Stanford Physicists Watched a Single Quantum of Sound Blink Out of Existence in Real Time. The Vibration Lasted Two Milliseconds, Which for a Normal Tuning Fork Would Be Several Hours, and It Ended in a Jump, Not a Fade.

To human ears, a struck bell fades. Its ring gets quieter and quieter until it is gone, and nothing about that decline looks abrupt. At the quantum level it is a different story: the vibrational energy of a resonator is supposed to fall in discrete steps, or jumps, the same way an electron drops between energy levels in an atom. A Stanford team has now watched that happen to a single quantum of sound, in real time, for the first time, and published the result in the journal Science.

Quantum jumps have been part of the theory since the early 1900s. They were first observed directly in trapped ions in 1986 and in photons, the particles of light, in 2007. Sound was harder. A quantum of light is one photon; a quantum of sound, called a phonon, is the coordinated motion of an enormous number of atoms, and catching one of them making a jump means holding a mechanical object in a fragile quantum state long enough to look at it repeatedly without destroying it. "What this study shows will allow us to move forward with developing new quantum technologies with sound," said Amir Safavi-Naeini, an associate professor of applied physics who led the work. "We have seen that vibrating objects can exhibit quantum behavior, which is the prerequisite for many of the operations needed by quantum computing and sensing."

The key was a mechanical resonator that rings for an unusually long time. The device is small but visible under a microscope, is built with standard chipmaking techniques, and works like a microscopic tuning fork that vibrates for about two milliseconds. That sounds brief, but scaled to the frequency of a full-sized tuning fork the same quality would keep it ringing for several hours. That long ringdown gave the team a window in which to take hundreds of readings and pin down the instant the vibration was no longer present, when the energy state dropped from one phonon to zero.

Reading a quantum system without disturbing it is the standing headache of the field. Co-first authors Takuma Makihara and Erik Szakiel solved it by coupling the resonator to a superconducting qubit, an electrical circuit that stores quantum information and can serve as a detector. The qubit repeatedly checked, over the two milliseconds, whether the resonator held one phonon or none. "We had to continually develop new processes to make this extremely long-lived, vibrating object and then integrate it with the qubit, which is our little electrical detector, without ruining either subsystem," said Makihara, a recent Stanford doctoral graduate.

Earlier experiments had found indirect evidence of phonon jumps. This one shows individual phonons jumping as it happens, and the group sees a direct path to applications. In many quantum computing architectures a quantum jump is an error, and the difficulty has always been knowing when one occurs; detecting jumps in sound is a step toward correcting them. The resonator's small size means many could be packed on a chip, and its sensitivity makes it a candidate for extremely precise measurement. Safavi-Naeini's lab is already working with Caltech physicist Michael Roukes to use the platform to detect and identify proteins inside cells.

"This shows we can have incredibly fine-tuned control of sound, which might mean that devices that use sound as a fundamental technology can get much better," said Szakiel, a doctoral student in the lab. Sound already runs the filters and sensors inside every smartphone. The claim here is that the quantum version of that technology is now observable, not just theoretical.

In plain terms: the researchers built a tiny vibrating object, kept it in a quantum state long enough to keep asking it whether it was still vibrating, and caught the exact moment its last packet of sound energy disappeared. It did not fade out. It jumped.