Charting the Post-Quantum Landscape

You could say that Caslav Brukner and Gregor Weihs are pioneers. Both are experts on quantum physics, yet they are not content to spend their days confined to the familiar territory set out by that theory of the subatomic realm. Two years ago, theoretical physicist Brukner sat down with Weihs, an experimentalist, to sketch out the landscape of hypothetical models that extend into the unchartered waters beyond quantum theory. That map would guide Weihs and colleagues through the long, dark hours of counting photons in quantum tests in the lab. In turn, the results of their experiments will be used to alter the map itself, warning future quantum explorers where not to venture.

At first sight, it may seem like a strange way to choose to spend your time. After all, quantum theory has been remarkably successful. Many of today’s gadgets, from sodium street lights, to the semiconductors in electronic devices, to the more complex MRI scanners in hospitals, stand as testaments to its power. So why bother to look over its horizon? For Brukner, at the University of Vienna, Austria, it is all about the bigger picture. By looking at more general hypothetical alternatives to the theory, he hopes to find out just what it is that makes quantum theory work so well. That’s an important question for those hoping to unite it with Einstein’s theory of gravity, general relativity, to come up with a description of the behavior of the universe at its birth and inside black holes, which is currently shrouded in mystery.

"The search for a quantum theory of gravity gives us good reason to believe that there is more out there than we know, and that there might actually be modifications of quantum theory in nature," notes Markus Mueller who works on quantum information theory at the Perimeter Institute, in Waterloo, Ontario.

One of the most important principles of quantum mechanics is that it tells us that at its core, reality is unpredictable. We cannot calculate in advance the precise outcome of an experiment, only the probability of getting a particular result. This indeterminism famously rankled Einstein, but today physicists are not only more comfortable with it, they are happy to search for alternatives that also include this strange feature. "People started to think, is it possible to have probabilistic theories that are different from quantum mechanics?" says Brukner. It turns out that the answer is yes. There is a whole landscape of probabilistic theories that share many of quantum theory’s weird characteristics that were previously thought to be unique to quantum mechanics. (See also, "Why Did Nature Choose Quantum Theory?")

So how do we know which one is right? Have we overlooked, for instance, the possibility that nature might be even spookier than we imagine? Lab tests are essential here to eliminate—or to help verify—these variants, and to probe whether there are modifications of quantum mechanics in nature.

Three-Slit Experiment

One such lab test in progress at Weihs’ lab in Innsbruck, Austria, is the three-slit experiment. This is a souped-up version of the classic double-slit experiment that traditionally helped to confirm wave-particle duality—the fact that quantum objects can behave either as a wave or as a particle, as the mood takes them. The standard experiment goes like this: Fire particles at a wall with two closely separated slits in it. If you decide to fire them one at a time and collect them after they have passed through the slits, on a screen beyond the wall, then over time they will build up to produce an interference pattern that looks like stripes on the screen. This evokes the ripple pattern you would see if you if you tracked the passage of a water wave through a similar two-slit set-up, creating two waves that interfere with each other beyond the wall. To reproduce such a pattern in the particle case, each particle must interfere with itself as it passes through the slits. Quantum physicists have a way of calculating exactly what this pattern of stripes should look like using a mathematical equation known as Born’s rule that describes how pairs of paths should interfere with each other. By extending the experiment to three-slits you can look for even more stripes of particles building up on the screen, called superinterference—or higher-order interference—that would mean Born’s rule had been violated In this event, our standard version of quantum theory may be too simple.

"We don’t know what it would look like, but we do know it would be really serious," says Weihs. It might mean that quantum mechanics needs modifying, raising the possibility of extending it to include quantum gravity. Mueller, who is not part of the Brukner or Weihs’ collaboration, notes: "This is definitely a worthwhile and very exciting experiment."

The experiment is already up and running, though Weihs and colleagues are actually using a three-path interferometer, rather than three slits, to carry out their test. This employs a layout of mirrors and beam splitters to divide a laser beam into three paths, so no laser light is discarded. The team can block or unblock the separate paths to create two or three-path interference. The intensity of the higher order interference is calculated by subtracting the sum of the intensities from the pairs of combinations of two-path interference from the full three-path intensity pattern.

If that doesn’t sound difficult enough, the experiments have to be done in total darkness to avoid stray light from the surroundings getting into the detector and creating a false measurement. "I remember when I did one of my first lab courses during my undergraduate degree and I thought, "wow, I could never go into optics and work in a dark lab the whole day, it would drive me crazy", but that’s exactly where I went, so never say never," laughs Weihs. The rewards make up for the hardship: "Compared to, say, high energy physics, where you work with a thousand people, we have a chance here in quantum optics to be a single person or a small team and actually answer interesting questions," says Weihs. "I think it’s the perfect place to be for someone who’s curious, who doesn’t want to crank out another digit of pi, say, but wants to ask questions that nobody’s asked before."

Arguably, finding that standard quantum theory needs adjusting would be even more significant a result than even the Large Hadron Collider could offer up. So has the three-slit experiment produced a discovery to rival the Higgs? Well, not quite. "It’s somehow sad because we’ve been looking for ways to unite the description of gravity that we have with quantum mechanics, and wherever it seems that there might be a new door then we do tests and we find that there’s not a way," says Weihs.

Rafael Sorkin, a theorist at Syracuse University in New York, who has studied the possibility of post-quantum, higher-order interference, urges persistence: "If multiorder interference exists at all, one wouldn’t really have expected to see it without going to the highest precision attainable," he says. "History is filled with cases where new insights showed up just at the limit of attainable precision."

Under Scrutiny

Weihs agrees that they still need to iron out some precision issues. "So far we’ve been dealing with a lot of technical issues that can make it look like quantum mechanics is not right, but in fact it still is," he says. Such issues include accounting for the characteristics of the detector, which can only pick up one photon at a time and needs to settle before it can detect the next one, and the stability of the experiment, which is sensitive to temperature fluctuations and vibrations. Nonetheless, the absence of higher-order interference is also reassuring—if their experiment indicated a breakdown in quantum mechanics his experiment would be under intense scrutiny.

As well as improving the accuracy with which they can rule out higher-order interference, the team want to extend their experiment to include four and five-path interference. If there is a higher-order effect, this would allow them to say something about how it behaves, rather than just whether it’s there or not. Some people have been thinking about doing the experiment with neutrons, although Weihs doesn’t think the experiment could attain nearly as good precision because neutron sources are so much weaker than lasers in terms of numbers of particles they can produce. With this sort of experiment, the more particles you count, the greater the precision, and every decimal place takes one hundred times as long to achieve. "The time it takes to do a PhD is probably your cut-off point!" laughs Weihs.

Many who know Brukner are impressed by his talent. He is clearly a man with a lot of ideas, highly regarded in the quantum theory community. "He is asking the right questions and proposing fascinating experiments to try to answer them," says Mueller, "and he is doing this in close connection to possible and actual experiments."

And Brukner really has a fantastically broad view of the theory landscape, adds Weihs. Though, as Brukner admits, he would be very surprised if something beyond quantum theory does show up, "but it’s worth testing quantum mechanics in every way possible, and this is a new way of testing it," he adds.