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FQXI ARTICLE

February 6, 2023

Charting the Post-Quantum Landscape

Laser experiments explore territory beyond the quantum horizon to investigate the theory’s limits.

FQXi Awardees: Gregor Weihs, Caslav Brukner

September 3, 2012

GREGOR WEIHS

University of Vienna

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.

The search for quantum gravity

gives us good reason to believe

that there might be modifications

of quantum theory.

gives us good reason to believe

that there might be modifications

of quantum theory.

- Markus Mueller

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.

QUANTUM THEORY UNDER THE LENS

Experiments at Innsbruck University explore the realm

beyond quantum mechanics.

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.

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STEVE AGNEW wrote on October 18, 2014

For each quantum object in the universe, there exists both an inner as well as an outer solution to the Schrodinger equation. Light represents the outer solution and each photon of light complements an inner binding solution for microscopic matter.

When we confine light, we create a hohlraum whose modes define the light that leaks out of any aperture. For a coherent hohlraum, a single photon as a wave leaks simultaneously out of any number of apertures onto separate world lines and yet we...

For each quantum object in the universe, there exists both an inner as well as an outer solution to the Schrodinger equation. Light represents the outer solution and each photon of light complements an inner binding solution for microscopic matter.

When we confine light, we create a hohlraum whose modes define the light that leaks out of any aperture. For a coherent hohlraum, a single photon as a wave leaks simultaneously out of any number of apertures onto separate world lines and yet we...

MARCEL-MARIE LEBEL wrote on October 17, 2014

What if...

Quantization appears when the freedom of the quantum object is squeezed, constrained

Light has speed and direction. The light beam direction is squeezed by the slits. 'direction' becomes a quantum number for the object and will now assume discrete values.

Direction is quantized and you get discrete directions on the screen ... a pattern.

Whenever we observe-interact with a quantum object we somehow contrains its freedom and cause the temporary...

What if...

Quantization appears when the freedom of the quantum object is squeezed, constrained

Light has speed and direction. The light beam direction is squeezed by the slits. 'direction' becomes a quantum number for the object and will now assume discrete values.

Direction is quantized and you get discrete directions on the screen ... a pattern.

Whenever we observe-interact with a quantum object we somehow contrains its freedom and cause the temporary...

RICHARD LEWIS wrote on January 2, 2014

It would be really interesting to see the results of this three slit experiment, and the interference pattern that is generated.

The experiment may help to verify the Spacetime Wave theory which is covered in:

The unification of physics

In the terminology of the Spacetime Wave theory the wave quantum (photon) passes through all paths of the interferometer as a spacetime wave and the interference pattern would reflect the wave magnitude at the detection screen. With more...

It would be really interesting to see the results of this three slit experiment, and the interference pattern that is generated.

The experiment may help to verify the Spacetime Wave theory which is covered in:

The unification of physics

In the terminology of the Spacetime Wave theory the wave quantum (photon) passes through all paths of the interferometer as a spacetime wave and the interference pattern would reflect the wave magnitude at the detection screen. With more...

read all article comments