The Quantum Truth Seeker

Joseph Emerson describes quantum theory as an "engineer’s theory"—one that gives us the tools to make certain predictions about the world, without giving us a clear picture of why it must be so and without letting us know what is going on "behind the veil." Almost a century since the theory was developed, physicists still debate whether it forces us to do away with the notion that there is an underlying objective reality. Emerson, a quantum physicist at the Institute for Quantum Computing at the University of Waterloo in Ontario, suspects that this is because the theory is not fundamental. And he has devised a series of experiments to help lift the veil.

Every physics student knows about the spooky double slit experiment: fire photons through a barrier with two slits, one at a time, and they behave in a very odd way. The photons splatter a screen on the far side as if a wave had passed through both slits at once, creating an interference pattern with a series of bright and dark spots. But if you try to catch the photons in the act, watching closely as each one passes through the slits, then this interference pattern goes away: now each photon seems to only pass through one slit at a time, like an ordinary particle, creating two patches of light on the screen. This bizarre behaviour can be seen even with large molecules, such as C-60 buckeyballs.

Researchers since Albert Einstein have long debated exactly what this demonstration of quantum weirdness means: does it imply that nature doesn’t assign things definitive properties that are independent of our observations, or does it mean there is a deeper set of rules describing objective existence that we haven’t yet worked out? And is quantum theory, which accurately describes this wacky two-slit behaviour, fully correct as it is written, or does it need tweaking?

Emerson is one of those trying to untangle the answers, by investigating more complicated experimental setups. He is the theorist behind a planned set of tests in which neutrons will be fired through a three-slit-type apparatus. The goal is to see if the neutrons behave as current quantum theory predicts, or in a more complex way that requires a re-write of the quantum rules.

Quantum Captain

Emerson’s main motivation lies in the philosophical questions of how to best understand the quantum world. "Most physicists long ago abandoned ship on the idea of an objective reality," he says: most follow in the footsteps of the famous Niels Bohr, who argued that the quantum world reveals there is no pure reality that exists independent of our observations. "But it became clear to me early on that many of them did so for what are probably bad reasons, others because they just like the idea of the world being inexplicable and still others just because Niels Bohr told them they had to," says Emerson. He, instead, sides with Einstein, in believing there may yet be a different way of understanding the quantum world that allows for truth to exist independent of our experiments. "I guess I’m like a good Captain who, even though the boat has taken on some water, is unwilling to concede the ship to the ocean depths until I know for sure she can’t be salvaged," he says.

Emerson started out doing a master’s degree in high energy experimental physics, but soon decided the more important questions lay in theoretical physics. "I want to know how the world works on the most fundamental level," says Emerson, "and by that I don’t mean I want to know the taxonomy of particles that they look for with billion dollar particle-smashers. What I want is an understanding of what kinds of objective properties the world might have, or whether quantum theory truly forces us to abandon this notion."

Emerson’s new work follows on the experiment that Gregor Weihs of the University of Innsbruck, in Austria, and colleagues did in 2010, in which they fired photons at a screen with three slits (Urbasi Sinha et al., Science 329, 418-421 (2010)). Quantum theory predicts that the resulting interference pattern should be no more or less than the simple sum of interference patterns from each pair of slits in the triad. And that is what the team saw. But this doesn’t rule out the possibility that something more complicated is going on that Weihs’s team did not pick up: there could be additional, tiny wiggles and peaks in the interference pattern that were too small to see within the sensitivity of the experiment, which could only detect signals about 1 per cent of the size of the normal interference pattern. (See "Charting the Post Quantum Landscape.")

Weihs is keen to see Emerson and others extend his work, though he notes that there’s only a slim chance that Emerson and colleagues see signs of an effect that they missed. "There’s no theory that predicts this extra complication. It’s a shot in the dark," says Weihs. But, he adds, that if Emerson’s team do spot something that violates the predictions of standard quantum theory, the repercussions would be immense. "I’d really like to see our results corroborated or refuted. If someone does find something, they probably have a Nobel Prize waiting," says Weihs. "It’s high risk, but it’s not very expensive."

Since 2010, Weihs’s team has switched from a screen with tiny slits to an interferometer—a device in which photon beams are split and travel along different paths before being recombined, at which point they interfere with each other. It is easier for physicists to control the paths that the photons are travelling along the interferometer and opens up their research to 5-slit-type studies. Their unpublished work shows no odd changes to the interference pattern down to 1000th of 1 per cent of the predicted signal, Weihs says.

Despite the absence of any strange results so far, Weihs has long recommended that others should try re-doing the experiment using something other than photons—like neutrons—to see if particles with mass behave any differently or produce a more complicated interference pattern that is easier to spot. "It is good to repeat the experiment with particles that have mass; that might change the results," agrees James Franson of the University of Maryland, Baltimore County, who isn’t involved with the work. That’s what Emerson is now doing, along with experimentalist colleagues, David Cory and Dmitry Pushin, also of the University of Waterloo, aided by an FQXi grant of more than $160,000.

The team will use a single crystal of pure silicon, cut into a series of thin blades, to make their "three slit" interferometer. A beam of single neutrons will be sent into the crystal to be refracted by the blades. It will probably take another 6 months to cut a crystal with the precision needed, says Emerson—the team has ordered a diamond turning machine that should be able to make an interferometer with unprecedented precision. And it will take another 6 months to calibrate and perform the experiment, which needs to be sheltered from any external vibration.

"There’s a very, very low probability that we’ll see anything beyond the precise quantum predictions. But we’re trying to push the theory where it hasn’t been tested carefully before," says Emerson.

Ultimately, the work might help those who are trying to perform the grand trick of unifying quantum theory with general relativity to create one overarching theory of everything. "If the two theories are really incompatible, maybe general relativity or quantum mechanics need to be tweaked," says Franson. "We need to know what ways of tweaking quantum mechanics can be ruled out. Experiments like this help to determine that."