Published online 22 July 2010 | Nature | doi:10.1038/news.2010.371

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A fresh take on a classic experiment makes no progress in unifying quantum mechanics and relativity.

A 3-slit experiment has confirmed a basic rule of quantum mechanics but failed to help physicists to reconcile the theory with relativity. Science/ AAAS

If you ever want to get your head around the riddle that is quantum mechanics, look no further than the double-slit experiment. This shows, with perfect simplicity, how just watching a wave or a particle can change its behaviour. The idea is so unpalatable to physicists that they have spent decades trying to find new ways to test it. The latest such attempt, by physicists in Europe and Canada, used a three-slit version — but quantum mechanics won out again.

In the standard double-slit experiment, a wide screen is shielded from an electron gun by a wall containing two separated slits. If the electron gun is fired with one slit closed, a mound of electrons forms on the screen beyond the open slit, trailing off to the left and right — the sort of behaviour expected for particles. If the gun is fired when both slits are open, however, electrons stack along the screen in comb-like divisions. This illustrates the electrons interfering with each other — the hallmark of wave behaviour.

Such a crossover in behaviour — known as wave–particle duality — is perhaps not too hard to swallow. But quantum mechanics gets weirder. Slow down the gun so that just one electron at a time reaches the screen, and the interference pattern remains. Does each electron pass through both slits at once and interfere with itself? The obvious way to answer this question is to watch the slits as the gun fires, but as soon as you do this the interference pattern disappears.

It's as if the electrons know when they're being watched and decide to behave as particles again. According to Nobel laureate Richard Feynman, the phenomenon "has in it the heart of quantum mechanics. In reality, it contains the only mystery".

Mind the gaps

The new three-slit version of the experiment, performed by Gregor Weihs at the University of Innsbruck in Austria and his colleagues, sought to uncover gaps in our understanding of quantum mechanics through which modern physics might make some headway. Perhaps the greatest problem in modern physics is how to reconcile quantum mechanics, which allows for seemingly instantaneous communication, with Einstein's theories of special and general relativity, which imply that nothing should travel faster than light.

Weihs's group thought that a route to reconciliation could lie in Born's rule, a central tenet of quantum mechanics that says interference should exist only between two paths, such as the two paths of the double-slit experiment. If there were any three-way interference in the three-slit version, Born's rule would break down and an area of quantum mechanics in which relativity might take hold would be exposed.

To perform their experiment, Weihs and colleagues aimed a source of single photons (which, like electrons, exhibit wave–particle duality) at a mask containing various open and closed combinations of three slits. The authors fired photons repeatedly through the mask, while building a probability distribution of photons arriving on a detector beyond it. From the probabilities of each combination, they could calculate a crucial interference term, which would highlight any three-path interference.

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As Weihs's group had secretly feared, the three-path interference term came to more or less zero1. Co-author Ray Laflamme of the University of Waterloo in Ontario, Canada, "always hoped for three-path interference", says Weihs. "But then he's more of a theoretician. If there was three-path interference, there would be a Nobel prize waiting."

It is true that the experiment has yielded little for theorists to work with, but it's not all bad news, as Markus Aspelmeyer at the University of Vienna points out. "The fact that one does not observe deviations from quantum theory also has profound implications," he says. "It suggests that the present theory is a good description of our physical world and that we have to work harder to understand its fundamental message."

Weihs is now considering a more rigorous test of Born's rule with an interferometer, a highly accurate device that employs a layout of mirrors and beam splitters in place of physical slits. Still, Weihs and his colleagues probably feel they have worked hard enough already. Their experiment involved the logging of billions of photons, a process that took over two years. "It's becoming a little tedious, I must stress," says Weihs.