John F. Clauser

John F. Clauser (born December 1, 1942, Pasadena, California, U.S.) is an American physicist who was awarded the 2022 Nobel Prize for Physics for his experiments with quantum entanglement. He shared the prize with French physicist Alain Aspect and Austrian physicist Anton Zeilinger. What happens to one particle in an entangled pair determines what happens to the other, even if they are really too far apart to affect each other. The laureates’ development of experimental tools has laid the foundation for a new era of quantum technology.

Clauser graduated from the California Institute of Technology in 1964 with a bachelor’s degree in physics. He continued his study of physics at Columbia University, where he earned a master’s degree in 1966 and a doctorate in 1969. He held postdoctoral positions from 1969 to 1975 at the University of California, Berkeley, and the Lawrence Berkeley National Laboratory, before serving as a research physicist at Lawrence Livermore National Laboratory until 1986. During the late 1980s Clauser worked in the private sector as a senior scientist at Science Applications International Corporation (SAIC) and as a private consultant and inventor. In 1990 he joined the physics faculty of the University of California, Berkeley, as a research scientist. Since 1997 he has been self-employed as a private consultant.

In quantum entanglement two particles are in a single entangled state such that measuring a property of one particle instantly determines that same property in another particle. For example, two particles are in a state where one is spin-up and the other is spin-down. Since the second particle must have the opposite value of the first particle, measuring the first particle results in a definite state for the second particle, notwithstanding the fact that the two particles may be millions of kilometres apart and are not interacting with each other at the time. In 1935, when Albert Einstein, Boris Podolsky, and Nathan Rosen devised this paradox, they thought that this conclusion was so obviously false that the quantum mechanical theory on which it was based must be incomplete. They concluded that the correct theory would contain some hidden variable feature that would restore the determinism of classical physics; that is, the particles must be in some definite spin even before they are measured.

In 1964 the Irish-born physicist John Stewart Bell devised mathematical relationships, Bell inequalities, that would be satisfied by a hidden variable theory in which measurement of one particle would not instantly affect the properties of the other particle. Clauser became interested in experimental testing of the Bell inequalities. He and his collaborators published work in 1969 that proposed a version of a Bell inequality that could be experimentally tested.

Clauser and Stuart Freedman used an apparatus from a previous experiment that used the decay of excited calcium atoms to generate pairs of photons that had opposite polarizations. Each photon was then measured by a polarizer. Their work, published in 1972, was the first experimental test of the Bell inequalities. They measured the rate of detection of both photons when the angle between the two polarizers was 22.5° and when it was 67.5°. In the Bell inequality they tested, those rates were divided by the rate of detection when both polarizers were removed; the absolute value of the difference between those two rates minus ¼ should have been less than or equal to zero to satisfy the Bell inequality. Their measurement of 0.05 ± 0.008 showed a clear violation of the Bell inequality, and their measurements at other angles were in accordance with those predicted by quantum mechanics and not with hidden variable theory.

Clauser is the recipient of the Reality Foundation Prize (1982, shared with Bell) and the Wolf Prize (2010, shared with Zeilinger and Aspect) and was named the Thompson-Reuters Citation Laureate in Physics (2011).