Leaning heavily on Alain Aspect’s and other similar experiments on entangled particle pairs, one often associates the notion of entanglement with an effect that is today clearly demonstrated at the smallest scale, but has little, if any, observable effects at the macroscopic scale, or even at the scale of microscopic materials. Even reproducing the entanglement of a few particles by a ‘classical’ computer is a daunting task, and becomes quickly impossible for more than a handful of elements, because of the exponential complexity of the correlations. In a recent Nature paper, a group led by Peter Zoller of the University of Innsbruck and the local Institute of Quantum Optics, used a quantum simulator of 51 ions arranged in a one-dimensional Heisenberg chain, to study the local entangled structures of the system Hamiltonian. The experiment works in parallel with a (classical) computer that generates correlated states, as input for the excited quantum state of the 51 particles; 20-particle subsets of the system are analyzed, to obtain their degree of correlation. I am getting older, and must admit that I had a very hard time reading this paper, whose complexity is second only to that of their Hamiltonian. However, an interesting take-home message is that they could interpret entanglement strength in terms of a kind of local temperature. While highly entangled regions of the quantum material appear “hot” in this method, weakly entangled regions appear “cold”. Moreover, the exact form of this locally-varying temperature field agrees with the predictions of an old (and absolutely obscure as well) result of quantum field theory, the Bisognano-Wichmann theorem. According to the authors of the study, such innovative technique opens the way to testing the full complexity of the properties of quantum materials on larger and larger scales.

Defeated by such overwhelming complexity, I will seek refuge in old-fashioned physics by telling you another little-known story. In 1949, Chien-Shiung Wu devised an experiment that firstly documented evidence of entanglement, but these findings remained ignored for more than 70 years. Wu is best known to the physics community for her discovery of the non-conservation of parity in weak interactions. She was a kind of “Chinese-madame-Curie”; however, the famed Sklodowska-Curie received two Nobel prizes for her discoveries, while Wu got none.

In November 1949 Chien-Shiung Wu and her graduate student, Irving Shaknov, descended to a laboratory below Columbia University’s Pupin Hall. They needed antimatter for a new experiment, so they made their own, using the local cyclotron. Wu and Shaknov used it to bombard a sheet of copper with deuterons, in the reaction 63Cu(d,p)64Cu, thereby generating an unstable isotope, that is 64Cu. This is a very weird isotope, which shares with very few other colleagues the singular ability to decay in at least three different weak-force pathways: it can do indifferently a beta–, beta+ and electron capture decay, with different branching ratios. For Wu, however, it was just a reliable source of positrons. When a positron and an electron collide they annihilate each other, releasing two photons that travel in almost exactly opposite directions. A few years earlier, John Wheeler had predicted that when matter and antimatter annihilate, the resulting photons would be orthogonally polarized. Wu and Shaknov were looking for conclusive proof of Wheeler’s theory. Their extremely precise data indicated that pairs of photons remained perfectly polarized at 90 degrees, as if somehow connected, even when the two photomultiplier tubes of the experiment were held at a very large distance from each other. Their experiment proved Wheeler’s “pair theory”, and Wu and Shaknov published their findings on New Year’s Day in 1950, in a one-page letter to the Physical Review. But it also became the first experiment to document evidence of something weirder: that the properties of entangled particles are always perfectly correlated, no matter how far apart they travel. Although not explicitly designed to rule out possible alternative explanations (like the Nobel-winning experiments of Aspect, Clauser and Zeilinger) Wu-Shaknov’s was the first experiment ever to show evidence of entanglement, against which Einstein, Podolsky and Rosen had unchained a decade-long polemic with the so-called “Copenaghen interpretation” of quantum mechanics. But the Nobel committee did not even cite Wu’s work, in their 2022 award motivation.

Chien-Shiung Wu was born the same year as the new revolutionary Republic of China, in a small town in the Yangtze River basin. Her father, Zhong-Yi Wu, was an intellectual and a revolutionary, but also a keen feminist, who had opened the first girls-only school in his region. At a time when most girls’ names suggested a delicate fragrance or beautiful flower, he named his daughter Chien-Shiung, which translates to “strong hero.”  At age 24, having reached the limit of what China could offer in physics training, our strong hero boarded the SS Hoover bound for California, to go study under Emilio Segrè, Ernest Lawrence and J. R. Oppenheimer. At Berkeley, Wu became a star student. Her dissertation research on the fission products of uranium was so sophisticated and sensitive that it was turned over to the military, and remained under embargo until the end of the war. Yet, Wu had trouble finding a job after graduation. At the time, none of the top research universities in the U.S. had a woman on the physics faculty, and moreover discrimination against Asian immigrants had intensified with the war. Eventually, she went to a teaching position at Smith College; the following year she became the first woman hired at Princeton University physics, and not long after she entered the Manhattan Project. Yet, Wu had to navigate repeated investigations by immigration authorities and threats of deportation for years. When she had left China in 1936, she expected to be away for only a short while. She never saw her family again.

In 1935 Einstein, Podolsky and Rosen had suggested that there had to be a better explanation for entanglement, rather than imagining information traveling faster than the speed of light that, according to them, proved that quantum theory was still incomplete. Like Einstein, David Bohm was sure there was a perfectly reasonable explanation for entanglement, which according to him could instead be attributed to hidden variables. In 1957 Bohm and his graduate student Yakir Aharonov described how photon experiments could exploit the EPR paradox to reveal these hidden variables. “There has been done an experiment which, as we shall see, tests essentially for this point, but in a more indirect way,” Bohm wrote. That experiment was the 1949 Wu-Shaknov experiment, as he acknowledged in a footnote to the article. Also Zeilinger, one of the 2022 Nobel laureates, wrote in 1999 that “an earlier experiment by Wu and Shaknov (1950) had demonstrated the existence of spatially separated entangled states.”

Interestingly, however, Wu and Shaknov in their 1950 PRL discussed Wheeler’s pair theory, but remained silent about entanglement. In 2012 F. J. Duarte, one of the fathers of laser physics, called Wheeler’s pair theory “the essence of entanglement.”, and other physicists and science historians had clearly spotted the connection. So why did Wu not mention quantum entanglement in her 1950 letter? It is suggested that she might have been hesitant to discuss evidence of entanglement because throughout the 1950s-60s, such quantum-foundations work was stigmatized as junk science. Back then, according to David Kaiser, a professor of physics and history of science at MIT, the idea of using an experiment to prove or disprove theories about quantum physics or to test for local hidden variables was “not even an inkling” for most physicists. Researchers who explored questions about entanglement often disguised their research because backlash could stymie a promising career. We are left to wonder whether Wu might have done so as well.

When Clauser published his proposed test of Bell’s theorem, in 1969, he explicitly distinguished the Wu-Shaknov experiment from his own. He wanted to prove that hidden variables were real; instead, in 1972, he disproved the existence of hidden variables, and demonstrated entanglement with even greater certainty. He had counted coincidences, as Bell suggested, but there were far more coincidences than hidden variables could ever explain. Clauser’s work prompted Aspect and Zeilinger’s later experiments, which closed up the final doubts and definitely supported entanglement. Together, those experiments led to their 2022 Nobel Prize, in which there was no mention of Wu’s early works.

In 1956 Tsung-Dao Lee, a colleague at Columbia, approached Wu for advice about a seemingly odd question: he and his partner Chen Ning Yang, wondered if parity symmetry was absolute. In response, Wu pointed them to a large body of research, and she described a handful of possible experiments to address the questions they posed. Like her brilliant father, Wu was willing to question mainstream thinking. So, she designed and led an experiment of her own, to address her theorist colleagues’ ideas. It meant canceling a trip to China that would have been her first visit home since 1936. By January 1957, always in close consultation with Yang and Lee, Wu made an astonishing discovery. Beta-decay particles from 60Co super-cold atoms were slightly more “left-handed” than “right-handed”, not perfectly symmetrical as current physics assumed. When Yang, Lee and Wu presented their findings at the American Physical Society meeting that same year, the New Yorker described it as “the largest hall … occupied by so immense a crowd that some of its members did everything but hang from the chandeliers.”  That October 1957, Yang and Lee became the first two Chinese Americans in history to win the Nobel Prize. But Wu was not included. It could hardly be more ironic, that the law of physics that Wu toppled was called the principle of parity.

The following year Columbia eventually promoted Wu to the rank of full professor. In his Nobel lecture that December, C.N. Yang told the committee how crucial Wu’s experiment had been, making a bold statement that the results were crucially due to Wu’s courage and skill. Also T.D. Lee would later plead with the Nobel Committee to recognize Wu’s work. Oppenheimer publicly stated that Wu should have shared in the 1957 prize. Segrè called the overthrow of parity “probably the major development of physics after the war.” In 1991 Douglas Hofstadter, the author of Gödel, Escher, Bach, organized scientists to write letters to the Nobel Committee recommending Wu for the physics prize. And in 2018, 1,600 researchers invoked Wu’s name in an open letter to CERN challenging current-day sexism in physics.

Later, Wu became the first woman prized by the National Academy of Sciences; the first female president of the APS; the first physicist to receive the Wolf Prize; and received many other honors, including a 2021 U.S. Mail post stamp featuring her portrait. Today, her parity-violation experiment is understood as a first step to what would become the Standard Model of particle physics, and even as one of the possible answers about why matter exists in our universe at all. Wu’s early entanglement work, however, remained in obscurity. The 2022 Nobel Prize celebrated a set of connected experiments that took place at great distance from one another. Extraordinary science, like entanglement itself, depends fundamentally on connections.

Parity is not equality

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