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WHAT keeps Nicolas Gisin pressing on with his experiments? He goes to extraordinary lengths to fire photons beneath the waters of Lake Geneva, sending them through fibre-optic telephone cables from his lab at the University of Geneva to another lab some 25 kilometres away. And time after time his results prove what we have known for nearly a century: that quantum theory makes good predictions about the properties of particles like Gisin’s photons. Why does he bother?

Over the past decade, Gisin and his colleagues at the university have been probing the peculiar features of quantum entanglement, the way the properties of two or more particles can become invisibly linked, even if they become separated. Do something to one, and quantum theory says that the other should be affected instantaneously. Gisin’s experiments and those of many others certainly seem to show the effect is real. But if they have proved that, what’s the point of carrying on? Well, says Gisin, something rather surprising is emerging from their results.

If Gisin’s experiments can be trusted – and most physicists believe they can – and quantum theory is right, then basic logic says there is something fundamentally wrong with our view of the universe. It would be good news for quantum theory, but bad news for some of our deepest preconceptions about the nature of reality. “Either space-time is an illusion,” Gisin says, “or free will is an illusion.”

But it may be more profound still. There is a third option, favoured by several physicists who have looked at Gisin’s work: that the laws of nature may somehow conspire to keep quantum theory’s strangest predictions from ever coming true, and that understanding why may point physicists toward a deeper theory from which quantum rules emerge. So you can begin to see why Gisin is still firing photons around Switzerland. “This,” he says, “is one of the most important questions in physics.”

Quantum entanglement has long produced as many questions as answers. Consider electrons, which have a quantum property called spin, which is always either “up” or “down”, along whichever direction you might choose to measure it – along the vertical, for instance, or east-west. When an experimenter creates two entangled electrons, they will have opposing spins: if one is up, the other is down. But the experimenter can also create entangled pairs of electrons with undefined spins. In this case each particle’s spin is a peculiar superposition of up and down at the same time.

And this is where entanglement creates some seriously strange phenomena. Quantum theory predicts that the value of either particle’s spin only becomes definite if you measure it. But if you measure the spin of one electron and find it to be up, say, that will instantaneously trigger the spin of the other electron to become definite as well, and point down.

Strange though it is, this prediction of quantum theory has survived test after experimental test. The most rigorous of these are based on theoretical work published in 1964 by physicist John Bell of CERN, the European particle physics lab near Geneva. Stimulated by an earlier thought experiment by Albert Einstein, Bell imagined an experiment in which you separate entangled pairs of electrons and send them to very distant places. Experimenters at these far apart locations could then measure their particles’ spins at the same time. What would they find?

Bell calculated the outcome based on three assumptions. First, that the two experimenters could freely choose to measure the spins using any axis they like. Second, he assumed that there is something about each electron, before it is measured, that helps determine what is likely to happen in a measurement; that is, the experimental results reflect some real, pre-existing property of the particles and their local environment. And third, that no influence can travel faster than light, so if the measurements take place at virtually the same time, what happens at one end cannot possibly affect what happens at the other.

Bell showed that, if these seemingly plausible assumptions hold true, certain experiments would flout the predictions of quantum theory. In mathematical terms, the three assumptions lead to a “Bell inequality” relating the outcomes of various experimental tests, whereas quantum theory says that the inequality is violated; in other words, if quantum theory holds, at least one of the assumptions must be wrong.

Many of the experiments testing Bell’s scenario use photons rather than electrons, but the logic is the same. In an experiment last year, for example, Gisin and colleagues created pairs of entangled photons, and sent each member of the pair in opposite directions along 25 kilometres of fibre-optic cable.

In this case the entanglement didn’t involve spin, but rather the precise timing of the photons. Just as an electron’s spin can be up or down, the photons produced by the team’s apparatus could be “early” or “late”, pushed forwards or backwards relative to some average time of arrival. Repeating the test for millions of photon pairs, Gisin and colleagues found that they acted just as quantum theory says they should, violating Bell’s inequality (Physical Review Letters, vol 93, p 180502).

“Logic says there is something wrong with our view of the universe”

At the University of Illinois at Urbana-Champaign, physicist Paul Kwiat and colleagues have just finished another experiment using photons and found similar results. “We’ve found some world-record violations,” Kwiat says. His experiments violate Bell’s inequality by more than 1000 standard deviations (where one standard deviation is the typical amount of random scatter). Clearly that’s not likely to be the result of chance.

“It could mean that relativity, Einstein’s description of space and time, is ripe for redrafting”

In every test, over more than two decades, quantum theory has come out intact. So relativity’s assumption that nothing can travel faster than the speed of light is flawed… or is it? Well, not necessarily: there are two other assumptions that have to be tested first.

The first is free will. Bell’s analysis only produces his inequality if the two experimenters have genuine freedom to choose how they set their detectors. In an experiment with spins, that means being able to make measurements along axes that they can choose independently. But maybe that isn’t possible. “The idea is that everything could be somehow determined at the beginning,” says Gisin. Perhaps the creation of the particle pairs and the experimenters’ choices are fixed by a vast web of cause and effect set up long ago, in which case the “choices” would be predetermined and beyond anyone’s control. Some fundamental law might mean that these choices always lead to a violation of Bell’s inequalities.

Unsurprisingly, not many physicists go for this idea. So what of the other assumption behind the inequality?

Bell’s analysis requires that reality is “out there” and has properties even when we don’t measure them. Rejecting this is another way to explain how it is that experiments seem to violate his inequality. It could be that prior to being measured, a quantum particle has no property that makes measurements come out one way rather than another (91av, 24 July 2004, p 30). Physicists do experiments and get results – lines on some photographic film, settings on a dial. Quantum theory describes these outcomes with perfect accuracy, but that is all there is to say.

That’s not as outrageous as it might seem – many physicists subscribe to this view. “Quantum theory isn’t a description of physical reality,” is how the late physicist Asher Peres at Technion, the Israel Institute of Technology, put it. “All it does is provide correct answers to meaningful questions about experiments.”

But many other researchers find this casual attitude toward reality less than satisfactory. “This philosophy certainly abolishes the paradox,” says mathematician Richard Gill of he University of Utrecht in the Netherlands, “but to my mind hardly explains it.” Gill and many others argue that reality simply must have properties that are independent of our decision to measure them, otherwise science is based on nothing concrete. As Einstein once said, it’s hard to believe that the moon isn’t there when we don’t look at it.

So, if we are satisfied that experimenters have free will and that reality has its own properties, we return to our original suspect, Gisin says. It seems entanglement allows distant parts of the universe, light years apart, to be linked up in a decidedly strange way. Some kind of influence appears to be travelling faster than light, and Gisin has even done experiments that put a lower limit on the speed of that weird influence (see “How much faster than light?”).

These faster-than-light links can’t be used for transmitting information, because they are based on random outcomes and so are uncontrollable. But this could still mean that relativity, Einstein’s famous description of space and time, is ripe for redrafting. After all, something has to give, and better relativity than human free will.

“It’s hard to believe that the moon isn’t there when we don’t look at it”

“Entanglement is a new explanation for how things happen in the universe,” Gisin says. “It’s on the same conceptual level as cause and effect: there’s cause, effect and entanglement.”

Loopholes

The results are still not quite clear-cut enough to entice all physicists to take this plunge, though. The dissenters suspect that it’s quantum theory that has something missing. Although a number of experiments have now found violations of Bell’s inequality – and made sure that only faster-than-light signals could have passed between the particles during the window of measurement – these experiments all suffer from an Achilles’ heel thanks to the imperfection of particle detectors.

In their most recent experiment at Lake Geneva, for example, in only about 1 per cent of photon pairs were both photons detected. While these detected pairs followed quantum theory, violating Bell’s mathematical condition, it is possible that they didn’t give a fair picture. Maybe the pairs that weren’t detected would have proved quantum theory wrong.

Four years ago, physicist Dave Wineland and colleagues from the National Institute of Standards and Technology in Colorado managed to close the detection loophole in an experiment using ions held in traps. But the catch was that the ions were so close together that the researchers couldn’t make their measurements quickly enough to ensure that any influence travelling between the ions would have had to move faster than light.

“Eventually the ions might be able to do the job,” says Wineland, “but they’d have to be separated by at least 30 kilometres, currently a tough proposition.” Kwiat hopes to close the loophole in the photon experiments by building better photon detectors, perhaps with efficiencies approaching 90 per cent, but this too appears to be some years off.

And so as physicist Emilio Santos of the University of Cantabria in Spain argues, it looks like the jury will be out until someone does an experiment that closes the loophole completely. “Most people think that the Bell inequality has been violated,” he says, “but this is misleading and in my view harmful for the progress of science.”

Santos might have a point: nature has surprised physicists many times in the past, and perhaps we shouldn’t be so quick to decide how experiments will “obviously” turn out. Physicist Ian Percival of Queen Mary, University of London, for example, speculates that a definitive loophole-closing experiment might be impossible as a result of some as yet unknown law of nature. This law might make it impossible to create some of the situations that quantum theory technically allows – and in particular those that would violate the Bell inequality – without leaving loopholes.

Percival suggests that if such experiments are impossible, then faster-than-light influences may be impossible too, and what Gisin is seeing is a mirage created by the open loopholes. Percival isn’t claiming that this is the case, merely pointing out that logic suggests it as a possibility. Gill agrees. “From a logical point of view,” he says, “all options are open until there has been a loophole-free experiment.”

And that openness could just possibly allow Einstein to have the last word. Quantum theory famously predicts only the probabilities of experimental outcomes, and most physicists believe that this reflects an ineradicable randomness of the world at the smallest scales. It’s an idea that Einstein never liked. Of similar mind Nobel prize-winning physicist Gerard ‘t Hooft, of the University of Utrecht in the Netherlands, is also convinced that quantum theory itself must be an approximation of some deeper theory, one that he believes should ultimately be deterministic, not random. If Percival’s suggestion is right and the detection loophole can never be closed, it could be precisely because some deeper deterministic level of physics makes sure of it, forbidding the existence of the quantum set-up that would truly violate Bell’s inequality.

‘t Hooft is trying to construct just such a deeper theory that would account for all the strangeness of quantum theory, detection loophole and all. To Gisin, Kwiat and many other experimenters, however, the detection loophole seems like a minor detail – little more than a mopping-up operation that future experiments will take care of. “I think it will be closed,” Gisin says, “quite possibly in my lifetime.”

Percival is not so sure. “This issue can only be resolved by experiment. That is why Bell experiments are so important.”