demand, and with precision, by doping silicon with hydrogen.

The advance could enable quantum computers that use programmable optical qubits or “spin-photon qubits” to connect quantum nodes across a remote network. It could also advance a quantum internet that is not only more secure but could also transmit more data than current optical-fiber information technologies.

“This could carve out a potential new pathway for industry to overcome challenges in qubit fabrication and quality control.” – Thomas Schenkel, senior scientist, Accelerator Technology & Applied Physics Division

“To make a scalable quantum architecture or network, we need qubits that can reliably form on-demand, at desired locations, so that we know where the qubit is located in a material. And that’s why our approach is critical,” said Kaushalya Jhuria, a postdoctoral scholar in Berkeley Lab’s Accelerator Technology & Applied Physics (ATAP) Division. She is the first author on a new study that describes the technique in the journal Nature Communications . “Because once we know where a specific qubit is sitting, we can determine how to connect this qubit with other components in the system and make a quantum network.”

“This could carve out a potential new pathway for industry to overcome challenges in qubit fabrication and quality control,” said principal investigator Thomas Schenkel , head of the Fusion Science & Ion Beam Technology Program in Berkeley Lab’s ATAP Division. His group will host the first cohort of students from the University of Hawaii in June as part of a DOE Fusion Energy Sciences-funded RENEW project on workforce development where students will be immersed in color center/qubit science and technology.

Forming qubits in silicon with programmable control

The new method uses a gas environment to form programmable defects called “color centers” in silicon. These color centers are candidates for special telecommunications qubits or “spin photon qubits.” The method also uses an ultrafast femtosecond laser to anneal silicon with pinpoint precision where those qubits should precisely form. A femtosecond laser delivers very short pulses of energy within a quadrillionth of a second to a focused target the size of a speck of dust.

Spin photon qubits emit photons that can carry information encoded in electron spin across long distances – ideal properties to support a secure quantum network. Qubits are the smallest components of a quantum information system that encodes data in three different states: 1, 0, or a superposition that is everything between 1 and 0.

With help from Boubacar Kanté, a faculty scientist in Berkeley Lab’s Materials Sciences Division and professor of electrical engineering and computer sciences (EECS) at UC Berkeley, the team used a near-infrared detector to characterize the resulting color centers by probing their optical (photoluminescence) signals.

What they uncovered surprised them: a quantum emitter called the Ci center. Owing to its simple structure, stability at room temperature, and promising spin properties, the Ci center is an interesting spin photon qubit candidate that emits photons in the telecom band. “We knew from the literature that Ci can be formed in silicon, but we didn’t expect to actually make this new spin photon qubit candidate with our approach,” Jhuria said.

An artistic depiction of a new method to create high-quality color-centers (qubits) in silicon at specific locations using ultrafast laser pulses (femtosecond, or one quadrillionth of a second). The inset at the top-right shows an experimentally observed optical signal (photoluminescence) from the qubits, with their structures displayed at the bottom. (Credit: Kaushalya Jhuria/Berkeley Lab)

The researchers learned that processing silicon with a low femtosecond laser intensity in the presence of hydrogen helped to create the Ci color centers. Further experiments showed that increasing the laser intensity can increase the mobility of hydrogen, which passivates undesirable color centers without damaging the silicon lattice, Schenkel explained.

A theoretical analysis performed by Liang Tan, staff scientist in Berkeley Lab’s Molecular Foundry, shows that the brightness of the Ci color center is boosted by several orders of magnitude in the presence of hydrogen, confirming their observations from laboratory experiments.

“The femtosecond laser pulses can kick out hydrogen atoms or bring them back, allowing the programmable formation of desired optical qubits in precise locations,” Jhuria said.

The team plans to use the technique to integrate optical qubits in quantum devices such as reflective cavities and waveguides, and to discover new spin photon qubit candidates with properties optimized for selected applications.

“Now that we can reliably make color centers, we want to get different qubits to talk to each other – which is an embodiment of quantum entanglement – and see which ones perform the best. This is just the beginning,” said Jhuria.

“The ability to form qubits at programmable locations in a material like silicon that is available at scale is an exciting step towards practical quantum networking and computing,” said Cameron Geddes, Director of the ATAP Division.

Theoretical analysis for the study was performed at the Department of Energy’s National Energy Research Scientific Computing Center (NERSC) at Berkeley Lab with support from the NERSC QIS@Perlmutter program.

The Molecular Foundry and NERSC are DOE Office of Science user facilities at Berkeley Lab.

This work was supported by the DOE Office of Fusion Energy Sciences.

Lawrence Berkeley National Laboratory (Berkeley Lab) is committed to delivering solutions for humankind through research in clean energy, a healthy planet, and discovery science. Founded in 1931 on the belief that the biggest problems are best addressed by teams, Berkeley Lab and its scientists have been recognized with 16 Nobel Prizes. Researchers from around the world rely on the Lab’s world-class scientific facilities for their own pioneering research. Berkeley Lab is a multiprogram national laboratory managed by the University of California for the U.S. Department of Energy’s Office of Science.

DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit energy.gov/science .

Members of the team that conducted the research in the Lab.

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The experiment is a step toward testing how quantum physics interfaces with gravity

Red squiggles representing photons are sent into a loop representing the optical fiber in an interferometer, which surrounds Earth on a starry backdrop

In a laboratory experiment, scientists sent entangled photons (red squiggles) into an interferometer (illustrated) that was sensitive enough to measure Earth’s rotation.

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Technical Review
Published: 19 December 2018

Entanglement certification from theory to experiment

Nicolai Friis ORCID: orcid.org/0000-0003-1950-8640 1 ,
Giuseppe Vitagliano 1 ,
Mehul Malik 1 , 2 &
Marcus Huber ORCID: orcid.org/0000-0003-1985-4623 1

Nature Reviews Physics volume 1 , pages 72–87 ( 2019 ) Cite this article

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Quantum information
Quantum optics

Entanglement is an important resource for quantum technologies. There are many ways quantum systems can be entangled, ranging from the two-qubit case to entanglement in high dimensions or between many parties. Consequently, many entanglement quantifiers and classifiers exist, corresponding to different operational paradigms and mathematical techniques. However, for most quantum systems, exactly quantifying the amount of entanglement is extremely demanding, if at all possible. Furthermore, it is difficult to experimentally control and measure complex quantum states. Therefore, there are various approaches to experimentally detect and certify entanglement when exact quantification is not an option. The applicability and performance of these methods strongly depend on the assumptions regarding the involved quantum states and measurements, in short, on the available prior information about the quantum system. In this Review, we discuss the most commonly used quantifiers of entanglement and survey the state-of-the-art detection and certification methods, including their respective underlying assumptions, from both a theoretical and an experimental point of view.

Entanglement detection and certification are of high significance for ensuring the security of quantum communication, improving the sensitivity of sensing devices, and benchmarking devices for quantum computation and simulation.

Recent years have seen continuous progress in the development of tools for entanglement certification and an increase in control over a wide variety of experimental setups suitable for entanglement creation.

Goals for the development of entanglement detection techniques are device-independence and assumption-free certification.

Current challenges include the extension of well-understood methods for two qubits to many-body and/or high-dimensional quantum systems and their application in entanglement experiments with ions, atoms and photons.

An important focus of recent research is the reduction in the number of measurements required for entanglement certification to cope with increasing system dimensions.

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Mirrored entanglement witnesses

Zero uncertainty states in the presence of quantum memory

Experimental few-copy multipartite entanglement detection

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Acknowledgements

The authors acknowledge support from the Austrian Science Fund (FWF) through the START project Y879-N27 and from the joint Czech–Austrian project MultiQUEST (I3053-N27 and GF17-33780L). N.F. acknowledges support from the FWF through project P 31339-N27. M.M. acknowledges support from the QuantERA ERA-NET co-fund (FWF Project I3773-N36) and from the UK Engineering and Physical Sciences Research Council (EPSRC) (EP/P024114/1). G.V. acknowledges support from the FWF through the Lise-Meitner project M 2462-N27.

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Chinese scientists harness power of ‘entanglement’ to fuel quantum engine

Breakthrough study is the first experimental realisation of a quantum engine with ‘entangled characteristics’, researchers said
The technology uses the mysterious phenomenon that allows a pair of separated light particles to remain intimately linked, regardless of the distance between the

The researchers, from the Chinese Academy of Sciences’ Innovation Academy of Precision Measurement Science and technology, said the breakthrough shows that quantum engines can use their own entangled states as a form of fuel.

“Our study’s highlight is the first experimental realisation 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, on Monday.

Unlike traditional engines that operate on thermal combustion, a quantum engine uses lasers to transition the particles between quantum states, converting light into kinetic energy.

Zhou, along with fellow corresponding author Feng Mang and the rest of the team, showed that the entanglement phenomenon increases the output efficiency of quantum engines, according to the study, published on April 30 by the journal Physical Review Letters.

Quantum engines could theoretically surpass the limits of classical thermodynamics, potentially achieving energy conversion efficiencies of more than 25 per cent – enough to power large-scale quantum computers and circuits.

Using ultra-cold 40Ca+ ions confined in an ion trap as the working substance for the quantum engine, the team designed a thermodynamic cycle that converts the external laser energy into the vibrational energy of the ions.

“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 said.

“We measured how well the engine works by looking at two things: conversion efficiency, which is how many vibrations (phonons) it produces for every bit of light (photons) it uses, and mechanical efficiency, which is how much of the energy we can actually use compared to all the energy it puts out.”

Chinese researchers claim brain-computer interface breakthrough using monkey brain signal

More than 10,000 experiments revealed that higher degrees of ion entanglement led to greater mechanical efficiency, although the conversion efficiency remained largely 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.

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

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

According to Zhou, while quantum batteries might not store as much energy as those used in electric vehicles, their real benefit would come from their ability 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 said.

Introducing two powerful new capabilities in Azure Quantum Elements: Generative Chemistry and Accelerated DFT

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Dr. Nathan Baker

Product Leader, Azure Quantum Elements, Microsoft

Azure Quantum Elements is making research in chemistry and materials science faster, easier, and more productive by integrating new tools based on generative AI and high-performance computing.

Microsoft’s mission is to empower every person and every organization on the planet to achieve more. Azure Quantum Elements contributes to this mission by offering scientific capabilities based on AI and cloud high-performance computing (HPC). These user-friendly tools greatly increase the productivity of scientific research and remove barriers on the path to scientific discovery by substantially reducing the effort and expertise needed to perform what were previously daunting tasks. Specifically, these capabilities use Copilot for Azure Quantum, a natural-language interface that can be used by experts and non-experts alike, making Azure Quantum Elements accessible to more people and increasing the speed at which complex scientific problems can be solved.

Unlocking solutions to our most pressing challenges will require the world’s collective genius, and we’re excited to put new capabilities into the hands of scientists, students, and organizations such as Unilever so that everyone can contribute to making scientific discoveries that change the world for the better.

Since its launch, Azure Quantum Elements has served an important role in helping scientists achieve breakthroughs that have paved the way for more sustainable batteries and innovations in the pharmaceutical industry . Today, we’re announcing two new purpose-built capabilities in Azure Quantum Elements that will further increase the productivity and accessibility of chemistry and materials science research— Generative Chemistry and Accelerated DFT .

Generative Chemistry helps scientists discover novel, synthesizable, and useful molecules quickly

There are hundreds of millions of known molecular compounds and substances and many more remain to be discovered. In the field of chemistry, a major challenge is narrowing down an enormous number of candidate molecules to find the few that are best suited for a particular application. This challenge results in the streetlight effect, which is when the vast number of possibilities is reduced to a reasonable size, not based on the properties of the compounds, but by focusing only on those that have been studied previously. Relying on databases to identify suitable compounds limits the search space so that only known compounds are revealed as candidates for specific applications. Generative AI helps bring to light a much larger portion of the estimated 10 60 possible combinations of atoms so that scientists are presented with novel candidates that are likely to serve the intended purpose.

Today, the Azure Quantum team at Microsoft is announcing Generative Chemistry—an upcoming capability that has the potential to greatly simplify this process and help scientists quickly discover and design new compounds with desired properties, thus increasing the productivity of product innovation.

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Generative Chemistry, which will be available through Azure Quantum Elements private preview, is an end-to-end workflow that involves multiple steps:

You provide information on desired molecular characteristics for your specific application. Additionally, you can supply reference molecules if you already have some possibilities in mind.
The information you provide is used to generate seed molecules from a dataset—those seed molecules are then used to initiate the guided AI generation of candidate molecules for your application. This is accomplished by applying multiple AI models and a unique methodology to identify new compounds that match your criteria. In this step, you have several options for configuration such as selecting the most relevant generative AI model, specifying the number of molecules to be generated, indicating the key molecular properties of interest, and screening compounds for toxicity.
AI-based screening models predict properties of the candidate molecules that are important for real-world applications—such as boiling point, density, or solubility. A feedback loop sends this information back to the guided AI generation (step 2) to modify the selection of candidate molecules. In this step, you also have the option to fine tune the AI models for your specific application.
The pool of potential candidates is further narrowed through a step that uses AI-guided synthesis planning to determine the feasibility of making the molecules in a laboratory—this is important because some novel molecules with desired properties may be difficult to synthesize. In this step, synthesis pathways are predicted, and candidate molecules are filtered based on how easy they are to make.
Highly accurate HPC simulations are performed on the top candidates. Accelerated DFT can be used to screen candidates for electronic properties such as dielectric constant, ionization potential, and polarizability. AutoRXN predicts chemical stability or reactivity, and it can also be used to provide insights on possible synthesis pathways.
You are presented with the final candidate molecules from which you can select the most promising for laboratory synthesis and testing.

This entire process takes only days, shaving months or even years off trial-and-error laboratory experiments that were previously required to arrive at this point. Generative Chemistry suggests entirely new compounds and gives scientists the freedom to focus on only those molecules that are fit for the desired purpose—saving time, money, and effort. This new capability will allow for rapid progress in the development of novel therapeutics, sustainable materials, and more.

Generative Chemistry

Webinar with American Chemical Society

Accelerated DFT offers substantial increases in speed compared to other density functional theory codes

Density functional theory (DFT) is one of the most popular methods in computational chemistry due to its efficiency and accuracy in modeling quantum-mechanical properties. It allows researchers to simulate and study the electronic structures of atoms, molecules, and nanoparticles as well as surfaces and interfaces, thus predicting properties such as dielectric constant, ionization potential, and polarizability. Scientists can then tailor those properties to optimize them for specific applications.

Although DFT is highly valuable for research and product design, most DFT codes need to be run directly by the user in HPC clusters, which can be a difficult undertaking. Additionally, DFT—when run on traditional HPC hardware—requires substantial compute power and becomes constrained as the molecules being studied or designed increase in complexity and size.

To simplify and improve this process, Azure Quantum and Microsoft Research designed and launched Accelerated DFT, a code used to simulate the electronic structure of molecules. Accelerated DFT can determine the properties of molecules with thousands of atoms in a matter of hours. It performs substantially faster than other DFT codes and offers a 20-fold average increase in speed compared to PySCF, a widely used open-source DFT code. 1

The setup of Accelerated DFT is made simple by providing the software as a service, requiring no code compilation or configuration on the user’s part, and it has a simplified API to streamline the computational process. Furthermore, a Python Software Development Kit (SDK) offers seamless integration into a wide variety of computational chemistry environments, which allows researchers to incorporate DFT calculations into complex chemistry workloads. Accelerated DFT is now available in Azure Quantum Elements private preview ; it will also become part of Generative Chemistry.

Accelerated DFT can substantially expedite research across a wide spectrum of chemical disciplines by employing the power of Azure’s cloud architecture. Accelerated DFT can also produce large and highly accurate datasets of molecular properties that can be used to improve AI models, which require large amounts of training data. By rapidly generating training data, new molecules can be discovered faster, and known molecules can be improved, leading to innovations in therapeutics, sustainable products, and more.

“ We have been very impressed by Accelerated DFT for simulating large systems with range-separated hybrid density functionals, where it speeds up the calculations considerably. Furthermore, the team at Microsoft has been very accommodating in quickly resolving technical challenges for our calculations of an active site of an enzyme .” —Tejs Vegge, Professor, Head of Section for Autonomous Materials Discovery, DTU Energy, Technical University of Denmark

“ Accelerated DFT has an easy-to-use Python interface, and the acceleration in calculations allows for the efficient use of novel hybrid functionals and a large basis set. Estimating key thermodynamic properties can thus be done in a matter of hours. AspenTech is excited to be partnering with Microsoft to evaluate the application of quantum chemistry in our physical properties system for process modeling, especially in our sustainability pathways solutions. We anticipate that working with Accelerated DFT will result in strong use cases for characterizing new molecules and materials that are crucial to decarbonizing and circularizing industrial and consumer materials .” —Heiko Claussen, Senior Vice President, Artificial Intelligence Technology at AspenTech

Azure Quantum Elements incorporates quantum computing

With the addition of each new feature, Azure Quantum Elements becomes a more effective tool for scientists by harnessing AI, HPC, and emerging hybrid-computing capabilities that bring the power of quantum computing to scientific challenges. Recently, we simulated a chemical catalyst by combining classical supercomputers, AI, and logical qubits created with Microsoft’s qubit-virtualization system and Quantinuum’s H1 hardware. In the coming months, we will introduce advanced logical qubit capabilities from Microsoft and Quantinuum to the private preview of Azure Quantum Elements. This classical-quantum hybrid computing offering follows our quantum computing milestone with Quantinuum in which we created the most reliable logical qubits on record with an error rate 800 times better than that of the corresponding physical qubits.

Advances in AI and quantum computing have the potential to help researchers solve global scientific challenges. In the future, we plan to offer a quantum supercomputer that can simulate interactions of molecules and atoms at the quantum level, which is beyond the reach of classical computers. This capability is expected to transform research and innovation across many industries. 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 how Azure Quantum Elements is revolutionizing chemistry and materials science

Read about our collaboration with Unilever in the Official Microsoft Blog.
Explore the product overview of Azure Quantum Elements .

1 Details on the performance of the private preview of Accelerated DFT can be found in the preprint of  Acceleration without Disruption: DFT Software as a Service .

How Microsoft and Quantinuum achieved reliable quantum computing

Today, Microsoft is announcing a critical breakthrough that advances the field of quantum computing by Read more

Responsible computing and accelerating scientific discovery across HPC, AI, and Quantum

The technological landscape can evolve quickly, and early adoption of governance and risk mitigation measures Read more

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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 ...
Proving that Quantum Entanglement is Real
Indeed, China's quantum-encrypted communications satellite, Micius, relies on quantum entanglement between photons that are separated by thousands of kilometers. The very first of these experiments was proposed and executed by Caltech alumnus John Clauser (BS '64) in 1969 and 1972, respectively.
'Spooky' quantum-entanglement experiments win physics Nobel
Three quantum physicists have won the 2022 Nobel Prize in Physics for their experiments with entangled photons, in which particles of light become inextricably linked. Such experiments have laid ...
Physics
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 . In this experiment, two sets of entangled photon pairs are ...
Proving that quantum entanglement is real: Researcher answers questions
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.
Explorers of Quantum Entanglement Win 2022 Nobel Prize in Physics
Alain Aspect, John F. Clauser and Anton Zeilinger won the 2022 Nobel Prize in Physics for their work using entangled photons to test the quantum foundations of reality. This year's Nobel Prize ...
Experiments on 'entangled' quantum particles won the physics Nobel Prize
Experiments on entanglement — a strange feature of quantum physics — have netted three scientists the 2022 Nobel Prize in physics. When two particles are entangled (illustrated), what happens to one determines what happens to the other — even when the second one is far away. For their tests of quantum weirdness and its real-world uses ...
Light from ancient quasars helps confirm quantum entanglement
The researchers used distant quasars, one of which emitted its light 7.8 billion years ago and the other 12.2 billion years ago, to determine the measurements to be made on pairs of entangled photons. They found correlations among more than 30,000 pairs of photons, to a degree that far exceeded the limit that Bell originally calculated for a ...
Quantum experiments with entangled photons win the 2022 Nobel Prize in
Entanglement is a delicate state of affairs and is difficult to maintain, but the results of the experiments of Clauser and Aspect show that quantum effects cannot be explained with any hidden ...
Particle, wave, both or neither? The experiment that ...
'Spooky' quantum-entanglement experiments win physics Nobel Aspect won a share of the 2022 Nobel prize in physics for his contribution to confirming the predictions of quantum mechanics ...
MIT researchers use quantum computing to observe entanglement
In this experiment, researchers sent a signal "through the wormhole" by teleporting a quantum state from one quantum system to another on the Sycamore 53-qubit quantum processor. To do so, the research team needed to determine entangled quantum systems that behaved with the properties predicted by quantum gravity — but that were also ...
Entangled Titans: unraveling the mysteries of Quantum Mechanics with
The CMS experiment has just reported the observation of quantum entanglement between a top quark and a top antiquark, simultaneously produced at the LHC. In quantum mechanics, a system is said to be entangled if its quantum state cannot be described as a simple superposition of the states of its constituents. If two particles are entangled, we ...
Advances in high-dimensional quantum entanglement
Entanglement swapping has become an important fundamental concept, with applications such as overcoming long distances in quantum networks 235,236 or in fundamental experiments regarding ...
What is quantum entanglement? A physicist explains the science of
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 ...
Press release: The Nobel Prize in Physics 2022
Press release. 4 October 2022. The Royal Swedish Academy of Sciences has decided to award the Nobel Prize in Physics 2022 to. "for experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science".
Rochester physicists find 'spooky action at a distance' at CERN
The researchers have confirmed that quantum entanglement persists between top quarks, the heaviest known fundamental particles. An experiment by a group of physicists led by University of Rochester physics professor Regina Demina has produced a significant result related to quantum entanglement—an effect that Albert Einstein called "spooky action at a distance."
The Nobel Prize in Physics 2022
Popular science background: How entanglement has become a powerful tool (pdf) Populärvetenskaplig information: Så blev sammanflätning ett kraftfullt verktyg (pdf) The Nobel Prize in Physics 2022. Using groundbreaking experiments, Alain Aspect, John Clauser and Anton Zeilinger have demonstrated the potential to investigate and control particles that are in entangled states.
Quantum entanglement
Quantum entanglement is the phenomenon of a group of particles being generated, interacting, or sharing spatial proximity in such a way that the quantum state of each particle of the group cannot be described independently of the state of the others, including when the particles are separated by a large distance. The topic of quantum entanglement is at the heart of the disparity between ...
China Shatters "Spooky Action at a Distance" Record, Preps for Quantum
Results from the Micius satellite test quantum entanglement, pointing the way toward hackproof global communications. ... as part of the nation's $100-million Quantum Experiments at Space Scale ...
PDF How entanglement has become a powerful tool
Experiments have shown that nature behaves as predicted by quantum mechanics. The balls are . grey, with no secret information, and chance determines which becomes black and which becomes white in an experiment. Quantum mechanics' most important resource. Entangled quantum states hold the potential for new ways of storing, transferring and ...
What is quantum entanglement? A physicist explains Einstein's 'spooky
A multitude of experiments have shown the mysterious phenomena of quantum mechanics to be how the universe functions. The scientists behind these experiments won the 2022 Nobel Prize in physics.
Light from ancient quasars helps confirm quantum entanglement
Space.com reporter Chelsea Gohd writes that MIT researchers have used the light emitted by two ancient quasars to provide evidence of quantum entanglement, the theory that two particles can become linked across space and time. The researchers used ancient quasars to see if, "the correlation between particles can be explained by classical mechanics stemming from earlier than 600 years ago."
What Is Quantum Entanglement? A Physicist Explains ...
Quantum entanglement is a phenomenon in which the quantum states of two or more objects become correlated, meaning that the state of one object can affect the state of the other (s) even if the objects are separated by large distances. This occurs because, according to quantum theory, particles can exist in multiple states at the same time (a ...
Unraveling Quantum Entanglement with Simulation
"The essential idea behind entanglement is that two quantum objects -; say, two particles -; can be correlated, ... " But countless experiments have shown that the spooky effect is real.
Quantum entanglement measures Earth rotation
Researchers carried out a pioneering experiment where they measured the effect of the rotation of Earth on quantum entangled photons. The work represents a significant achievement that pushes the ...
New Technique Could Help Build Quantum Computers of the Future
Key Takeaways. Berkeley Lab researchers have reported a major advancement that could bring us closer to a scalable quantum computer. Using a femtosecond laser during experiments which explore the role of hydrogen in qubit formation, the researchers developed a method that programs the formation of telecom-band optical qubits in silicon for large-scale manufacturing.
Physicists measured Earth's rotation using quantum entanglement
The experiment is a step toward testing how quantum physics interfaces with gravity. ... Experimental observation of Earth's rotation with quantum entanglement. Science Advances. Published ...
Entanglement certification from theory to experiment
Nature Communications (2023) Entanglement is an important resource for quantum technologies. There are many ways quantum systems can be entangled, ranging from the two-qubit case to entanglement ...
Chinese scientists harness power of 'entanglement' to fuel quantum
More than 10,000 experiments revealed that higher degrees of ion entanglement led to greater mechanical efficiency, although the conversion efficiency remained largely unaffected by the level of ...
Introducing two powerful new capabilities in Azure Quantum Elements
Azure Quantum Elements incorporates quantum computing . With the addition of each new feature, Azure Quantum Elements becomes a more effective tool for scientists by harnessing AI, HPC, and emerging hybrid-computing capabilities that bring the power of quantum computing to scientific challenges.

Proving that Quantum Entanglement is Real

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Nobel Prize : Quantum Entanglement Unveiled

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

Experimental Entanglement Swapping: Entangling Photons That Never Interacted

Experimental Test of Local Hidden-Variable Theories

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Proving that quantum entanglement is real: Researcher 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?

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

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

Can you help us understand exactly what your experiments showed?

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

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An alternative way to manipulate quantum states

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

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Existing in multiple states at once

Two entangled particles

Disproving a theory