Phonons, the smallest pieces of sound, have a lot in common with photons. Just as turning off a light bulb reduces the number of photons, we reduce the number of phonons by muting a loudspeaker. The quietest sounds are made up of separate and indivisible phonons. However, unlike photons, sound particles cannot move in space, they need a medium: air, water or, as in this study, an elastic material. Phonons cannot be permanently broken down into smaller pieces, but a new experiment has shown that this can be done for a while using quantum mechanics.

How does it work

A team of physicists from the University of Chicago accomplished this task by using a sound beam splitter, a device that transmits about half of the incoming phonons and reflects the rest back. But when only one phonon enters this divider for a certain period of time, it enters a special quantum state in which both variants of the development of events occur simultaneously – simultaneously passing in both directions. The reflected and past phonon interacts with itself in the interference process.

A laboratory demonstration of this effect was performed inside a device cooled to temperatures very close to absolute zero, millions of times louder than the human ear can hear. Using bits of quantum information instead of speakers and microphones, the scientists threw a phonon from one qubit to the next. On the way, the phonon hit the splitter of the sound beam.

Adjusting the setup parameters changed the way the reflected and transmitted parts of the phonon interact with each other. This allowed the researchers to quantum-mechanically alter the probability that an entire phonon would return to the qubit that started the phonon, or to a qubit on the other side of the beam splitter.

An interesting point is that after the phonon is broken, both particles then recombine to the same qubit. Since they have the opportunity to fall on anyone at random, they are always in the same qubit as another particle and then clump together.

The next logical step in this experiment will be to show that we can make a quantum gate with phonons. This will be a gate from a group of gates necessary for actual calculations.
– says Andrew Cleland, one of the researchers.

Sound devices are unlikely to surpass photon-based quantum computers, but it could lead to new quantum applications that are not yet clear.