Computing with Quantum Cats

by John Gribbin

  Shimony did not follow up the implications immediately, but a little later he was visited by a graduate student, Michael Horne, looking for a problem to work on for his PhD. Shimony showed Horne Bell's paper, along with those of Bohm and Aharonov, and Wu and Shaknov, and suggested that he might try to design an experiment to test Bell's theorem. As a preliminary, Horne quickly found that he could construct a simple hidden variables model that would account for the Wu-Shaknov results, but that some much more sophisticated experiment would be needed to provide a proper measurement of Bell's inequality, using polarization measurements at a variety of different angles on pairs of photons produced by a single source. The good news, though, was that you didn't need gamma rays to do the job—ordinary photons of visible light would suffice. Indeed, it is easier to measure the polarization of such “ordinary” photons. All of this formed part of Horne's PhD thesis, accepted in 1970; but by then the experimental side was moving on.

  In 1968, Shimony and Horne learned of experiments carried out at the University of California, Berkeley, by Carl Kocher and Gene Commins. They had measured the polarization of photons produced from calcium atoms in a process known as a cascade. They had taken measurements for just two polarizations at right angles to each other, in a simple experiment originally intended as a demonstration for an undergraduate physics course, and were unaware of Bell's work, so their results were inconclusive as far as testing hidden variables theory went; but clearly such an experimental setup could be adapted for testing Bell's theorem. Kocher and Commins weren't interested in pursuing this possibility, but Shimony and Horne now had a clear idea of what kind of experiment they wanted done. All they had to do was find a laboratory with the right kind of apparatus and an experimenter willing to do the job. They found the apparatus (actually using a mercury cascade rather than a calcium cascade) in the laboratory of Frank Pipkin, a professor at Harvard; and they found their experimenter in the form of Richard Holt, a graduate student at Harvard. With Pipkin's approval, Holt (who already knew of Bell's papers) set out on what turned out to be a mammoth project to measure Bell's inequality. But he had scarcely started when the team was hit by a bombshell, in the form of the program for the spring 1969 meeting of the American Physical Society. Included in that bulletin was the abstract of a paper to be presented at the meeting by John Clauser, of Columbia University, who, the abstract revealed, was already working on the design of a similar experiment.

  Clauser had graduated in physics from Caltech in 1964, then moved on to Columbia to work for an MA (1966) and PhD (1969). He was an experimenter, and his PhD work was in astrophysics—specifically, radio astronomy—but he was also interested in quantum physics, and had been astonished by Bell's papers, which he came across in 1967. His reaction was that the implications could not possibly be true, and he tried to find a counter-argument, but failed.19 Realizing that the puzzle could only be definitively resolved by carrying out an experiment, he began scouring the scientific literature for papers describing such experiments. He also wrote to Bell, Bohm and de Broglie asking if they knew of any such experiments; the answers, of course, were all “no.” Bell later said that this was the first communication he had ever received about the paper he had published in 1964. Having failed to find any relevant experiments (although he did find the paper by Kocher and Commins), Clauser began to work out how he could do such an experiment himself, to the frustration of his thesis supervisor, who told him he was wasting his time and should concentrate on his radio astronomy work. From the outset, his objective was to find evidence that Bell's inequality was not violated, and that the world really did operate in accordance with local reality. So he prepared a paper describing the kind of experiment he planned to do, and arranged to present it at the spring 1969 meeting of the American Physical Society.

  Clauser's abstract prompted Shimony to telephone him with the news that his team was thinking along the same lines, and a suggestion that they might join forces. Clauser was not keen on the idea, until he learned that Holt already had the basis of the experimental apparatus needed to do the job. The four scientists (Clauser, Horne, Shimony and Holt) met up at the American Physical Society gathering, and got on well enough together to produce a joint paper (usually referred to in the trade by their initials, CHSH). This paper presented a generalization of Bell's theorem and gave practical details of the kind of experiment needed to test it, using polarized photons. But before the collaboration could proceed further, Clauser was offered a job at Berkeley to work in radio astronomy under the laser pioneer and Nobel Prize winner Charles Townes. This was a plum position in itself, but Clauser was just as interested in the fact that the Kocher and Commins apparatus was still at Berkeley, and might be adapted to carry out a proper test of Bell's theorem. Commins was not keen on the idea, because he regarded entanglement as a known feature of quantum mechanics and saw no need to test it; but Townes, whose opinion carried more weight, was supportive, in spite of the fact that Clauser was supposed to be working on radio astronomy. The upshot was that Commins allowed a new graduate student, Stuart Freedman, to work on the project with Clauser, and Clauser never actually did any radio astronomy work worth mentioning. “Without Townes,” says Clauser, “I could never have done that experiment.” Clauser and Freedman were now competing with Holt, in a race to carry out the first proper test of Bell's theorem. But both teams had seriously underestimated the amount of time and effort this would take.

  Fortunately, we do not have to go over all the trials and tribulations that the experimenters encountered. The bottom line is that in April 1972 Freedman and Clauser were able to publish a paper in the journal Physical Review Letters reporting that Bell's inequality was violated. “We consider,” they said, “these results to be strong evidence against local hidden-variable theories.” Remember that this was the opposite of what Clauser had set out to prove; it is, somehow, more compelling when experimenters find their expectations wrong than when they find what they hoped to find. Freedman was awarded his PhD for his part in this work in May 1972.

  Meanwhile, Holt had found the opposite result! His experiment implied, but not very strongly, that Bell's inequality was not violated. A wealth of other experiments have since confirmed that he was wrong and Clauser and Freedman were right, but nobody has ever found out exactly what went wrong with Holt's experiment; the most likely explanation is that a glass tube in the apparatus had an un detected curve in it which affected the polarization of the photons passing through it. Nevertheless, Holt received his PhD in 1972. Clauser, in a heroic effort to resolve the confusion, carried out his own version of Holt's experiment and found results in disagreement with local realism. These results were published in 1976. The same year, another researcher, Ed Fry at Texas A&M University, had carried out a third test of Bell's theorem, using a laser-based system, and also found a violation of Bell's inequality.

  In 1978, Clauser and Shimony published a review summarizing the situation, and concluding: “It can now be asserted with reasonable confidence that either the thesis of realism or that of locality must be abandoned…The conclusions are philosophically startling: either one must totally abandon the realistic philosophy of most working scientists or dramatically revise our concepts of space-time.”

  As these words make clear, by then it was almost impossible to find a loophole which would allow for the possibility of local reality. Almost, but not quite. One notable loophole remained; but it was about to be closed by an experiment carried out in Paris by a team headed by Alain Aspect.

  CLOSING THE LOOPHOLE

  The essence of the experiments to test Bell's theorem is that photons from a single source fly off in opposite directions, and their polarizations at various angles across the line of sight are measured at detectors as far away as possible from the source. The angle of polarization being measured can be chosen by setting one detector—a polarizing filter—at a particular angle (let's call it filter A), and another filter (filter B) at another carefully chosen angle on the
other wing of the experiment. The number of photons passing through filter A can be compared with the number of photons passing through filter B. The results of the first-generation experiments, including those of John Clauser, showed that the setting of filter A affected the number of photons passing through filter B. Somehow, the photons arriving at B “knew” the setting of A, and adjusted their behavior accordingly. This is startling enough, but it does not yet prove that the communication between A and B is happening faster than light (non-locally), because the whole experimental setup is determined before the photons leave the source. Conceivably, some signal could be traveling between A and B at less than the speed of light, so that they are in some sense coordinated, before the photons reach them. This would still be pretty spooky, but it would not be non-local.

  John Bell expressed this clearly, in a paper first published in 1981.20 After commenting that “those of us who are inspired by Einstein” would be happy to discover that quantum mechanics might be wrong, and that “perhaps Nature is not as queer as quantum mechanics,” he went on:

  But the experimental situation is not very encouraging from this point of view. It is true that practical experiments fall far short of the ideal, because of counter inefficiencies, or analyzer inefficiencies, [or other practical difficulties]. Although there is an escape route there, it is hard for me to believe that quantum mechanics works so nicely for inefficient practical set-ups and yet is going to fail badly when sufficient refinements are made. Of more importance, in my opinion, is the complete absence of the vital time factor in existing experiments. The analyzers are not rotated during the flight of the particles. Even if one is obliged to admit some long range influence, it need not travel faster than light—and so would be much less indigestible. For me, then, it is of capital importance that Aspect is engaged in an experiment in which the time factor is introduced.

  That experiment bore fruit soon after Bell highlighted its significance. But it had been a long time in the making.

  Alain Aspect was born in 1947, which makes him the first significant person in this book to be younger than me (by just a year). He was brought up in the south-west of France, near Bordeaux, and had a childhood interest in physics, astronomy and science fiction. After completing high school, he studied at the École Normale Supérieure de Chachan, near Paris, and went on to the University of Orsay, completing his first postgraduate degree, roughly equivalent to an MPhil in the English-speaking world and sometimes known in France as the “little doctorate,” in 1971. Aspect then spent three years doing national service, working as a teacher in the former French colony of Cameroon. This gave him plenty of time to read and think, and most of his reading and thinking concerned quantum physics. The courses he had taken as a student in France had covered quantum mechanics from a mathematical perspective, concentrating on the equations rather than the fundamental physics, and scarcely discussing the conceptual foundations at all. But it was the physics that fascinated Aspect, and it was while in Cameroon that he read the EPR paper and realized that it contained a fundamental insight into the nature of the world. This highlights Aspect's approach—he always went back to the sources wherever possible, studying Schrödinger's, or Einstein's, or Bohm's original publications, not second-hand interpretations of what they had said. But it was only when he returned to France, late in 1974, that he read Bell's paper on the implications of the EPR idea; it was, he has said, “love at first sight.”21 Eager to make a contribution, and disappointed to find that Clauser had already carried out a test of Bell's theorem, he resolved to tackle the locality loophole as the topic for his “big doctorate.”

  Under the French system at the time, this could be a large, long-term project provided he could find a supervisor and a base from which to work. Christian Imbert and the Institute of Physics at the University of Paris-South, located at Orsay, agreed to take him on, and as a first step he visited Bell in Geneva early in 1975 to discuss the idea. Bell was enthusiastic, but warned Aspect that it would be a long job, and if things went wrong his career could be blighted. In fact, it took four years to obtain funding and build the experiment and two more years to start to get meaningful results, and Aspect did not receive his big doctorate (doctorat d’état) until 1983. But it was worth it.

  Such an epic achievement could not be attained alone, and Aspect led a team that included Philippe Grangier, Gérard Roger and Jean Dalibard. The key improvement over earlier tests of Bell's theorem was to find, and apply, a technique for switching the polarizing filters while the photons were in flight, so that there was no way relevant information could be conveyed between A and B at less than light speed. To do this, they didn't actually rotate the filters while the photons were flying through the apparatus, but switched rapidly between two different polarizers oriented at different angles, using an ingenious optical-acoustic liquid mirror.

  In this apparatus, the photons set out on their way towards the polarizing filters in the usual way, but part of the way along their journey they encounter the liquid mirror. This is simply a tank of water, into which two beams of ultrasonic sound waves can be propagated. If the sound is turned off, the photons go straight through the water and arrive at a polarizing filter set at a certain angle. If the sound is turned on, the two acoustic beams interact to create a standing wave in the water, which deflects the photons towards a second polarizing filter set at a different angle. On the other side of the experiment, the second beam of photons is subjected to similar switching, and both beams are monitored; the polarization of large numbers of photons is automatically compared with the settings of the polarizers on the other side. It is relatively simple to envisage such an experiment, but immensely difficult to put it into practice, matching up the beams and polarizers, and recording all the data automatically—which is why the first results were not obtained until 1981, and more meaningful data not until 1982. But what matters is that the acoustic switching (carried out automatically, of course) occurred every 10 nanoseconds (1 ns is one billionth of a second), and it occurred after the photons had left their source. The time taken for light to get from one side of the experiment to the other (a distance of nearly 13 meters) was 40 ns. There is no way that a message could travel from A to B quickly enough to “tell” the photons on one side of the apparatus what was happening to their partners on the other side of the apparatus, unless that information traveled faster than light. Aspect and his colleagues discovered that even under these conditions Bell's inequality is violated. Local realism is not a good description of how the Universe works.

  Partly because there had been a groundswell of interest in Bell's work since the first pioneering Clauser experiment, and partly because of the way it closed the “speed of light” loophole, Aspect's experiment made a much bigger splash than the first-generation experiments, and 1982 is regarded as the landmark year (almost “year zero” as far as modern quantum theory is concerned) in which everything changed for quantum mechanics. One aspect of this change was to stimulate research along lines that led towards quantum computers. Another result of Aspect's work was that many other researchers developed ever more sophisticated experiments to test Bell's theorem ever more stringently; so far, it has passed every test. But although experiments like the ones carried out by Aspect and his colleagues are couched in terms of “faster than light signaling,” it is best not to think in terms of a message passing from A to B at all. What Bell's theorem tells us, and these experiments imply, is that there is an instantaneous connection between two (or more) quantum entities once they have interacted. The connection is not affected by distance (unlike, say, the force of gravity or the apparent brightness of a star); and it is specific to the entities that have interacted (only the photons in Aspect's experiment are correlated with one another; the rest of the photons in the Universe are not affected). The correlated “particles” are in a real sense different aspects of a single entity, even if they appear from a human perspective to be far apart. That is what entanglement really means. It is
what non-locality is all about.

  It's worth reiterating that this result, non-locality, is a feature of the Universe, irrespective of what kind of theory of physics is used to describe the Universe. Bell, remember, devised his theorem to test quantum mechanics, in the hope of proving that quantum mechanics was not a good description of reality. Clauser, Aspect and others have shown that quantum mechanics is a good description of reality; but, far more important than that, they have shown that this is true because the Universe does not conform to local reality. Quantum physics is a good description of the Universe partly because quantum mechanics also does not conform to local reality. But no theory of physics that is a good description of the Universe can conform to local reality.

  This is clearly Nobel Prize–worthy stuff. Unfortunately, John Bell did not live long enough to receive the prize. On September 30, 1990, just a few days after receiving a clean bill of health at a regular checkup, he died unexpectedly of a cerebral hemorrhage. He was just sixty-two. Unbeknown to Bell, he had, in fact, been nominated for the Nobel Prize in physics that year, and although it usually takes several years of nominations for a name to rise to the top of the list, there is no doubt he would have got one sooner rather than later. The surprise is that Clauser and Aspect have not yet been jointly honored in this way.

  As is so often the case in quantum physics, there are several different ways of understanding how we get from Bell's theorem and entanglement to quantum computation. One way of getting an insight into what is going on—the one I prefer—is to invoke what is called the “Many Worlds” (or “Many Universes”) Interpretation of quantum mechanics—in everyday language, the idea of parallel universes. Although he was not attracted by the idea, Bell admitted to Paul Davies that: