The early history of efforts to test the Einstein-Podolsky-Rosen (EPR) paradox is discussed in part 1. Both parts previously appeared together in the Fall/Winter 2025 issue of AIP’s semiannual History Newsletter. This version has been edited for length and style.

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The quest to test Bell’s theorem has become reasonably well-trodden historical territory, having been covered, for instance, by David Wick in his 1995 book The Infamous Boundary and Olival Freire in his 2015 book The Quantum Dissidents. The retelling here draws heavily on interviews from AIP’s collections that historian Joan Bromberg conducted with John Clauser and Abner Shimony in the early 2000s.

Abner Shimony’s early interest in Bell’s theorem

John Bell, a theoretical physicist at CERN, agreed with David Bohm that quantum mechanics likely obscured hidden variables that determined its seemingly random outcomes. However, he kept his interest in quantum foundations largely to himself, and it was only when on leave at Stanford University, Brandeis University, and the University of Wisconsin that he wrote up his thinking about an EPR-style experiment that could distinguish whether hidden variables are at work.

Bell’s theorem appeared in print toward the end of 1964 in a brand-new boutique journal called Physics, and for years it received very little attention, even among the few people working on quantum foundations. Abner Shimony was one of the very few to take quick notice.

Shimony received a PhD in philosophy from Yale University in 1953 and a second PhD in 1962, this time in physics, working under Eugene Wigner at Princeton. With Wigner, Shimony attended the 1962 Xavier University conference on quantum foundations, which introduced him to the small community that was interested in the subject.

Having landed a philosophy job at MIT, Shimony learned of Bell’s paper when a colleague at nearby Brandeis mailed him a draft. He told Bromberg:

“I thought, ‘Here’s another kooky paper that’s come out of the blue.’ I’d never heard of Bell, and it was badly typed, and it was on the old multigraph paper, with the blue ink that smeared. There were some arithmetical errors. I said, ‘What’s going on here?’ But I re-read it, and the more I read it, the more brilliant it seemed. And I realized, ‘This is no kooky paper. This is something very great.’”

Taking his cues from Bohm and Yakir Aharonov, Shimony started searching the physics literature for evidence that tested Bell’s theorem, beginning with the Wu-Shaknov experiment, which he quickly realized did not include the needed measurements.

Shimony also consulted with Aharonov, who was now at Yeshiva University in New York. He recalled Aharonov as saying that his 1957 paper with Bohm had settled the matter:

“Aharonov is a very fast thinker and a very fast talker, and I was in awe of him, and thought, ‘He’s right… Maybe he’s right, but maybe he isn’t right.’ The more I thought of it, the less convinced I was.”

Ultimately, Shimony concluded a new experiment was needed, but the opportunity only arose when he moved to Boston University’s physics department in 1968 and introduced the idea to graduate student Mike Horne. Together, they consulted colleagues at Harvard University, who told them about Carl Kocher’s experiment at Berkeley. Like the Wu-Shaknov experiment, it was an EPR-style setup that was not designed to test Bell’s theorem, but they felt it could be adapted and were soon introduced to Harvard experimenter Frank Pipkin and his student Dick Holt, who was working with a similar apparatus.

Converging efforts

Meanwhile, John Clauser was working on a doctorate at Columbia in astrophysics. However, in his self-guided efforts to understand quantum mechanics better, he soon became enthralled by the idea of hidden variables, which led him to Bell’s paper.

Meditating on a possible apparatus that could test Bell’s theorem, Clauser secured an invitation to present his ideas at MIT, where Dan Kleppner was undertaking related experiments. Shimony, already at Boston University, was not in attendance. However, Kocher, freshly arrived at MIT as a postdoc, was there and told Clauser about his work.

Clauser recalled that Kocher asserted his experiment had already accomplished what he was proposing to do:

“Carl actually didn’t have any reprints available, but he wrote down the reference to it, and as soon as I got back to my office in New York, I went to the library and picked it up and read it. As soon as I read it, I realized, ‘No, this is a crock. He hasn’t done it at all.’”

Realizing that Kocher’s setup could be the basis for a decisive experiment, he drafted a short description of one in an abstract for an upcoming American Physical Society meeting that was printed in the APS Bulletin.

Shimony told Bromberg that he and Horne missed the deadline to submit their own abstract:

“I remember telling Mike, ‘It doesn’t matter. Nobody is working on this sort of thing. This is so far out. We’ll write up a full paper with all our calculations, and that will be much better than one paragraph in a bulletin.’ Then the Bulletin came out, and there was our work, and we felt pretty low. We really felt pretty sick.”

On Wigner’s advice, Shimony reached out to Clauser and enticed him into a collaboration as Holt had a clear path to conducting the experiment at Harvard. Together, Clauser, Horne, Shimony, and Holt wrote a paper reworking the idealized experiment in Bell’s paper so that it could be tested on a practical apparatus.

Then, in late 1969 Clauser moved on to a postdoctoral position at Berkeley, where he persuaded his supervisor, the eminent Charles Townes, as well as Kocher’s PhD advisor, Eugene Commins, to let him adapt the device that Kocher used. In Clauser’s recollection, although Commins was unenthusiastic, the weight of Townes’s support led him to make another of his students, Stuart Freedman, available to work on the experiment.

Clauser remembered the experiment as demanding a major overhaul of Kocher’s device:

“Once we started the design on the new experiment, each time we’d try to think, ‘Okay, well, how much of this stuff can we use?’ And as the design progressed, more and more pieces became inappropriate. So, they got pitched out, and we kind of joked about it. At the end, we looked back and we went, ‘Well, what are we really using out of all of Kocher’s hardware?’ And it was almost nothing.”

By 1972, the work was done and Clauser and Freedman’s experiment had secured results that seemed to clearly rule out predetermination by hidden variables. Although this dashed Clauser’s hopes, the experiment’s meaning was still up for grabs. Clauser told Bromberg that for some who never believed in hidden variables, the experiment’s agreement with quantum mechanics meant it was all a waste of time.

Measuring experimental progress

What, then, did the Freedman-Clauser experiment accomplish?

Initially, it was not certain it had even worked properly. Back at Harvard, Holt’s results soon came in and contradicted predictions from quantum mechanics. While the presumption was something had gone wrong, the result lurked like a specter, prompting Clauser to replicate Holt’s design, yielding results agreeing with his experiment with Freedman. By the time Clauser and Shimony published a landmark review in 1978 surveying what had become a full constellation of experiments, the outlook was clearer.

Then there was the question of what exactly any given experiment was or was not adequate to demonstrate. Notably, back at Columbia, Chien-Shiung Wu had revived her interest in her experiment with Shaknov and tasked graduate student Leonard Kasday with replicating it, now with Bell’s theorem in mind. The result he obtained in 1970 accorded with Clauser and Freedman’s later result, but Clauser and others regarded it as unconvincing due to limitations that prevented it from clearly ruling out hidden variables.

Of course, Clauser and Freedman’s experiment had its own limitations that prevented it from ruling out signaling between its separated photon detectors; this “loophole” was only overcome by the experiments in France that Alain Aspect completed in the early 1980s. But given so few physicists took occult mechanisms like hidden variables seriously, the question arises of why closing off loopholes became such an important measure of experimental significance. From that perspective, the Wu-Shaknov, Kocher, and Kasday experiments might themselves be reasonably regarded as crucial steps in demonstrating entanglement.

Still another way of looking at the Freedman-Clauser experiment is that successfully testing Bell’s theorem was an important step in being able to identify, isolate, and control exotic quantum phenomena in the laboratory. From this view, it was not one experiment in a line, but rather part of a broad landscape of experiments that was beginning to emerge in its time. Historian Climério Silva Neto has recently examined the instrumentation used in the early tests of Bell’s theorem and how they served as a bridge with other research fields like atomic, molecular, and optical physics. Following Bromberg, historian Gautier Depambour has looked specifically at the links between the Bell experiments and the emerging field of quantum optics.

It is an exciting time to be piecing together our historical map of quantum science, and there is much left to be done. Last year, the Nobel Prize in Physics was awarded to John Clarke, Michel Devoret, and John Martinis for work in the mid-1980s on macroscopic quantum tunneling—another quantum behavior that is integral to today’s quantum technologies but has yet to find its historians. Hopefully the prize will further energize scholars to keep expanding their work in new directions.

William Thomas
American Institute of Physics
wthomas@aip.org

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