PHYSICS Trapping the Tiniest Sound

Controlling the smallest unit of sound could have applications in quantum information

Researchers have gained control of the elusive “particle” of sound, the phonon. Although phonons—the smallest units of the vibrational energy that makes up sound waves—are not matter, they can be considered particles the way photons are particles of light. Photons commonly store information in prototype quantum computers, which aim to harness quantum effects to achieve unprecedented processing power. Using sound instead may have advantages, although it would require manipulating phonons on very fine scales.

Until recently, scientists lacked this ability; just detecting an individual phonon destroyed it. Early methods involved converting phonons to electricity in quantum circuits called superconducting qubits. These circuits accept energy in specific amounts; if a phonon’s energy matches, the circuit can absorb it—destroying the phonon but giving an energy reading of its presence.

In a new study, scientists at JILA (a collaboration between the National Institute of Standards and Technology and the University of Colorado Boulder) tuned the energy units of their superconducting qubit so phonons would not be destroyed. Instead the phonons sped up the current in the circuit, thanks to a special material that created an electric field in response to vibrations. Experimenters could then detect how much change in current each phonon caused.

“There’s been a lot of recent and impressive successes using superconducting qubits to control the quantum states of light. And we were curious—what can you do with sound that you can’t with light?” says Lucas Sletten of U.C. Boulder, lead author of the study published in June in Physical Review X. One difference is speed: sound travels much slower than light. Sletten and his colleagues took advantage of this to coordinate circuit-phonon interactions that sped up the current. They trapped phonons of particular wavelengths (called modes) between two acoustic “mirrors,” which reflect sound, and the relatively long time sound takes to make a round trip allowed the precise coordination. The mirrors were a hair’s width apart—similar control of light would require mirrors separated by about 12 meters.

Sound’s “slowness” also let the experimenters identify phonons of more than one mode. Typically, Sletten says, quantum computers increase their capacity through additional superconducting qubits. But having just one qubit process information with multiple modes could achieve the same result.

“This is definitely a milestone,” says Yiwen Chu, a physicist at ETH Zurich, who was not involved in the study. Analogous experiments with light were a first step toward much of today’s work on quantum computers, she notes.

Similar applications for sound are far off, however: among other things, scientists must find a way to keep phonons alive much longer than they currently can—about 600 nanoseconds. Eventually, though, the research could open new paths forward in quantum computing.