2023 Segrè Lecture, featuring John Clauser
October 30, 2023 at 5:30 pm
Chevron Auditorium at International House
A Reception was held in the Slusser Room at 4:00 pm
View photos from the event HERE
Experimental proof that nonlocal quantum entanglement is real
I describe experiments and theory that provide the first experimental proof that nonlocal quantum entanglement is real. Sadly, the experimental results also undermine Albert Einstein’s basic platform for providing a mathematical description of nature in a space-time framework, and make quantum mechanics very difficult to understand. Quantum entanglement now provides the basis for the second quantum revolution. It is used by quantum encryption and quantum computers.
Einstein, Podolsky, and Rosen (EPR, 1935) proposed Local Hidden Variable Theories as a possible completion of Quantum Mechanics for entangled-state systems. John Bell (1964) ironically showed mathematically that the existence of EPR’s local hidden variables is incompatible with the predictions of Quantum Mechanics. Inspired by his result, Clauser, Horne, Shimony, and Holt (CHSH, 1969) designed a specific experiment employing entangled photon pairs. Assuming that a general Local Hidden Variable Theory governs the experiment, they derive the CHSH inequality as an experimental prediction for it. Alternatively, they show that Quantum Mechanics predicts measurably different results for the experiment. Hence, the two theories are experimentally distinguishable. The 1969 CHSH inequality is the first experimentally testable so-called “Bell inequality”. The first experimental test of it was by Freedman-Clauser (1972), performed at the UC Berkeley Physics Dept. and LBNL. That experiment measured the polarization correlation of entangled photon pairs produced by an atomic cascade in Calcium. The second experimental test was also at UC Berkeley [Clauser (1976)]. It used photon pairs produced by Mercury. These two experiments were the first observations of a violation of the CHSH - Bell inequality, and disproved general Local Hidden Variable Theories.
Given limitations of available technology at that time, fully conclusive tests of the CHSH – Bell inequality were not possible, and highly plausible auxiliary assumptions were additionally offered to facilitate the tests. The need for these auxiliary assumptions identified various associated so-called loopholes associated with the experiment. Subsequent experimental efforts eventually closed these loopholes.
In 1974, Clauser and Horne formulated the theory of Local Realism as a significant generalization of EPR’s Local Hidden Variable Theories, and derived its important experimental prediction - the 1974 Clauser-Horne (CH) inequality. Additional contributions were subsequently added to the theory by Abner Shimony and John Bell. The 1974 CH inequality is the second experimentally testable “Bell inequality”. It is loophole-free. Given even milder assumptions than those used by CHSH, the CH inequality reduces to the CHSH inequality, and the Freedman-Clauser (1972) and Clauser (1976) experiments then also refute Local Realism. The CH inequality is experimentally much more demanding to test than is the CHSH inequality when the auxiliary assumptions are not imposed. Observations of its violation have occurred only more recently in 2015 and 2017.
In addition to closing several so-called “loopholes” left by CHSH inequality tests, Local Realism and CH inequality tests provide an important test of the minimal elements of Einstein’s whole theoretical platform for providing a mathematical description of nature in a “laboratory” space-time framework. Local Realism minimally assumes that nature consists of matter, a.k.a. “elements of reality”, objectively real objects, i.e. “stuff” or “matter” that is distributed throughout space and evolves in time. Stuff may evolve either deterministically or stochastically, and may or may not have a finite mass-energy density. Local Realism simply assumes that experimental results are definite, whether or not they are observed. (That is, the cat is either dead or alive. We just may not know which.) It further assumes that the presence and properties of the stuff determine the probabilities of the results of experiments performed locally. Finally, Local Realism also prohibits super-luminal signals from propagating and thereby influencing these probabilities. Surprisingly, these simple assumptions are sufficient for a derivation of the CH inequality, and, in turn, are now refuted by experiment.
John Clauser
John Francis Clauser is an American experimental and theoretical physicist. He is best known for his contributions to the foundations of quantum mechanics, in particular for the Clauser – Horne – Shimony - Holt (CHSH - 1969) inequality, for the first experimental proof that non-local quantum entanglement is real (Freedman – Clauser - 1972), and for the formulation by Clauser and Horne of the theory of Local Realism and its associated inequality (CH - 1976). Together, the 1969 CHSH and 1976 CH experimental predictions, the 1972 Freedman - Clauser experiment, and subsequent experiments by Clauser and others confirm that quantum mechanics is correct, and pave the way for quantum computers, quantum networks and quantum encrypted communication. His work was performed while he was a postdoc at the UC Berkeley Physics Dept. and LBNL. For this work, Clauser was awarded the 2022 Nobel Prize in Physics, jointly with Alain Aspect and Anton Zeilinger "for experiments with entangled photons, establishing the violation of Bell inequalities and pioneering quantum information science".