Levitating sand escapes classical world, enters quantum ground state

We're close to being uncertain about where hundreds of millions of atoms are.

How big can something get and still display quantum behavior? It's a fundamental question in physics, and it gets at the nature of reality itself. All sorts of weird behavior goes on in the quantum world: particles behave like they're in two places at once, there are limits to how certain we can be of where things are, and so on. But once things get bigger than a handful of atoms, we get the nice, orderly behavior of our familiar world, where things exist in definite locations.

The transition between the quantum and the familiar seems to be set by environmental interactions. Once an object gets big enough, it's constantly bumping into atoms and absorbing photons, any of which can push it out of a well-defined quantum state. So the question becomes one of how big we can let things get while still controlling their interactions with the environment.

For the last decade, the gold standard in this area has been a physical resonator linked to hardware that lets us control it with photons of a specific wavelength. To keep stray bits of energy from messing with the resonator, the whole device is kept very close to absolute zero. But now, researchers have found that they can control the interactions of a tiny bead that's levitated on laser light. While it's smaller than the resonators, the setup works at room temperature and doesn't require that the bead be physically linked to any special hardware.

Floating on light

The Austrian-US research team behind the work decided to use a small silica bead, less than 200 nanometers across. While that's well below visible, it contains roughly 10⁸ atoms, so it's definitely not your typical quantum object. The bead can hold units of vibrational energy called phonons, which behave somewhat analogously to photons. If you squeeze the last energy out of the bead by eliminating all the phonons, it will be in a quantum ground state. From there, you could test its quantum behavior. Create a situation where the bead may or may not absorb a single phonon, and it will be in a quantum superposition of vibrating and not vibrating.

All that interesting stuff, however, requires reproducibly getting the bead into a ground state in the first place. And if that were easy, someone would have done it already.

Part of the problem is that beads tend to sit on surfaces, which even when chilled down as close to absolute zero as we can get them will still contain stray phonons that they can transfer to the bead. To avoid that, the researchers turned to optical tweezers, which use lasers to generate a small force by both inducing and interacting with electromagnetic fields in objects. In the case of the bead, the optical tweezers could be used to levitate it, freeing it from contact with any surface. Another major source of phonons, collisions with air molecules, was reduced by pulling a vacuum.

That solves some of the problems, but it doesn't deal with photons (which can impart vibrational energy) or the process of pulling phonons out of the bead to begin with. For this, the authors levitated the bead in an optical cavity, a device that has dimensions similar to those of some wavelengths of light, which limits the wavelengths that can exist inside the cavity. It also creates a static interference pattern in the light from the lasers of the optical tweezers that levitate it. The particle will find its way to a location in this interference pattern where the energy of this light is minimized.

Bad vibrations

Or would, if it wasn't vibrating. Its vibrations cause it to twitch out of the energy minimum, at which point it can have its energy altered by absorbing photons from the laser of the optical tweezers and then emitting new photons that have slightly more or less energy than the ones it absorbs. This is where the fact that the optical cavity favors certain wavelengths comes in. The research team structured the cavity so the photons that have less energy than the optical tweezers' light aren't favored by the cavity. By contrast, the photons that are higher energy than the optical tweezers' light match up well with the cavity's structure.

The net result of this is that, as long as the bead is vibrating, it will be exchanging energy with the environment via photons. But it's rare for it to pick up energy from the environment, and far more frequent for it to lose a bit of energy. Over time, the loss of energy will cause the bead to stop vibrating. At this point, it's in the quantum ground state, with no vibrational energy whatsoever. And the researchers can tell it got there simply by looking at any photons that are scattered out of the cavity.

The researchers found that they could typically get the bead down to a temperature where it had a 70-percent chance of being in the quantum ground state. And they could do this at room temperature.

They suspect that the majority of the uncertainty here is due to the fact that they didn't test on top-end vacuum hardware, and pulling a better vacuum would help eliminate a few stray collisions with gas molecules. If they cooled the hardware below room temperature, it could also improve matters by limiting vibrations in the equipment that's controlling the whole setup. Better optical cavities could help control limit noise as well.

Still, even in its present state, they can imagine doing weird things with the bead. For example, they could shut the optical tweezers off, leaving the particle in free fall. Except during the time it's falling, we'll grow increasingly uncertain about its location. Based on the amount of time it remains in the ground state while trapped, the researchers estimate that the uncertainty would rise from 3 picometers to 10 picometers. If they put the setup in an ultra-high vacuum and kept the apparatus in liquid nitrogen, the authors estimate that the uncertainty would be similar in size to the radius of the particle itself.

And that's just one of the odder things the authors of the new paper thought about doing. If this setup works well enough to have other labs adopt it, they'll undoubtedly come up with even odder things to try.

Science, 2020. DOI: 10.1126/science.aba3993  (About DOIs).