Quantum Legacies: Dispatches From an Uncertain World Read online

  Knowing of my interest in John Bell’s work and in cosmology, Andy shared with me what he and Jason had brainstormed together, and soon all three of us set to work—Andy now ensconced at MIT just down the hall from my office, and Jason from his new perch at the bottom of the world.10 Our biggest stroke of luck came just as we were finishing our paper proposing the new protocol for experimental tests of Bell’s inequality. Seemingly by chance—or perhaps thanks to the subtle gears of entanglement—Austrian physicist Anton Zeilinger was scheduled to visit MIT and deliver a lecture for the physics department. Over a remarkable career, Zeilinger had designed and conducted ever more ingenious experiments to test some of the strangest, most beguiling features of quantum theory, including Bell’s inequality.11 I immediately secured time on Zeilinger’s schedule during his visit. At our appointed hour, Andy and I pitched to him our idea of using uncorrelated, astronomical sources of randomness for Bell tests. Midway through our spiel, Anton sported a grin as wide as Andy’s and mine. He and his group in Vienna had recently completed a significant project on aspects of the freedom-of-choice loophole, and he seemed tickled by our novel twist.12 Before long, we had put together a team: our “Cosmic Bell” collaboration was born.

  Figure 4.1. The Cosmic Bell collaboration takes form, nourished by a working lunch near MIT in October 2014. From left to right: Andrew Friedman, Jason Gallicchio, Anton Zeilinger, and David Kaiser. (Source: Photograph from the author’s collection.)

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  We performed our first experiment in April 2016, spread out in three locations across Schrödinger’s native Vienna. A laser in Zeilinger’s laboratory at the Institute for Quantum Optics and Quantum Information supplied our entangled photons. About three-quarters of a mile to the north, Thomas Scheidl and his colleagues set up two telescopes in a different university building. One was aimed at the institute, ready to receive the entangled photons, and one was pointed in the opposite direction, fixed on a star in the night sky. Several blocks south of the institute, at the National Bank of Austria, a second team, led by Johannes Handsteiner, had a comparable setup. Their second telescope, the one that wasn’t looking at the institute, was turned to the south.

  Our group’s goal was to measure pairs of entangled particles while ensuring that the type of measurement we performed on one particle of the pair had nothing to do with how we assessed the other. Handsteiner’s target star was six hundred light-years from Earth, which meant that the light received by his telescope had been traveling for six hundred years. We selected the star carefully, such that the light it emitted at a particular moment all those centuries ago would reach Handsteiner’s telescope first, before it could cover the extra distance to either Zeilinger’s lab or Scheidl’s station at the university. (In this way, Ellie’s waiter would offer her a dessert chosen on the basis of an event that had occurred quadrillions of miles from Earth, which neither Ellie nor Toby nor Toby’s waiter could have foreseen.) Scheidl’s target star, meanwhile, was nearly two thousand light-years away. Both teams’ telescopes were equipped with special filters that could distinguish extremely rapidly between photons that were redder or bluer than a particular reference wavelength. If Handsteiner’s starlight in a given instant happened to be more red, then the instruments at his station would perform one type of measurement on the entangled photon, which was just then zipping through the night sky, en route from Zeilinger’s laboratory. If Handsteiner’s starlight happened instead to be bluer, then the other type of measurement would be performed. The same went for Scheidl’s station. The detector settings on each side changed every few millionths of a second, on the basis of new observations of the stars.13

  Figure 4.2. Johannes Handsteiner sets up equipment on the top floor of the National Bank of Austria to get ready for our first Cosmic Bell test, April 2016. The telescope pointing out the window would gather light from a bright star within our Milky Way galaxy, while equipment on the other side of the hallway would detect and measure entangled photons fired through the night sky from the roof of Anton Zeilinger’s laboratory. (Source: Photograph by Sören Wengerovsky.)

  By placing Handsteiner’s and Scheidl’s stations relatively far apart, we were able to close the locality loophole even as we addressed the freedom-of-choice loophole. (Since we detected only a small fraction of all the entangled particles that were emitted from Zeilinger’s lab, though, we had to assume that the photons we did measure represented a fair sample of the whole collection.) We conducted two experiments that night, aiming the stellar telescopes at one pair of stars for three minutes, then another pair for three more. In each case, we detected about a hundred thousand pairs of entangled photons. The results from each experiment showed beautiful agreement with the predictions from quantum theory, with correlations far exceeding what Bell’s inequality would allow.14

  How might a devotee of Einstein’s ideas respond? Perhaps our assumption of fair sampling was wrong, or perhaps some strange, unknown mechanism really did exploit the freedom-of-choice loophole, in effect alerting one receiving station of what question was about to be posed at the other. We can’t rule out such a bizarre scenario, but we can strongly constrain it. To account for our experimental results with some explanation other than quantum mechanics, any hypothetical mechanism that could have coordinated all those measurement settings and outcomes would need to have been set in motion before the starlight that our teams observed that night had been emitted. By selecting the measurements to be performed at each receiving station on the basis of events that had occurred long ago and far away, our experiment in Vienna improved upon previous efforts to address the freedom-of-choice loophole by sixteen orders of magnitude, a factor of ten million billion. When the starlight that Handsteiner’s group observed that night had first been emitted—six hundred years ago—Joan of Arc was so young, her friends still called her Joanie.

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  Six hundred years is a long time in human terms, but a mere blink in cosmic history. After all, our observable universe has been expanding for nearly fourteen billion years. On the strength of our result with the Vienna test, Anton was able to secure precious telescope time for our group at the world-class Roque de los Muchachos Observatory on La Palma, in the Canary Islands. The observatory is home to some of the largest optical telescopes in the world. Whereas we had used inexpensive, hobby telescopes for our pilot test in Vienna—Andy, Jason, and I had literally pointed to a back-page ad in Sky and Telescope magazine during one of Anton’s visits to MIT, to describe the type of equipment we would need for our first test—on La Palma we were able to use enormous telescopes, each with a mirror that stretched four meters across. With such huge light-gathering surfaces, these telescopes could collect light from much fainter, more distant objects.

  Our chance came in January 2018. The large telescopes on La Palma are in such high demand among professional astronomers—and our group would need to commandeer two of them, to be used in concert during the same observing periods—that our allotted windows came during the relative off-season for most astronomers. We quickly found out why: our first few scheduled observing nights were washed out, foiled by freezing rain and hail. On our first evening, in fact, the professional telescope operators warned us that if we did not leave the mountaintop right away and retreat to the observatory headquarters (at a modestly lower elevation), then the makeshift road back to headquarters would become impassable for our rental cars, which did not have ice-gripping chains on the tires. Nonetheless, on our final evening at the observatory, we caught a lucky break with the weather. Both telescopes functioned flawlessly, enabling our team to perform real-time measurements of light from two different quasars: monstrous, black-hole-powered primordial galaxies that are so far away that the light we observed that evening had been emitted eight and twelve billion years ago, respectively.

  Figure 4.3. Two of the large telescopes at the Roque de los Muchachos Observatory on La Palma. On the left is the Telescopio Nazionale Galileo, which our group used during our
Cosmic Bell test in January 2018. (Source: Photograph by Calvin Leung.)

  Figure 4.4. Members of the Cosmic Bell collaboration discuss observing options in the control room of the William Herschel Telescope at the observatory on La Palma, January 2018. Anton Zeilinger sits with his back to the camera. Others shown are (from left to right) Christopher Benn (leaning), Thomas Scheidl, Armin Hochrainer, and Dominik Rauch. (Source: Photograph by the author.)

  As in our Vienna test, we generated pairs of entangled photons with a laser, housed in a makeshift laboratory on the mountaintop, and beamed the particles half a kilometer in each direction toward the enormous telescopes.15 While the entangled particles were in flight, fast electronics at each receiving site took in light from the extragalactic quasars and, on the basis of the color of the quasar observed, prepared to perform one or another measurement on its member of the entangled pair. Once again we found the “spooky” correlations, just as quantum theory predicts. But this time, any alternative mechanism that might have exploited the freedom-of-choice loophole to set up the correlations that we measured would need to have been set in motion at least eight billion years ago—long before there was a planet Earth, let alone quantum physicists to ponder the ultimate laws of nature.16

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  Experiments like ours harness some of the largest scales in nature to test its tiniest, and most fundamental, phenomena. Beyond that, our explorations could help shore up the security of next-generation devices, such as quantum-encryption schemes, which depend on entanglement to protect against hackers and eavesdroppers.

  For me, the biggest motivation remains exploring the strange mysteries of quantum theory. The world described by quantum mechanics is fundamentally, stubbornly different from the worlds of Newtonian physics or Einsteinian relativity. Indeed, quantum theory seems to force us to grapple with questions of chance, randomness, and contingency—rather like our study of history. Do events strewn across space and time unfold according to some grand, hidden plan? Or are we always chasing accidents, beset by uncertainty? If Ellie’s and Toby’s dessert orders are going to keep lining up so spookily, I want to know why.

  CALCULATING

  5

  From Blackboards to Bombs

  Early in the morning of 6 August 1945, a mushroom cloud billowed skyward, hovering eerily above the smoldering city of Hiroshima. Three days later, a similar cloud lingered above Nagasaki. For the first time—and, to date, the only time—nuclear weapons had been used in combat. Days after the bombs were dropped, Japan surrendered and the Second World War lumbered to a close.

  The war marked an unprecedented mobilization of scientists and engineers and a turning point in the relationship between science, technology, and the state. By the end of the war, the Allied nuclear weapons project, code-named the Manhattan Engineer District or “Manhattan Project,” had enrolled 125,000 people, working at thirty-one secret installations scattered across the United States and Canada. Isotope separation plants in Oak Ridge, Tennessee, stretched the length of a city block; the nuclear reactor facilities at Hanford, Washington, required more than half a billion cubic meters of concrete. Allied efforts on radar, likewise top secret at the time, swelled to comparable scale during the war.1

  The drama with which the war ended—the detonation of nuclear weapons over cities—cemented the reputation of the Second World War as “the physicists’ war.” In 1949, for example, Life magazine profiled physicist J. Robert Oppenheimer, who had served as scientific director of the wartime Los Alamos laboratory, a central node of the Manhattan Project. Referring to massive military projects like the bomb and radar, the reporter invoked “the popular notion” that the Second World War had been “a physicists’ war.”2 By that time the First World War, with its notorious battlefield use of poison gases like chlorine and phosgene, had long since been known as “the chemists’ war.” The bomb and radar presented a logical counterpoint.

  News of the bombings thrust American physicists into the spotlight. As early as May 1946, a commentator in Harper’s observed, “Physical scientists are in vogue these days. No dinner party is a success without at least one physicist.” Scholars who “in the pre-atomic age were considered hopelessly out of touch,” the commentator continued, “now find themselves consulted as oracles on questions ranging from the supply of nylons to international organization.” Another observer noted soon after the war that physicists young and old—including those who had played no role in the secret, wartime projects—found themselves “besieged with requests to speak before women’s clubs” and “exhibited as lions at Washington tea-parties.”3

  Physicists’ mundane travels suddenly became draped with strange new fanfare. Police motorcades escorted twenty young physicists on their way to a private conference on Shelter Island, off the northern tip of Long Island, in June 1947; a local booster sponsored a steak dinner en route for the startled guests of honor. B-25 bombers began to shuttle elite physicists-turned-government-advisers between Cambridge, Massachusetts, and Washington, DC, when civilian modes of transportation proved inconvenient. Physics department chairs across the country received a steady inflow of mail throughout the 1950s from grade-school students who now dreamed of becoming nuclear physicists. Other letters, some full of questions and others loaded with theories all their own, streamed in from architects, industrial engineers, Navy officers at sea, prisoners, and patients in tuberculosis wards. By the early 1960s, Americans ranked “nuclear physicist” the third most prestigious profession in a nationwide poll, behind only Supreme Court justice and physician.4

  Each of these developments seemed to be an inevitable corollary of “the physicists’ war.” Yet the term originally had nothing to do with bombs or radar and had been introduced as early as November 1941—weeks before the surprise attack on Pearl Harbor and years before the bombings of Hiroshima and Nagasaki. James B. Conant had explained in a newsletter of the American Chemical Society that the conflict then raging in Europe was “a physicist’s war rather than a chemist’s.”5 Conant was well placed to know: he was president of Harvard University, chair of the US National Defense Research Committee, and a veteran of earlier chemical weapons projects.6

  When Conant first wrote about “the physicist’s war,” no one could know whether the bomb or radar would play any significant role in the war. The Radiation Laboratory, or “Rad Lab,” at the Massachusetts Institute of Technology—which served as headquarters for the Allied effort to improve radar—was just one year old. A prototype radar device had recently been rejected by a US Army review board, and funding for the project from the National Defense Research Committee had nearly been revoked. Meanwhile, the Manhattan Project didn’t exist yet; Los Alamos still housed a private boys’ school. The Army Corps of Engineers requisitioned the site’s mud-caked ranch houses to begin setting up the new laboratory several months after Conant had written about “the physicist’s war.”

  Beyond the question of timing, there is the matter of secrecy. Conant oversaw both radar development and the nascent nuclear weapons program; information about each was strictly classified. An experienced, high-ranking government adviser like Conant surely did not intend to disclose some of the nation’s most closely guarded secrets in a public newsletter. And there is the nature of the radar and bomb projects themselves. Though each was directed by physicists, they teemed with specialists of many stripes. By the end of the war, physicists were a small minority—only about 20 percent—of the Rad Lab staff. At Los Alamos, the wartime organization chart displayed the various groups (metallurgy, chemistry, ballistics, ordnance, and electrical engineering in addition to physics) arranged in a circle, connected by spindly links. No group appeared on top directing the others. Researchers at both the Rad Lab and Los Alamos forged new kinds of hybrid, interdisciplinary spaces during the war. Neither facility could be categorized simply as a physics laboratory.7 So what was Conant talking about?

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  To most scientists and policymakers in the early 1940s, “the
physicists’ war” referred to a massive, urgent educational mission: to teach elementary physics to as many enlisted men as possible. In January 1942, the director of the American Institute of Physics (AIP), Henry Barton, citing Conant, began issuing bulletins entitled “A Physicist’s War.” Barton reasoned that “the conditions under which physicists can render services to their country are changing so rapidly” that academic leaders and heads of laboratories needed some means of keeping abreast of evolving policies and priorities.8 The bimonthly bulletins focused on two main topics: how to secure draft deferments for physics students and personnel and how academic departments could meet the sudden demand for more physics instruction.

  Modern warfare, it seemed, demanded rudimentary knowledge of optics and acoustics, radio and circuits. Before the war, the US Army and Navy had trained technical specialists from within their own ranks, at their own facilities. The sudden entry of the United States into the war required new tactics. University physicists, consulting with Army and Navy officials, reported early in the conflict that enrollments in high school physics classes would need to jump 250 percent to meet the new demand. Their goal: half of all high school boys in the country should spend at least one class period per day focusing on electricity, circuits, and radio. The challenge was significant, since at the time less than half of the nation’s high schools offered any instruction in physics at all. “New courses in biology and chemistry are not needed,” concluded scientists advising officials in the US Office of Education. But the need for physics instruction was urgent: “It is now, each day, each month, this fall, this year,” that educators across the country needed to “encourage schools to offer and capable students to take physics,” demanded an Office of Education official.9