A pendulum that swings on its own, without any winding — in the quantum world, such 'pendulums' are called time crystals. In them, rhythmic motion arises spontaneously and does not decay. But until now, controlling this rhythm was nearly impossible: any intervention threatened to destroy the fragile order.
In the experiment, a tiny cavity made of gallium aluminum arsenide was used. It was filled with exciton-polaritons — particles that behave simultaneously like light and like matter. Under non-resonant laser pumping, stable oscillations at a frequency of several gigahertz spontaneously emerged in this mixture — a genuine solid-state time crystal. To tame it, physicists employed two tools: a weak control laser and the interaction of polaritons with acoustic phonons (high-frequency sound) within the same cavity.
The sound channel proved no less important. At a certain setting, the laser excited mechanical vibrations of the lattice, and feedback arose: polaritons began to move in step with the sound. This optomechanical locking sharply increased temporal coherence — irregularities in the rhythm smoothed out, and the spectral line narrowed. The frequency of the time crystal was fixed by phonons, just as an anchor holds a boat in a stormy sea.
Such dual control, by light and sound, not only tames a wayward quantum system. It transforms the time crystal from a laboratory curiosity into a potential tool for precise measurements and signal processing in the gigahertz range. The pendulum that once swung on its own now obediently ticks under the direction of an external conductor.
The oscillation frequency of the time crystal in this experiment, several gigahertz, is comparable to the clock speed of a processor in a modern smartphone.