Chill a sliver of crystal to within a whisker of absolute zero and something strange happens to the electrons inside. They stop rattling around randomly, shed most of their thermal energy, and start behaving less like billiard balls and more like waves on a pond. Push an electrical current through that crystal hard enough, and those wave-like electrons do something stranger still: they break the speed of sound. Not the speed of sound in air, which is a sedate 340 metres per second, but the speed at which vibrations travel through the crystal lattice itself, roughly 3 kilometres per second. Cross that threshold and the electrons begin shedding energy in concentrated, rhythmic bursts of sound-like vibrations. Bursts that, it turns out, physicists can tune and control.
That is the finding from a team at McGill University in Montreal, working with colleagues at the National Research Council of Canada and Princeton University. Published in Physical Review Letters, the research could be a significant step toward phonon lasers, devices that do with sound roughly what optical lasers do with light.
Phonons are the quantum packets of vibrational energy that carry sound through a solid. The idea of a phonon laser has circulated in physics for decades, in the same way that the idea of a conventional laser circulated before anyone actually built one. The attraction is practical: in some environments, coherent sound beats coherent light. “Modern communication is largely based on light, including electromagnetic waves and electrical currents. In a medium such as oceans, sound can travel, whereas light and electrical currents cannot,” says Michael Hilke, associate professor of physics at McGill and a co-author of the study. The same logic applies inside the human body, where ultrasound already does work that X-rays cannot, and where a finely controllable phonon source might do considerably more.
The Problem with Phonons
The difficulty is that phonons are genuinely unruly. They scatter, they dissipate, they bleed into every available vibrational mode. Generating them in a controlled, tunable way has proved obstinate.
The McGill device attempts a different approach. Rather than coaxing phonons from some external source, it makes electrons produce them directly, the way a plane produces a sonic boom when it crosses the sound barrier. The device channels electrons through a two-dimensional layer of crystal, a channel just a few atoms thick, and drives a current through it. The material used in this experiment was grown at Princeton and is ultrahigh-mobility, meaning electrons in it travel with unusually little scattering, which gives the team the control they need. At low enough temperatures (the experiments ran between 10 millikelvin and 3.9 kelvin, so cold that quantum effects dominate everything), the drift velocity of those electrons can be pushed past the speed of sound in the crystal.
Hilke explains that at absolute zero temperatures, no sound is created unless electrons travel collectively at the speed of sound or above. Once they clear that barrier, something shifts sharply. Instead of the gradual, temperature-sensitive phonon scattering seen in the subsonic regime, the team observed strong, sharp oscillations in the device’s electrical resistance, oscillations that barely changed as the temperature varied. Those features are the fingerprint of resonant phonon emission: electrons losing energy in distinct, quantized steps as they couple to the vibrating crystal.
Quantizing the Boom
The quantization comes, in part, from the magnetic field the researchers applied to the device. A magnetic field forces electrons in a 2D system into discrete energy levels, called Landau levels. When a supersonic electron drops between those levels, it doesn’t just shed energy as a blurry smear of vibration; it emits phonons at specific, well-defined frequencies. The result is a kind of quantum selection rule that imposes order on a process that would otherwise be chaotic. Helpfully, the team were also able to extract a concrete measure of how strongly the electrons and phonons were coupling: a dimensionless constant of about 0.0016, which will give theorists something to anchor future models to.
What also surprised the researchers was evidence for something theorists had predicted but not yet seen experimentally: the phase of those resistance oscillations flips as the drift velocity crosses the sound barrier. It’s perhaps a subtle detail, but physically it marks the transition between two fundamentally different phonon-emission regimes, and it tells physicists that their models are tracking reality.
Hilke is careful not to oversell where the technology sits right now. “Phonons are hard to generate and harness in a controlled way, so we are exploring new regimes,” he says. The existing theory, he adds, needs updating: “Our study goes further by pushing the system well beyond that point and showing that existing theories need to be reassessed by considering that electrons can be very hot even if the host crystal is close to absolute zero temperature.” That detail, electrons remaining energetically hot even inside a crystal that is thermally cold, is a wrinkle that standard models hadn’t properly accounted for, and it matters for anyone trying to build reliable phonon-emitting devices.
The next step, the team reckons, is to swap the current crystal for graphene, which could allow the device to operate at considerably higher speeds and, potentially, at less extreme temperatures. Whether graphene delivers that is still open; it’s a plausible bet, but materials science has a habit of throwing surprises.
From Physics to Medicine
The broader ambition is spelled out plainly enough. Phonon lasers could find roles in high-speed communications through media that block light, in sensing tools that exploit the mechanical properties of sound, and in medical diagnostics that need to interrogate soft tissue at microscopic resolution. “At a broad level, this is about how electrical current and energy moves and is converted inside advanced electronic materials,” Hilke says, which is perhaps a modest way of describing research that is trying, in a rather fundamental sense, to teach electrons to sing.
The full scope of what that might eventually enable is genuinely hard to predict. Which is, arguably, exactly where physics should be.
Study: Resonant Magnetophonon Emission by Supersonic Electrons in Ultrahigh-Mobility Two-Dimensional Systems, Physical Review Letters, 2026.
Frequently Asked Questions
What is a phonon laser and why would one be useful?
A phonon laser emits coherent, controllable bursts of sound-like vibrations rather than light. That matters in environments where light can’t penetrate: deep water, soft biological tissue, or certain electronic materials. A practical phonon laser could improve underwater communication, medical ultrasound resolution, or the sensing tools used to probe the microscopic structure of new materials.
Why do the electrons have to go supersonic for this to work?
Below the sound speed of the crystal, phonon emission is suppressed at low temperatures and extremely sensitive to how cold the system is. Cross the sound barrier and the physics changes: electrons shed energy in sharp, resonant bursts that are far more stable and far less dependent on temperature, which makes them much easier to harness. The McGill team found the transition is abrupt enough to flip the phase of the electrical oscillations they were measuring, a clear physical marker of two distinct regimes.
How cold does the device actually need to be?
The experiments ran between 10 millikelvin and 3.9 kelvin, temperatures achieved using dilution refrigerators and far below anything found in nature on Earth. That said, the team’s next aim is to test the approach in graphene, which could allow the same supersonic electron behavior at somewhat less extreme conditions, though how much of a practical difference that makes remains to be seen.
Could this eventually lead to sound-based medical imaging better than ultrasound?
It’s early to say, but the underlying logic is sound (so to speak). Current medical ultrasound uses relatively coarse, broadband vibration sources; a device that generates phonons at precise, tunable frequencies could in principle target specific tissue structures with far greater selectivity. The technology would need to be made practical at room temperature first, which is a significant engineering challenge, but one the phonon-laser field is actively working toward.
What does it mean that the existing theory needs to be updated?
Standard models of electron-phonon interaction at low temperatures assumed that if the crystal lattice is cold, the electrons are too. The McGill experiments suggest that’s wrong in the supersonic regime: electrons can carry substantial energy even inside a near-zero-temperature crystal, and that changes the predicted emission behavior in measurable ways. Updating the theory matters because anyone trying to design a practical phonon-emitting device will need accurate models to predict how the device will actually behave.
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