This article previously appeared in the Fall/Winter 2025 issue of AIP’s semiannual History Newsletter. This version has been lightly edited for length and style.

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Quantum entanglement is a phenomenon wherein two or more particles are united within a single quantum system. Although notoriously hard to control, such particles’ interlocked behaviors are poised to have revolutionary applications in areas such as computing, cryptography, and sensing. Entanglement is, therefore, of absolute importance in today’s flourishing landscape of quantum information science and the nascent quantum technology industry.

The idea of entanglement is commonly traced to a 1935 Physical Review paper referred to as “EPR” after its authors: Albert Einstein, Boris Podolsky, and Nathan Rosen. But for many years, the physics community mustered little interest in the subject, relegating it to the status of an intellectual curiosity. It was only toward the end of the last century that work on entanglement gained real momentum—a shift punctuated by the award of the 2022 Nobel Prize for Physics to John Clauser, Alain Aspect, and Anton Zeilinger for experiments confirming entanglement’s quantum mechanical nature.

Scholars have become increasingly interested in how entanglement moved from the outer edges of physics to the center. In his 2011 book How the Hippies Saved Physics, David Kaiser stressed the importance of the 1970s counterculture as an environment amenable to discussions of foundational issues in quantum physics that were anathema elsewhere in the profession. That culture also helped popularize quantum phenomena through works such as The Tao of Physics, but its enthusiasm for subjects like extrasensory perception risked dragging entanglement deeper into the scientific fringe.

This article is about another narrative thread focused on experiments that, while certainly conducted on the margins of physics, also had significant connections with the mainstream. About 20 years ago, historians Olival Freire and Joan Bromberg began reconstructing how efforts to test entanglement not only sharpened understanding of quantum behaviors but also pointed the way toward building devices that exploit those behaviors. Today, historical research on this subject is burgeoning as new generations of scholars take up the reins.

Thinking about thought experiments

There were initially no substantive connections between entanglement and the laboratory, even though the heart of the EPR paper was a so-called “thought experiment.” The EPR experiment was one in a series imagined during a years-long dispute between Einstein and Niels Bohr that, far from being serious proposals for actual tests, were more like weapons in a duel of ideas. Einstein’s and Bohr’s goal was to resolve conceptually whether the indeterminate features of quantum mechanics reflect limitations in its theoretical framework or describe the fundamental nature of reality, at least insofar as it is accessible to human inquiry.

While Bohr and his allies firmly insisted on the latter interpretation, Einstein was loath to concede that objects could, for example, have a position that was truly indeterminate until measured, or that an event could happen without any immediate cause. The EPR paper sought to seal the case for Einstein’s view by positing a situation in which two particles with intertwined but indeterminate properties are separated. Then, the property of one is fixed by measurement. Since that particle could no longer affect the other, the fact that the other particle’s property would presumably always correlate with that of the first suggested their properties had been jointly determined from the beginning.

The point of the EPR thought experiment was to reveal an ostensible incompleteness lurking within quantum mechanics. The only seeming alternative was if one supposed that both particles’ properties had truly been undetermined until the measurement of one instantaneously discovered what value the property of the other would have when it was measured—a phenomenon Einstein later dismissively called “spooky action at a distance.”

That the thought experiment posed questions that could be settled by real experiments was a possibility that went unexplored for years. The first person to press the idea of an actual experimental test was David Bohm.

Like Einstein, Bohm was skeptical that indeterminacy in quantum mechanics reflected reality, and he sought to develop a theory that allowed for “hidden variables” governing quantum behaviors. Having left the United States after falling victim to anticommunist persecution, Bohm spent several years in Brazil and arrived at the newly established physics department at the Technion in Israel in 1955. There, he addressed the prospect of testing the EPR “paradox” in a paper he published in 1957 with graduate student Yakir Aharonov.

Bohm & Aharonov and Wu & Shaknov

Philosopher Guy Hetzroni has been interviewing Aharonov, who went on to a highly successful career and is now in his 90s. According to Aharonov, Bohm asked him to calculate the correlation of polarization between photons emitted by the mutual annihilation of positrons and electrons. This was a variation on a version of the EPR thought experiment that Bohm published in 1951 featuring measurements of particles’ spin. Polarization, like spin, is a binary up-or-down property that was simpler to handle theoretically and experimentally than nonbinary properties like position or momentum.

Bohm then discovered a Physical Review paper published in 1950 by Columbia University physicist Chien-Shiung Wu and graduate student Irving Shaknov, which presented experimental data about just such photon correlations. Sure enough, these aligned with Aharonov’s calculation. Per Aharonov, this led to the 1957 paper, which dealt broadly with test designs that could determine whether the EPR paradox could be escaped by supposing that correlations between spatially separated particles might break down spontaneously—an outcome forbidden in quantum mechanics. The Wu-Shaknov experiment showed the correlations, and quantum mechanics, remained intact.

The Wu-Shaknov paper did not itself mention EPR, framing its experiment as a test of a prediction John Wheeler had made about the polarization properties of photons in electron–positron annihilation. However, in just the past few years, the paper has attracted an avalanche of renewed attention in pieces written by Indianara Silva; Michelle Frank; Chon-Fai Kam, Cheng-Ning Zhang, and Da Hsuan Feng; and Yu Shi, who stress its importance as the first experiment to clearly demonstrate entanglement.

These authors all regard the experiment as a neglected contribution by Wu, who is otherwise best known for her 1956 experiment demonstrating the nonconservation of the parity property in radioactive decays. As we have little historical documentation on the Wu-Shaknov experiment beyond the paper, many of these authors are also hunting for evidence that Wu and Shaknov had the EPR thought experiment in mind but declined to mention it, perhaps because the subject was seen as disreputable.

In any event, while it has long been widely felt Wu was unjustly denied the Nobel Prize for the parity nonconservation discovery, her career was extremely successful by any other measure. She was one of only a handful of women and Chinese Americans of her generation to reach the top echelons of the US physics profession, and in 1975 alone she received the National Medal of Science and became the first woman to be president of the American Physical Society.

Irving Shaknov’s story was unfortunately tragic. After completing his doctorate, he joined the Operations Evaluation Group, an MIT-administered contractor organization that conducted studies for the US Navy. While flying on a mission in Korea in 1952, he was killed when his plane was shot down by enemy fire. He was posthumously awarded the Medal of Freedom.

Sparks of interest in the 1960s

If the Wu-Shaknov experiment has become less familiar with time, it remained a standard citation as interest in testing the EPR paradox grew during the 1960s and beyond. Asher Peres and Paul Singer, who both earned their doctorates under Nathan Rosen at the Technion, published a paper on EPR in 1960. They argued that analysis of photon behavior, and thus the Wu-Shaknov experiment, could not resolve whether correlations between properties were preserved, and they suggested alternative approaches. Although the paper made few waves, Peres maintained his interest in quantum foundations and was one of the theorists who developed the concept of quantum teleportation in 1993.

“Action at a distance” (sans the “spooky” descriptor) was the first topic on the agenda of a small, landmark conference on quantum foundations that Boris Podolsky organized in 1962 at Xavier University in Cincinnati. Podolsky and Aharonov both cited the relevance of the Wu-Shaknov experiment in addressing the phenomenon.

Around 1964, Eyvind Wichmann, a theorist at the University of California, Berkeley, suggested to Carl Kocher, a graduate student working under Eugene Commins, that he try constructing a device to measure correlations in the polarization of photons. In a recent retrospective, Kocher recalled that Wichmann thought of it as a “tabletop” apparatus for instructing undergraduate students, but it ultimately became Kocher’s thesis, which he completed in 1967. While the thesis confined a discussion of the EPR paradox and the Wu-Shaknov experiment to an appendix, an article that Kocher and Commins published the same year made the connections clear in the second paragraph.

After Kocher completed his work, two people independently seized on his experiment: Abner Shimony, a philosopher-physicist working at MIT, and John Clauser, a graduate student at Columbia University. While they did not regard Kocher’s result as especially significant in itself, they saw it as a jumping-off point for testing an obscure new theorem that promised to firmly prove or disprove the reality of quantum indeterminacy.

The second and final part of this article will arrive next week.

William Thomas
American Institute of Physics
wthomas@aip.org

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