Detecting a Single Unit of Sound

Researchers can now measure individual phonons -- a single unit of sound.

Yuen Yiu, Staff Writer

(Inside Science) -- Just as light waves have a particle version -- the photon -- sound waves do too. Called a phonon, it's a way to quantify incredibly tiny packets of sound, or vibrations, and is particularly important for understanding certain properties of solids such as electric resistivity. And just like how the study of light brought us modern-day technologies such as fiber optics, the study of phonons may one day revolutionize applications such as quantum computing.

Last year, a group of researchers from the University of Colorado Boulder built a device that can detect phonons with unprecedented precision. Their device used an acoustic cavity, which is essentially a vibrating drumhead that contains the phonons. You can picture it as a bunch of kids bouncing on a trampoline, except the trampoline is smaller than the cross section of a human hair. The phonons trapped in the cavity interact with a connected superconducting circuit that gives out a readable electrical signal.

Since even the lightest whisper can produce a huge number of phonons, the researchers had to put their contraption inside a vacuum chamber to isolate it from all the outside phonons bouncing around. They also had to cool it down to just a few thousandths of a degree Celsius above absolute zero to minimize vibrations from heat. Even so, they were only able to count with a precision of around seven phonons.

Now, the same group of researchers has improved their previous design enough to detect individual phonons. By optimizing the transducer, a device used to convert phonon-induced strain in the acoustic cavity into electric signals, they were able to improve the link between the cavity and the superconducting circuit, which increased the sensitivity. They reported their latest invention in the journal Physical Review X.

Phonons are slower than photons, and this, together with the possibility of measuring phonons without destroying them, makes sound particles a unique contender for quantum computing applications. One caveat, however, is that the current device can hold on to the phonons only for a short time before they escape, which is less than ideal for certain applications, such as data storage. The researchers look to overcome this problem by adding acoustic reflectors to the cavity, which other scientists have previously built to successfully increase the lifetime of the phonons.