Physicists Build a Chip That Fires Bursts of Sound One Quantum at a Time

Physicists at McGill University have built a tiny device that generates precisely controlled bursts of sound-like particles called phonons by forcing electrons to race through an ultrathin crystal at temperatures a whisker above absolute zero. The work, published in the journal Physical Review Letters, points toward technologies that would use sound rather than light to carry and process information.

Phonons are the quantum units of vibration, the acoustic equivalent of the photons that make up light. Getting them to appear on demand, in neat and adjustable bursts, has long been a challenge. The McGill device solves it with what amounts to a microscopic racetrack: a two-dimensional crystal, only a few atoms thick, carved into a narrow channel. When an electrical current is pushed through that channel, electrons are trapped and forced to sprint forward, and when they are driven hard enough they shed their excess energy as tunable pulses of phonons.

To coax that orderly behavior out of the electrons, the team ran their experiments at temperatures between roughly 10 millikelvin and 3.9 kelvin, a regime near absolute zero that demands sophisticated cryogenic equipment. At those temperatures the electrons move in a coherent, wavelike fashion rather than the chaotic jostling seen at room temperature, allowing the researchers to control exactly when and how the sound particles are emitted.

Michael Hilke, the McGill physics professor who led the study, said the appeal lies in doing with sound what modern technology already does with light. "Modern communication is largely based on light," he said. "In the human body, sound waves can also be a useful tool." Sound travels where light and electrical currents cannot, including through dense media such as tissue and water, which is part of why phonon-based devices are worth building. The project drew on collaborators at the National Research Council of Canada and Princeton University, which synthesized the pristine crystals required.

The researchers say the breakthrough could eventually enable phonon lasers, which would emit coherent beams of sound the way an ordinary laser emits light, along with faster communications hardware, improved medical imaging and highly sensitive detectors. For now the demonstration is a laboratory milestone rather than a consumer gadget, but by showing that phonons can be generated cleanly and controllably, the team has cleared a hurdle that had stood in the way of turning the physics of quantum sound into practical machines. The result also deepens the basic physics of how fast-moving electrons interact with a material, since the emission depends on driving the electrons to speeds beyond the natural velocity of sound within the crystal, a supersonic regime that had rarely been probed so cleanly before.