*Proving that Quantum Entanglement is Real* *September 20, 2022*
*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 <https://www.johnclauser.com/_files/ugd/36ef59_e9b993b0864c45148ca0c25c5ddaaf71.pdf>, 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 <https://www.johnclauser.com/_files/ugd/36ef59_05bf40eb0ab4457cac6bf6b0dc8d7077.pdf> has been cited more than 8,500 times on Google Scholar <https://scholar.google.com/citations?user=BDm2SGcAAAAJ&hl=en>.) 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 <https://www.johnclauser.com/_files/ugd/36ef59_3f20f8b70cc64e6aaf8b6cb40e65ffea.pdf>, 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 <https://www.johnclauser.com/_files/ugd/36ef59_05bf40eb0ab4457cac6bf6b0dc8d7077.pdf> CHSH paper and the 1972 <https://www.johnclauser.com/_files/ugd/36ef59_6dbcb33d133740be886e7cd0d8643ac7.pdf> 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 <https://www.johnclauser.com/_files/ugd/36ef59_941d93aaa6fb442981a62e84f9fc543f.pdf>]. Additional contributions to it were subsequently offered by John Bell <https://www.johnclauser.com/_files/ugd/36ef59_3d2bdc922185483db655f4d5da49e459.pdf> and Abner Shimony <https://www.johnclauser.com/_files/ugd/36ef59_62b5987bd00d4d64bd74b1f84ecd67cd.pdf>, so perhaps it is more properly called Bell–Clauser–Horne–Shimony Local Realism <https://www.johnclauser.com/_files/ugd/36ef59_4708380b594c4ed7a8c06ebed4b2f0f4.pdf>. 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 <https://galcit.caltech.edu/about/vonkarman>. 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. *The Double-Slit Experiment* Perhaps the most definitive experiment in the field of quantum physics is the double-slit experiment <https://www.pbs.org/video/pbs-space-time-quantum-experiment/>. This experiment, which involves shooting particles such as photons or electrons though a barrier with two slits, was originally used in 1801 to show that light is made up of waves. Since then, numerous incarnations of the experiment have been used to demonstrate that matter can also behave like a wave and to demonstrate the principles of superposition, entanglement, and the observer effect. The field of quantum science may seem mysterious or illogical, but it describes everything around us, whether we realize it or not. Harnessing the power of quantum physics gives rise to new technologies, both for applications we use today <https://scienceexchange.caltech.edu/topics/quantum-science-explained/quantum-technology> and for those that may be available in the future <https://scienceexchange.caltech.edu/topics/quantum-science-explained/quantum-computing-computers> . *quantum theory* Quantum theory is the theoretical basis of modern physics that explains the nature and behavior of matter and energy on the atomic and subatomic level. The nature and behavior of matter and energy at that level is sometimes referred to as quantum physics and quantum mechanics. Organizations in several countries have devoted significant resources to the development of quantum computing <https://www.computerweekly.com/news/252486912/Finland-government-funds-work-on-quantum-leap>, which uses quantum theory to drastically improve computing capabilities beyond what is possible using today's classical computers. In 1900, physicist Max Planck <https://www.techtarget.com/whatis/definition/Plancks-constant> presented his quantum theory to the German Physical Society. Planck had sought to discover the reason that radiation from a glowing body changes in color from red, to orange, and, finally, to blue as its temperature rises. He found that by making the assumption that energy existed in individual units in the same way that matter does, rather than just as a constant electromagnetic wave - as had been formerly assumed - and was therefore *quantifiable*, he could find the answer to his question. The existence of these units became the first assumption of quantum theory. Planck wrote a mathematical equation involving a figure to represent these individual units of energy, which he called *quanta* <https://www.techtarget.com/whatis/definition/quantum>. The equation explained the phenomenon very well; Planck found that at certain discrete temperature levels (exact multiples of a basic minimum value), energy from a glowing body will occupy different areas of the color spectrum. Planck assumed there was a theory yet to emerge from the discovery of quanta, but, in fact, their very existence implied a completely new and fundamental understanding of the laws of nature. Planck won the Nobel Prize in Physics for his theory in 1918, but developments by various scientists over a thirty-year period all contributed to the modern understanding of quantum theory. *The Development of Quantum Theory* - In 1900, Planck made the assumption that energy was made of individual units, or quanta. - In 1905, Albert Einstein theorized that not just the energy, but the radiation itself was *quantized* in the same manner. - In 1924, Louis de Broglie proposed that there is no fundamental difference in the makeup and behavior of energy and matter; on the atomic and subatomic level either may behave as if made of either particles or waves. This theory became known as the *principle of wave-particle duality*: elementary particles of both energy and matter behave, depending on the conditions, like either particles or waves. - In 1927, Werner Heisenberg proposed that precise, simultaneous measurement of two complementary values - such as the position and momentum of a subatomic particle - is impossible. Contrary to the principles of classical physics, their simultaneous measurement is inescapably flawed; the more precisely one value is measured, the more flawed will be the measurement of the other value. This theory became known as the uncertainty principle, which prompted Albert Einstein's famous comment, "God does not play dice." *The Copenhagen Interpretation and the Many-Worlds Theory* The two major interpretations of quantum theory's implications for the nature of reality are the Copenhagen interpretation and the many-worlds theory. Niels Bohr proposed the Copenhagen interpretation of quantum theory, which asserts that a particle is whatever it is measured to be (for example, a wave or a particle), but that it cannot be assumed to have specific properties, or even to exist, until it is measured. In short, Bohr was saying that objective reality does not exist. This translates to a principle called superposition <https://www.techtarget.com/whatis/definition/superposition> that claims that while we do not know what the state of any object is, it is actually in all possible states simultaneously, as long as we don't look to check. To illustrate this theory, we can use the famous and somewhat cruel analogy of Schrodinger's Cat <https://www.techtarget.com/whatis/definition/Schrodingers-cat>. First, we have a living cat and place it in a thick lead box. At this stage, there is no question that the cat is alive. We then throw in a vial of cyanide and seal the box. We do not know if the cat is alive or if the cyanide capsule has broken and the cat has died. Since we do not know, the cat is both dead and alive, according to quantum law - in a superposition of states. It is only when we break open the box and see what condition the cat is that the superposition is lost, and the cat must be either alive or dead. The second interpretation of quantum theory is the *many-worlds* (or *multiverse* theory. It holds that as soon as a potential exists for any object to be in any state, the universe of that object transmutes into a series of parallel universes equal to the number of possible states in which that the object can exist, with each universe containing a unique single possible state of that object. Furthermore, there is a mechanism for interaction between these universes that somehow permits all states to be accessible in some way and for all possible states to be affected in some manner. Stephen Hawking and the late Richard Feynman are among the scientists who have expressed a preference for the many-worlds theory. *Quantum Theory's Influence* Although scientists throughout the past century have balked at the implications of quantum theory - Planck and Einstein among them - the theory's principles have repeatedly been supported by experimentation, even when the scientists were trying to disprove them. Quantum theory and Einstein's theory of relativity form the basis for modern physics. The principles of quantum physics are being applied in an increasing number of areas, including quantum optics, quantum chemistry, quantum computing <https://www.techtarget.com/whatis/definition/quantum-computing>, and quantum cryptography <https://www.techtarget.com/searchsecurity/definition/quantum-cryptography>. The first applications of quantum mechanics to physical systems were the algebraic determination of the hydrogen spectrum by Wolfgang Pauli and the treatment of diatomic molecules by Lucy Mensing. *Condensed-matter-physics*[edit <https://en.wikipedia.org/w/index.php?title=Quantum_field_theory&action=edit§ion=8> ] Although quantum field theory arose from the study of interactions between elementary particles, it has been successfully applied to other physical systems, particularly to many-body systems <https://en.wikipedia.org/wiki/Many-body_system> in condensed matter physics <https://en.wikipedia.org/wiki/Condensed_matter_physics>. Historically, the Higgs mechanism of spontaneous symmetry breaking was a result of Yoichiro Nambu <https://en.wikipedia.org/wiki/Yoichiro_Nambu>'s application of superconductor <https://en.wikipedia.org/wiki/Superconductor> theory to elementary particles, while the concept of renormalization came out of the study of second-order phase transitions <https://en.wikipedia.org/wiki/Phase_transition> in matter.[23] <https://en.wikipedia.org/wiki/Quantum_field_theory#cite_note-23> Soon after the introduction of photons, Einstein performed the quantization procedure on vibrations in a crystal, leading to the first quasiparticle <https://en.wikipedia.org/wiki/Quasiparticle>—phonons <https://en.wikipedia.org/wiki/Phonon>. Lev Landau claimed that low-energy excitations in many condensed matter systems could be described in terms of interactions between a set of quasiparticles. The Feynman diagram method of QFT was naturally well suited to the analysis of various phenomena in condensed matter systems.[24] <https://en.wikipedia.org/wiki/Quantum_field_theory#cite_note-wilczek-24> Gauge theory is used to describe the quantization of magnetic flux <https://en.wikipedia.org/wiki/Magnetic_flux> in superconductors, the resistivity <https://en.wikipedia.org/wiki/Resistivity> in the quantum Hall effect <https://en.wikipedia.org/wiki/Quantum_Hall_effect>, as well as the relation between frequency and voltage in the AC Josephson effect <https://en.wikipedia.org/wiki/Josephson_effect>.[24] <https://en.wikipedia.org/wiki/Quantum_field_theory#cite_note-wilczek-24> Albert Einstein famously said that quantum mechanics should allow two objects to affect each other’s behaviour instantly across vast distances, something he dubbed “spooky action at a distance” <https://www.nature.com/news/quantum-spookiness-passes-toughest-test-yet-1.18255> 1 <https://www.nature.com/articles/d41586-020-00120-6#ref-CR1>. Decades after his death, experiments confirmed this. But, to this day, it remains unclear exactly how much coordination nature allows between distant objects. Now, five researchers say they have solved a theoretical problem that shows that the answer is, in principle, unknowable. Niels Bohr and Max Planck, two of the founding fathers of Quantum Theory, each received a Nobel Prize in Physics for their work on quanta. Einstein is considered the third founder of Quantum Theory because he described light as quanta in his theory of the Photoelectric Effect, for which he won the 1921 Nobel Prize. Quantum physics usually deals with things that are very very small, like particles and atoms. This view of the universe is known as the Quantum Realm. It's not a different place; it's a different way of looking at why things are the way that they are. Invisible forces influencing the physical. The term field in quantum physics represents invisible moving forces that influence the physical world. Spirituality represents invisible moving forces that influence the physical world. This is the view that reality isn't fundamentally a collection of objects – particles, atoms – spread out in three-dimensional space or even four-dimensional spacetime, but instead, reality is fundamentally a wave function, a field-like object that exists in some higher-dimensional quantum reality. Quantum theory does provide a workaround if you're patient. And a team of researchers from the University of Innsbruck in Austria has finally seen the quantum tunneling in action in a world-first experiment measuring the merger of deuterium ions with hydrogen molecules. Tunneling is a quirk of the quantum universe that makes it seem like particles can pass through obstacles <https://www.sciencealert.com/physicists-shed-light-on-the-mystery-of-how-particles-ghost-through-walls> that are ordinarily too hard to overcome. In chemistry, this obstacle is the energy required for atoms to bond with one another, or with existing molecules. Yet theory says that, in extremely rare instances, it's possible for atoms in close proximity to 'tunnel' their way through this energy barrier and connect without any effort. "Quantum mechanics allows particles to break through the energetic barrier due to their quantum mechanical wave properties, and a reaction occurs," says <https://www.eurekalert.org/news-releases/980849> first author Robert Wild, an experimental physicist from the University of Innsbruck. Quantum waves are the ghosts that drive the behaviors of objects like electrons, photons, and even entire groups of atoms, blurring their existence before any observation so they sit not in any one precise place but occupy a continuum of possible positions. This blurring is insignificant for larger objects like molecules, cats, and galaxies. But as we zoom in on individual subatomic particles, the range of possibilities expands, forcing the location states of various quantum waves to overlap. When that happens, particles have a slight chance of appearing where they have no business being, tunnelling into regions that would otherwise require a great deal of force to enter. One of those regions for an electron might be within the bonding-zone of a chemical reaction, welding together neighboring atoms and molecules without the boom-crash-crush of heat or pressure. Understanding the role quantum tunneling plays in the building and rearrangements of molecules could have important ramifications in the calculations of energy release in nuclear reactions, such as those involving hydrogen in stars and fusion reactors here on Earth <https://www.sciencealert.com/fusion-technology-is-reaching-a-turning-point-that-could-change-the-energy-game> . While we've modeled this phenomenon <https://journals.aps.org/pra/abstract/10.1103/PhysRevA.95.022706> for examples involving reactions between a negatively charged form of deuterium – an isotope of hydrogen containing a neutron – and dihydrogen or H2, proving the numbers experimentally requires a challenging level of precision. To accomplish this, Wild and his colleagues cooled negative deuterium ions to a temperature that brought them close to a standstill before introducing a gas made of hydrogen molecules. Without heat, the deuterium ion was far less likely to have the energy required to force hydrogen molecules into a rearrangement of atoms. Yet it also forced the particles into sitting quietly near one another, giving them more time to bond through tunneling. "In our experiment, we give possible reactions in the trap about 15 minutes and then determine the amount of hydrogen ions formed. From their number, we can deduce how often a reaction has occurred," Wild explains <https://www.eurekalert.org/news-releases/980849>. That figure is just over 5 x 10-20 reactions per second taking place in each cubic centimeter, or around one tunneling event for around every hundred billion collisions. So not a lot. Though the experiment does back up previous modeling, confirming a benchmark that can be used in predictions elsewhere. Given tunneling plays a fairly important role in a diverse range of nuclear and chemical reactions <https://www.sciencealert.com/a-surprising-number-of-mutations-occur-thanks-to-a-quirk-of-quantum-physics>, much of which is also likely to occur out in the cold depths of space, getting a precise grip on the factors at play gives us a more solid grounding to base our predictions on. This research was published in *Nature* <https://www.nature.com/articles/s41586-023-05727-z>. 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