Physicists split bits of sound using quantum mechanics

You can’t divide the indivisible, unless you use quantum mechanics. Physicists have now turned to quantum effects to split phonons, the smallest bits of sound, researchers report in the June 9 Science.

It’s a breakthrough that mirrors the sort of quantum weirdness that’s typically demonstrated with light or tiny particles like electrons and atoms (SN: 7/27/22). The achievement may one day lead to sound-based versions of quantum computers or extremely sensitive measuring devices. For now, it shows that mind-bending quantum weirdness applies to sound as well as it does to light.

“There was no one that had really explored that,” says engineering physicist Andrew Cleland of the University of Chicago. Doing so allows researchers “to draw parallels between sound waves and light.”

Phonons have much in common with photons, the tiniest chunks of light. Turning down the volume of a sound is the same as dialing back the number of phonons, much like dimming a light reduces the number of photons. The very quietest sounds of all consist of individual — and indivisible — phonons.

Unlike photons, which can travel through empty space, phonons need a medium such as air or water — or in the case of the new study, the surface of an elastic material. “What’s really kind of, in my mind, amazing about that is that these sound waves [carry] a very, very small amount of energy, because it’s a single quantum,” Cleland says. “But it involves the motion of a quadrillion atoms that are all working together to [transmit] this sound wave.”

Phonons can’t be permanently broken into smaller bits. But, as the new experiment showed, they can be temporarily divided into parts using quantum mechanics.

Cleland and his team managed the feat with an acoustic beam splitter, a device that allows about half of an impinging torrent of phonons to pass through while the rest get reflected back. But when just one phonon at a time meets the beam splitter, that phonon enters a special quantum state where it goes both ways at once. The simultaneously reflected and transmitted phonon interacts with itself, in a process known as interference, to change where it ultimately ends up.

The lab demonstration of the effect relied on sound millions of times higher in pitch than humans can hear, in a device cooled to temperatures very near absolute zero. Instead of speakers and microphones to create and hear the sound, the team used qubits, which store quantum bits of information (SN: 2/9/21). The researchers launched a phonon from one qubit toward another qubit. Along the way, the phonon encountered a beam splitter.

Adjusting the parameters of the setup modified the way that the reflected and transmitted portions of the phonon interacted with each other. That allowed the researchers to quantum mechanically change the odds of the whole phonon turning up back at the qubit that launched the phonon or at the qubit on the other side of the beam splitter.

A second experiment confirmed the quantum mechanical behavior of the phonons by sending phonons from two qubits to a beam splitter between them. On their own, each phonon could end up back at the qubit it came from or at the one on the opposite side of the beam splitter.

If the phonons were timed to arrive at the beam splitter at the exact same time, though, they travel together to their ultimate destination. That is, they still unpredictably go to one qubit or the other, but they always end up at the same qubit when the two phonons hit the beam splitter simultaneously.

If phonons followed the classical, nonquantum rules for sound, then there would be no correlation in where the two phonons go after hitting the beam splitter. The effect could serve as the basis for fundamental building blocks in quantum computers known as gates.

“The next logical step in this experiment is to demonstrate that we can do a quantum gate with phonons,” Cleland says. “That would be one gate in the assembly of gates that you need to do an actual computation.”

Sound-based devices are not likely to outperform quantum computers that use photons (SN: 2/14/18). But phonons could lead to new quantum applications, says Andrew Armour, a physicist at the University of Nottingham in England who was not involved in the study.

“It’s probably not so clear what those [applications] are at the moment,” Armour says. “What you’re doing is extending the [quantum] toolbox…. People will build on it, and it will keep going, and there’s no sign of it stopping any time soon.”