Quantum Legacies: Dispatches From an Uncertain World
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How stark the contrast must have seemed between the quantum world and the rules of human affairs. Fluid, uncertain identities had no easy place at a time when McCarthy-era investigators pressed, “Are you now or have you ever been . . . ?”14 Yet Pontecorvo himself had ricocheted among several distinct identities in rapid order, from the young “Puppy” of Fermi’s group in Rome to “Academician Bruno Maximovitch Pontecorvo” for the KGB.
One of the earliest dividends of Pontecorvo’s theory concerned physicists’ understanding of the Sun. The core of the Sun is a massive nuclear reactor, and physicists could exploit their theories of nuclear physics to predict to high precision how many neutrinos from the Sun should be detected on Earth. Yet more sensitive follow-up experiments to the original Reines-Cowan test had found only about one-third the expected number of solar neutrinos. Amid the early stirrings of détente between the United States and the Soviet Union in the late 1960s, Pontecorvo was able to share his latest ideas directly with colleagues in the West. He now calculated that neutrinos should oscillate among three distinct flavors. If so, the solar neutrino detectors, which were sensitive to only one of the flavors, should register about the number of neutrinos that the experimentalists kept finding. Years’ more data confirmed the pattern and eventually convinced the skeptics.15
The solar neutrino readings provided only indirect evidence that neutrinos oscillate. The next challenge was to try to catch them in the act. Groups around the world built ever-bigger detectors buried deep underground, ultimately thousands of times larger than Reines and Cowan’s original design. During the late 1990s and early 2000s, teams at the Super Kamiokande facility in Japan, and separately at the Sudbury Neutrino Observatory (SNO) in Ontario, Canada, amassed compelling evidence of neutrino oscillations. The existence of oscillations indicated that neutrinos could not be massless particles, as was predicted by the prevailing theory of the time. The origin and nature of neutrino mass remains a major, ongoing area of exploration in physics. Physicists also continue to test whether only three flavors of neutrinos exist in nature. Any more than three would provide decisive evidence that physicists’ Standard Model of particle physics—which has successfully described every experiment involving elementary particles for more than forty years—is incomplete.
The SNO and Super Kamiokande projects netted the Nobel Prize for physicists Arthur McDonald and Takaaki Kajita, the leaders of the two groups, in October 2015. Three weeks later, the annual Breakthrough Prize in Fundamental Physics disbursed $3 million among the nearly 1,400 physicists who had worked on the teams.16 My friend and colleague at MIT, Joseph Formaggio, a member of the SNO collaboration, used his share of the prize money to buy a nice bottle of wine—something a few price points north of his usual purchases.
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Research on neutrinos seems more exciting than ever, offering a tantalizing route to press beyond the Standard Model. My own interest in them, however, was sparked when Joe suggested that we might put neutrinos to work in a different way: to test one of the central tenets of quantum theory.
Pontecorvo suggested back in the 1950s that neutrinos’ flavor-changing ways were directly analogous to Schrödinger’s half-dead/ half-alive cat. If so, then neutrino oscillations could provide a powerful way to explore the validity of superposition. Joe realized that we could analyze how the mix of neutrino flavors changes as the particles travel, finally settling into a single flavor when measured. Together with two marvelous students, undergraduate Talia Weiss and graduate student Mykola Murskyj, Joe and I set out to investigate.
Pontecorvo’s theory of neutrino oscillations, based squarely on the notion of quantum superpositions, provides an excellent match to the latest experimental data. But we wondered: could the same data be compatible with alternate theories? Perhaps a theory more like the type that Einstein and Schrödinger had held out hope for—in which superpositions are absent and particles always possess definite properties at each instant of time—could account equally well for the data. Joe’s key insight was that if neutrinos are truly governed by quantum superposition—if they zoom through space as “both-and” rather than “either-or”—then the likelihood of measuring particular flavors at a detector should be quantitatively different than if each neutrino possessed a definite identity at any given moment and merely oscillated among distinct identities over time.
Though our analysis became a bit baroque, in essence it boiled down to a simple observation. According to quantum mechanics, the probability of detecting a particular flavor of neutrino spreads out through space and time like a wave. The wave associated with one neutrino flavor evolves with a slightly different frequency than the wave for another flavor. For a neutrino in a superposition state, those not-quite-identical waves can interfere with each other, in much the way that overlapping waves can interfere with each other on the surface of a pond. At some points along the neutrino’s journey, the crests of each probability wave will align, while at others the trough from one wave will cancel out a crest from another.17
All of this leads to a measurable effect. Where crests meet, the probability of detecting a particular flavor rises; where troughs cancel crests, that probability falls. Moreover, the interference pattern—those spots where crests add with crests—should shift with the neutrino’s energy. On the other hand, in rival theories that lack superposition, such as those sought by Einstein and Schrödinger, no such interference pattern should occur. We calculated the different patterns predicted for the number of neutrinos that should be detected in a given flavor as one varied their energy, depending on whether the neutrinos made their journey in a superposition state or not. Then we compared these calculations with data from the Main Injector Neutrino Oscillation Search, or MINOS, an experiment that had been shooting beams of neutrinos from Fermilab toward the Soudan mine in Minnesota since 2005.
Not only did the quantum-mechanical calculation match the MINOS data beautifully, but the Einstein-like version didn’t come close. Even taking into account the uncertainties and statistical flukes that can skew experimental results, we found the odds that the neutrinos were genuinely governed by an Einstein-like theory of matter, with no superpositions, to be less than one in a billion.18
Whereas quantum effects like superposition are usually manifest only over incredibly short distances of tens or hundreds of nanometers, our test demonstrated unmistakable quantum strangeness over a distance of 450 miles. And that may be just the beginning. After all, we are awash in neutrinos from the Sun, and cutting-edge experiments, like the IceCube Neutrino Observatory at the South Pole, can now detect primordial neutrinos that have been traveling through space for billions of years, ever since the big bang. Perhaps neutrinos like these, which have traversed cosmic distances, can also be coaxed to reveal telltale signs of quantum superposition. Then we could test this central feature of quantum theory across the vastness of space itself.
In the meantime, by puzzling through the strange dance of oscillating neutrinos, my colleagues and I have found that for all the fairy-tale strangeness of quantum theory, its predictions hold up across human-sized distances. Perhaps it is fitting that the neutrinos’ journey from Fermilab to the Soudan mine is about the same distance that Pontecorvo himself traveled during his storied lifetime, bounding from Rome to Paris or sneaking from Helsinki to Moscow. Across distances like these, we can say with confidence, the world really is governed by quantum superpositions.
4
Quantum Theory by Starlight
The headquarters of the National Bank of Austria, in central Vienna, are exceptionally secure. During the week, in the basement of the building, employees perform quality-control tests on huge stacks of euros. One night in April 2016, however, part of the bank was given over to a different sort of testing. A group of young physicists, with temporary ID badges and sensitive electronics in tow, were allowed up to the top floor, where they assembled a pair of telescopes. One they aimed skyward, at a distant star in the Milky Way. The other they pointe
d toward the city, searching for a laser beam shot from a rooftop several blocks away. For all the astronomical equipment, though, their real quarry was a good deal smaller. They were there to conduct a new test of quantum theory.
It is difficult to overstate the weirdness of quantum physics. Even Albert Einstein and Erwin Schrödinger, both major architects of the theory, ultimately found it too outlandish to be wholly true. For one thing, unlike Newtonian physics and Einstein’s relativity, which elegantly explained the behavior of everything from the fall of apples to the motion of galaxies, quantum theory offered only probabilities for various outcomes, not rock-solid predictions. Einstein objected that quantum theory treated objects in the real world as mere puffs of possibility—both there and not there or, in the case of Schrödinger’s famous imaginary cat, both alive and dead. Strangest of all was what Schrödinger dubbed “entanglement.” In certain situations, the equations of quantum theory implied that one subatomic particle’s behavior was bound up with another’s, even if the second particle was across the room or on the other side of the planet or in the Andromeda galaxy. They couldn’t be communicating, exactly, since the effect seemed to be instantaneous, and Einstein had already demonstrated that nothing could travel faster than light. In a letter to a friend, Einstein dismissed entanglement as “spooky actions at a distance”—more ghost story than respectable science.1 But how to account for the equations?
Physicists often invoke twins when trying to articulate the more fantastical elements of their theories. Einstein’s relativity, for instance, introduced the so-called twin paradox, which illustrates how a rapid journey through space and time can make one person age more slowly than her twin. (Schrödinger’s interest in twins was rather less academic, centering on his exploits with the Junger sisters, who were half his age.)2 I am a physicist, and my wife and I actually have twins, so I find it particularly helpful to think about them when trying to parse the strange dance of entanglement.
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Let us call our quantum twins Ellie and Toby. Imagine that, at the same instant, Ellie walks into a restaurant in Cambridge, Massachusetts, and Toby walks into a restaurant in Cambridge, England. They ponder the menus, make their selections, and enjoy their meals. Afterward, their waiters come by to offer dessert. Ellie is given the choice between a brownie and a cookie. She has no real preference, being a fan of both, so she chooses one seemingly at random. Toby, who shares his sister’s catholic attitude toward sweets, does the same. Both siblings like their restaurants so much that they return the following week. This time, when their meals are over, the waiters offer ice cream or frozen yogurt. Again the twins are delighted—so many great options!—and again they choose at random.3
In the ensuing months, Ellie and Toby return to the restaurants often, alternating aimlessly between cookies or brownies and ice cream or frozen yogurt. But when they get together for Thanksgiving, looking rather plumper than last year, they compare notes and find a striking pattern in their selections. It turns out that when both the American and British waiters offered baked goods, the twins usually ordered the same thing—a brownie or a cookie for each. When the offers were different, Toby tended to order ice cream when Ellie ordered brownies, and vice versa. For some reason, though, when they were both offered frozen desserts, they tended to make opposite selections—ice cream for one, frozen yogurt for the other. Toby’s chances of ordering ice cream seemed to depend on what Ellie ordered, an ocean away. Spooky, indeed.
Einstein believed that particles have definite properties of their own, independent of what we choose to measure. (He famously pressed a colleague, while strolling at night through Princeton, whether the colleague really believed that the Moon was only in the sky when someone happened to look.)4 Einstein believed with equal fervor that local actions can produce only local effects. In describing our quantum twins, in other words, Einstein would have insisted that Toby had some definite dessert preference on his own each evening, regardless of what type of dessert Ellie’s waiter happened to offer. After all, given that no information can travel faster than light, it seemed self-evident that Ellie’s order should have no bearing on Toby’s actions, once the twins had traveled sufficiently far apart. If relativity really set an absolute speed limit on how quickly A could influence B, then Toby would need to carry all his own information with him as he traveled to his restaurant; there would be no time to receive an update on what his dessert order should be based on whatever had just happened with Ellie.
In 1964, the Irish physicist John Bell identified the statistical threshold between Einstein’s world and the quantum world.5 If Einstein was right, then the outcomes of measurements on pairs of particles should line up only so often; there should be a strict limit to how frequently Toby’s and Ellie’s dessert orders are correlated. But according to quantum theory, Bell went on to show, the correlations should occur significantly more often. For the past four decades, scientists have tested the boundaries of Bell’s theorem. In place of Ellie and Toby, they have used specially prepared pairs of particles, such as photons of light. In place of friendly waiters recording dessert orders, they have used instruments that can measure some physical property, such as polarization—whether a photon’s electric field oscillates along or at right angles to some direction in space. To date, every single published test has been consistent with quantum theory.6
From the start, however, physicists have recognized that their experiments are subject to various loopholes, circumstances that could, in principle, account for the observed results even if quantum theory was wrong and entanglement merely a chimera. One loophole, known as locality, concerns information flow: could a particle on one side of the experiment, or the instrument measuring it, have sent some kind of message to the other side before the second measurement was completed? Another loophole concerns statistics: what if the particles that were measured somehow represented a biased sample, a few spooky dessert orders amid thousands of unseen boring ones? Physicists have found clever ways of closing one or the other of these loopholes over the years, and beginning in 2015, several beautiful experiments have managed to close both at once.7
But there is a third major loophole, one that Bell overlooked in his original analysis. Known as the freedom-of-choice loophole, it concerns whether some event in the past could have nudged or previewed the choice of measurements to be performed and thereby affected the behavior of the entangled particles—in our analogy, the desserts being offered and the selections that Ellie and Toby made. If the twins knew ahead of time the exact order in which, say, Toby would be offered baked goods or frozen goods, then they could have devised a plan so that their dessert orders would betray certain patterns. (As Schrödinger himself commented in 1935, one would hardly be surprised if a student aced an exam if he had received a copy of the questions ahead of time.)8 Where the locality loophole imagines Ellie and Toby, or their waiters, communicating with each other while various desserts are being offered in each restaurant, the freedom-of-choice loophole supposes that some third party could have either guessed in advance what one of the waiters would offer or—stranger still—somehow forced the waiter’s hand. It was this third loophole that my colleagues and I recently set out to address.
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I began thinking about the freedom-of-choice loophole in the autumn of 2012. I had recently finished a book on the early history of Bell’s theorem and the first efforts to test quantum entanglement in the laboratory, in an era before the topic had entered physicists’ mainstream.9 With the book done, I began to work with a new postdoctoral researcher at MIT, Andy Friedman; our plan had been to focus together on various theoretical models of the early universe, trying to account for the behavior of our cosmos around the time of the big bang. Just as he was getting settled at MIT, Andy had dinner in Harvard Square with a friend of his from graduate school, Jason Gallicchio. Jason’s new office was about to be a good bit further away: he had begun working with the South Pole Telescope collaboration and would soon de
ploy to Antarctica to serve as station science leader for the “winterover” shift. (Astronomers like to joke about the winterover position: you need to work only one night, although, at the pole, that night happens to last six months.)
Before leaving Cambridge for Antarctica, Jason had been thinking about the great vastness of space, and all that astronomers and cosmologists had learned in recent decades about the structure of spacetime. In human terms, the speed of light is enormous—nearly seven hundred million miles per hour—and yet our universe is so large, and has been expanding for so long, that some faint pinpricks of light in the night sky, which astronomers have carefully measured and cataloged, hail from objects so far away that the light has been traveling, uninterrupted, for most of the history of the universe.
Jason and Andy mused that night over burgers: Could we somehow exploit these large-scale features of the universe to test quantum theory? What if we made astronomical measurements of the light from very distant objects and used the outcomes of those observations to determine which measurements to perform, here on Earth, on pairs of entangled particles? In that case, rather than flipping a coin in the kitchen to determine which dessert option to offer, Ellie’s and Toby’s waiters would offer choices of dessert based on events that had occurred long ago and far away. Whereas a coin in the kitchen might have been tampered with by some hidden mechanism right up to the moment that the waiters took Ellie’s and Toby’s orders, the astronomical signals would hail from opposite sides of the universe.