Quantum mechanics has so many counterintuitive features that it seems possible to learn a new one every month. Today's lesson involves particles that are set into the same quantum state and effectively become indistinguishable. Once they are indistinguishable, they start behaving that way, showing up in the same place even when we'd expect to see them distributed at random. In today's issue of Nature, a paper describes getting atoms to behave this way, blurring the lines between a quantum probability function and what we think of as a physical object.

The original issue of indistinguishability was highlighted in an experiment done decades ago using photons. Called the Hong, Ou, and Mandel experiment, it involved sending photons in the same quantum state into a partial mirror along two different paths. The partial mirror, called a beam splitter, has a 50/50 chance of reflecting a photon, shifting it from one path to the second.

Based on the 50/50 chance, you'd expect three different outcomes. Half the time, when the beamsplitter reflects neither or both of the photons, you'd expect one photon in each of the output paths. When the beamsplitter reflects only one photon, you'd see both photons in one or the other path (with a 25 percent chance for each).

Of course, this is quantum mechanics, so that doesn't actually happen. Both always show up in one output channel or another. It's as if reality dictates that, since you can't tell the photons apart, they behave as if they're the same object.

Of course, these are photons, massless particles that readily behave as waves. But quantum mechanics has also shown that things like electrons and heavier particles—even molecules—can behave as waves when given the chance. So a team from Universite Paris Sud decided to try to replicate the Hong, Ou, and Mandel experiment with something a bit more substantial than a photon. They chose 4-helium atoms, which are relatively easy to set in an identical quantum state.

The experiment involved holding the atoms over a sensor using an optical trap. Because of the way the trap was set up, the atoms would start off moving upward at two different velocities until the influence of gravity started pulling them back downward.

While they were moving upward, the atoms were hit with photons that would exchange their momenta. This causes them to cross paths before they begin to fall. At the precise point where the paths cross, the atoms were hit with photons again. But this time, the photons were only half the intensity, leading to a 50/50 chance that their momenta would change. In essence, this acted like a beam splitter for atoms.

As a result, the atoms became indistinguishable—within the experiment, we'd have no idea of when we'd expect them to impact the detector.

Our classically trained expectations would predict a Gaussian (bell) curve, with momenta distributed around the two starting speeds (7 and 12 cm/sec) due to random error and noise. And that's similar to what the results look like. Except if you look at the correlations between when particles land, you see the same sort of bias that Hong, Ou, and Mandel saw: the atoms seem to show up at one or the other of the two speeds, but not both. Since they were indistinguishable, they acted that way and travelled together.

The behavior isn't perfect—there are more atoms traveling along both paths than we'd expect if the experiment were operating perfectly. But the authors ascribe the difference to experimental noise, and the team notes this behavior is still radically different from what classical mechanics would predict. And having gotten this to work with atoms, it's possible the technique could be expanded to work with larger particles, allowing us to probe the boundaries between the quantum and classical worlds.

Nature, 2015. DOI: 10.1038/nature14331 (About DOIs).