First experimental evidence that quantum entanglement is real

Explain quantum entanglement

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

Q&A with Caltech alumnus John Closer on his first experimental evidence of quantum entanglement.

When scientists, including Albert Einstein and Erwin Schrödinger, first discovered the phenomenon of entanglement in the 1930s, they were baffled. Worryingly, entanglement requires two separate particles to remain connected without being in direct contact. In fact, Einstein called entanglement “spooky action at a distance,” because the particles seemed to communicate faster than the speed of light.

Born on December 1, 1942, John Francis Clauser is an American theoretical and experimental physicist known for his contributions to the foundations of quantum mechanics, in particular the Clauser-Horn-Schmonie-Hult inequality. Clauser was jointly awarded the 2022 Nobel Prize in Physics with Alan Aspect and Anton Zeilinger “for experiments with entangled photons, proving the violation of Bell’s inequality and pioneering in quantum information science.”

To explain the strange effects of entanglement, Einstein, along with Boris Podolsky and Nathan Rosen (EPR), argued that “hidden variables” must be added to quantum mechanics. These can be used to explain entanglement, restoring ‘local’ and ‘causal’ particle behaviour. The local area states that objects are affected only by their immediate surroundings. Causality states that an effect cannot occur before its cause, and that causal signals cannot propagate faster than the speed of light. Niels Bohr opposed the famous EPR argument, while Schrödinger and Wendell Fury, in response to EPR, independently hypothesized that entanglement vanishes with large-scale particle separation.

Unfortunately, at that time, there was no experimental evidence for or against large-scale particle-separated quantum entanglement. Experiments since then have proven that entanglement is very real and fundamental to nature. Moreover, it has now been shown that quantum mechanics works, not only at very short distances but also at very large distances. In fact, China’s quantum-encoded communications satellite, MICUS, (part of the QUESS Project) relies on quantum entanglement between photons thousands of kilometers apart.

John Clauser's second experiment on quantum entanglement

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

The first of these experiments was proposed and implemented by Caltech alumnus John Clauser (BS ’64) in 1969 and 1972, respectively. His conclusions are based on Bell’s theory, which he devised CERN The view is John Bell. In 1964, Bell ironically demonstrated that the EPR argument actually led to the opposite conclusion of what EPR had originally intended to show. Bell explained that quantum entanglement is, in fact, incompatible with the EPR idea of ​​locus and causation.

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

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

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

John Clauser Yacht Club

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

Here, John Clauser answers questions about his historical experiments.

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

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

In my correspondence with John Bell, expressed the exact opposite sentiments and strongly encouraged me to experiment. John Bell’s 1964 seminal work on Bell’s theory was originally published in the final issue of Obscure Journal, Physicsand in a secret physics journal, Cognitive messages. It wasn’t even yet 1969 CHSH and . paper 1972 The results of Freedman – Clauser have been published in physical review messages That John Bell finally discussed his work frankly. He was aware of the taboo in questioning the foundations of quantum mechanics and did not discuss it with his counterpart[{” attribute=””>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 bet scientifically, you bet with quantum physicist Yakir Aharonov on the outcome of the Friedman-Klauser experiment. Oddly enough, he paid $1 for $2. I lost the bet and attached a $2 bill and congratulated me when I sent him a hard copy of our findings.

I was very sad to see that my own experience had proven Einstein wrong. But the experiment gave him a result of 6.3 sigma against him [a five-sigma result or higher is considered the gold standard for significance in physics]. But then the experiment of rival Dick Holt and Frank Pepkin at Harvard University (never published) had the opposite result. I wondered if I had overlooked some important details. I went alone at UC Berkeley to do three more experimental tests of quantum mechanics. All yielded the same conclusions. Bohr was right, and Einstein was wrong. The Harvard result was not repeated and was wrong. When I called back the 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 showed I was a reasonably talented experimental physicist. This fact alone has given me a job at Lawrence Livermore National Laboratory doing controlled amalgamation[{” attribute=””>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 were made later by John Bell And the Abner Shmoniso it might be called more appropriately Domestic Realism Bill Clauser Horn Shimoni. Domestic realism was short-lived as a viable theory. In fact, it was empirically disproved even before it was fully formulated. However, local realism is empirically important because it explains in detail what quantum mechanics is Not.

Domestic realism assumes that nature is made up of things, of objectively real things, i.e. things that you can put inside a box. (The square here is an imaginary closed surface that defines the separate inner and outer volumes.) It is also assumed that objects exist whether we notice them or not. Likewise, definite experimental results are supposed to be obtained, whether we look at them or not. We may not know what the objects are, but we assume that they exist and that they are distributed throughout space. Things may develop either inevitably or randomly. Local realism assumes that things inside the box have intrinsic properties, and that when someone performs an experiment inside the box, the probability of any outcome obtained is influenced in some way by the properties of the things inside that box. If one performs a different experiment with different experimental parameters, a different result is assumed. Now suppose one contains two widely separated chests, each containing objects. Local realism also assumes that the experimental parameter selection made in one square cannot affect the experimental result in the far square. So local realism forbids remote scary work. It imposes an Einsteinian causal relationship that prohibits any such non-local cause and effect. Surprisingly, those very simple and reasonable assumptions are sufficient alone To allow the derivation of a second important empirical prediction that limits the correlation between the experimental results obtained in the separate squares. This prediction is the 1974 Clauser–Horne (CH) inequality.

The 1969 CHSH inequality derivation required several secondary complementary assumptions, sometimes called ‘gaps’. The derivation of the CH inequality cancels out those complementary assumptions and is thus more general. There exist quantum entangled systems that are inconsistent with the predictions of CH, where local realism is amenable to experimental refutation. Both CHSH and CH inequalities have been violated, not only by the first Freedman-Clauser experiment in 1972 and my second experiment in 1976 but now by hundreds of confirmed independent experiments. Various labs have now entangled and violated the CHSH inequality with photon pairs, beryllium ion pairs, and ytterbium-rubidium ion pairs.[{” attribute=””>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] They were PhD students at Caltech under Theodor von Karmann. Frances Closer was Clark Blanchard Millikan Professor of Engineering at Caltech (80’s Distinguished College Award) and Chair of the Department of Engineering and Applied Sciences at Caltech. Milton J Clauser’s son, Milton J Clauser [PhD ’66]and grandson of Karl Closer [BS ’86] Both went to Caltech. My mom, Catherine MacMillan Closer was a librarian for the humanities at Caltech, where she met my dad. Her brother, Edwin Macmillan [BS ’28, MS ’29], is a California Institute of Technology and ’51 Nobel Prize laureate. The family now maintains Caltech’s “Milton and Francis Doctoral Award” given at Caltech commencement ceremonies.

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