In a Stanford laboratory, a tiny mechanical resonator suddenly stopped vibrating not smoothly, but in a jolt — exactly as quantum mechanics predicted. For the first time, scientists captured this moment in real time: a quantum of sound, or phonon, transitioned from one energy state to another in fractions of a millisecond.
Phonons are discrete portions of vibration, analogous to photons of light. In the ordinary world, sound fades gradually, like a bell after a strike. At the quantum level, energy changes in leaps, and now researchers led by Amir Safavi-Naini have managed to catch such a leap "on the fly."
The key to success was a microscopic resonator capable of "ringing" for a full two milliseconds — for its size, that is an enormous span of time. During this time, a superconducting qubit connected to the resonator managed to make hundreds of measurements and precisely determine the moment when the phonon disappeared, transitioning from state "1" to "0."
The experiment opens the way to new quantum technologies. In quantum computers, such leaps often become a source of errors; the ability to track them is an important step toward error correction. Moreover, the sensitivity of the device will make it possible to create ultra-precise sensors, for example for detecting individual proteins inside cells.
In the future, control over quantum sound will also improve everyday gadgets: smartphones, acoustic sensors and other devices where sound plays a key role. The study is published in the journal Science and completes a century-long search that began back in the early twentieth century.
Now quantum acoustics is moving from theory into practice, and this changes the understanding of how vibrations can be controlled at the most fundamental level.
