Counting the Quanta of Sound

At the origin of every musical note is a mechanical oscillator that resonates at a specific frequency. But what the ear cannot distinguish is that the energy of these vibrations is discretized into an integer number of quanta of motion, or phonons. Most vibrating objects contain an uncountable number of phonons, but researchers have, for some time now, been able to prepare massive mechanical oscillators in their quantum ground state, where the average phonon number is smaller than one. This hard-won accomplishment not only involved getting rid of all thermal excitations in the oscillator through intense cooling, but it also required inventing a system of motion detection with a sensitivity at the quantum level [1]. An emerging technique consists of coupling the oscillator motion to another quantum object: a superconducting qubit, which can serve a role in the detection as well as the manipulation of states of motion [2–4]. Using such a “qubit sound system,” two separate teams have managed to measure the number of phonons directly in a macroscopic mechanical oscillator. In one case, the oscillator is a membrane whose center of mass vibrates like a drumhead [5], while in the other case, the oscillator is a type of sound-wave cavity called an acoustic wave resonator [6]. By demonstrating unprecedented control over states of motion, these results may open the door to the use of oscillators as gravity sensors and quantum memory devices.