A Whole New Quantum Ball Game

Polystyrene isn’t just useful packaging. You can also use it to test a pillar of modern physics—should you want to. That’s just what Hendrik Ulbricht is attempting to do, by making ever bigger polystyrene balls that could reveal where the quantum realm ends and where the classical world begins.

When quantum mechanics arrived on the scene in the early part of the twentieth century it wrought havoc with our common sense view of the world. Think that light is a wave and electrons are tiny billiard-ball-shaped particles? Think again, because experiments showed that light can sometimes behave like a stream of bullets, known as photons, while small particles, such as electrons, can display wave-like properties. The question is, can we find the line beyond which objects are too big to show this dual-quantum behavior?

With an FQXi grant of around $140,000 in hand, Ulbricht, at the University of Southampton, UK, is searching for this quantum limit—if it exists—by recreating one of those classic experiments using microscopic balls of polystyrene of increasing size. His polystyrene test has all the elements of Thomas Young’s nineteenth-century two slit experiment, in which light from a single source is shone through a pair of slits and onto a screen, where an interference pattern of light and dark bands appears. This happens because the light wavefronts have spread out from each of the two slits and interfered with one another, canceling out some parts and reinforcing others—similar to the way concentric ripples from two raindrops on a puddle interfere.

Pushing Boundaries

Over the years, interference patterns have been created by firing single electrons, atoms, and even large molecules at the slits. Those experiments confirm that these particles have a wave-like nature, somehow interfering with themselves before landing on the collection plate. Ulbricht hopes to push the quantum-classical boundary a big step further by demonstrating interference using polystyrene balls that are a thousand times heavier than the largest molecules tested so far.

Ulbricht traces his interest in physics to school, where a teacher lent him Stephen Hawking’s A Brief History of Time. At university, he resolved to try to solve a problem that Hawking had outlined: the unification of quantum mechanics and gravity. "But I soon realised that this is all very heavy stuff that I couldn’t solve quickly," says Ulbricht. "Also I had the feeling that experiments are the thing, and for this question there are no good experiments."

Instead Ulbricht opted for an experimental PhD, which eventually led him to a molecular interference lab at the University of Vienna, Austria, with Markus Arndt and FQXi-member Anton Zeilinger.

"Ulbricht has a good theoretical background and a good feeling for implementing things to make a successful experiment," says Arndt, who has been carrying out interference experiments with particles over the last decade.

Ulbricht’s main challenge in carrying out his tests is to create a coherent beam of polystyrene balls. The balls come in a solution, which Ulbricht squirts into the focus of an optical trap, where a laser pins down the balls and holds them in an electric potential between mirrors. There the balls are cooled to stop them jiggling, and then released, so that they drop under gravity through an optical grating. The grating—the equivalent of Young’s double slit—is composed of a standing light wave, similar to a standing wave on a guitar string, that is created by shining a laser at a mirror. The electromagnetic potential varies from high to low along the optical grating, like a series of slits and barriers in a standard grating.

The final step is to examine the distribution of the polystyrene balls on a glass plate beneath the grating, using a microscope. "Nobody has done it with polystyrene before but it looks very promising," says Ulbricht.

Quantum Breakdown

Steven Carlip, a quantum gravity theorist at UC Davis, notes that the technology has only recently become good enough to test these big masses. "A lot of us hope that something interesting will happen when you get to high enough masses," says Carlip. "Maybe quantum mechanics will break down."

Talking about the breakdown of one of the most successful theories of modern physics may sound dramatic. But some quantum paradoxes make people uncomfortable. For instance, most interpretations of quantum mechanics cannot explain how we get from a wavefunction—the mathematical description of an object before it is observed, when it can be in two places at the same time—to seeing a quantum object in one place, when we measure it. The standard, not-too-fussy explanation is that the wavefunction collapses when you observe it. But this extra ingredient shouldn’t be necessary if everything is quantum mechanical.

To fill the gap there are lots of non-standard theories that attempt to explain the transition. The most popular is decoherence—the idea that interactions with the environment, for example, through collisions with gas atoms or radiation, in effect, measure the particle, causing it to localise and become classical.

"There is no question that decoherence exists," says Ulbricht. "But the question is whether that is really the reason why we don’t observe macroscopic quantum objects. Is it new physics or is it a technological issue, of being able to stop the particle interacting with the environment?"

Spaceballs

Rival theories of wavefunction collapse predict a quantum breakdown at different masses between a million and ten billion times that of a hydrogen atom and both Ulbricht and Arndt hope to discriminate between them. But there’s a problem: At the upper end of that mass scale, gravity could affect the experiment by accelerating the particles towards the grating, making their wavelength too small for interferometry.

To get round this, the tests may have to be carried out in microgravity. "Ulbricht’s experiment is preparing the way for such space-based experiments," says Ardnt.

If Ulbricht’s polystyrene balls do make it into space years from now, it will be good news for quantum mechanics, because it means that the theory will have held up in all his Earth-based tests.

Carlip’s money is on quantum mechanics’ continuing success. "I would bet that Ulbricht will continue to see interference as long as his experiment is accurate enough," he says.