Stanford Physicists Watch Sound Take Its First Quantum Jump

Physicists at Stanford University have watched a single quantum of sound vanish from one energy state and reappear in another in real time, the first direct observation of a "quantum jump" in a mechanical object. The result, published in the journal Science, closes a chapter of quantum theory that began with light and matter more than a century ago and now extends, for the first time, to sound.

A phonon is the smallest possible unit of vibration, the quantum equivalent of a musical note produced by the coordinated motion of a large group of atoms. In the new experiment, researchers built a microscopic mechanical resonator, a sliver of crystal engineered to vibrate at a single, precise pitch, and coupled it to a superconducting qubit that acted as a listening device. The qubit repeatedly interrogated the resonator without collapsing its fragile quantum state, and captured the exact instant a single phonon disappeared, a jump from an energy level of one to an energy level of zero.

The numbers behind the jump

The resonator's vibration lasted about two milliseconds before decaying, a window so brief it is easy to underestimate, according to the Stanford research team's account of the work. Scaled up to the size of an ordinary tuning fork, a vibration that persistent would ring for several hours rather than fade in seconds, a measure of how effectively the team suppressed the everyday noise and friction that normally destroy quantum behavior in anything large enough to see. Within that two-millisecond span, the superconducting qubit took hundreds of individual measurements, stitching them into a continuous record precise enough to pinpoint the moment of the transition rather than merely confirming it happened before and after the fact.

The team was led by Amir Safavi-Naeini, an associate professor of applied physics at Stanford, with Takuma Makihara, a recent Stanford doctoral graduate, and Erik Szakiel, a doctoral student in applied physics, sharing lead authorship. David I. Schuster, a Stanford professor of applied physics, and several other Stanford and SLAC National Accelerator Laboratory researchers contributed to the design and measurement of the device.

A century-old question, answered piece by piece

Quantum jumps, the idea that a system moves between discrete energy levels abruptly rather than sliding smoothly between them, have been part of quantum theory since Niels Bohr's early atomic models in the 1910s. For decades the concept remained a mathematical abstraction. Physicists first watched a quantum jump happen directly, rather than inferring it statistically, in trapped ions in 1986. Photons, the particles of light, followed in 2007, when researchers tracked individual light quanta jumping between states inside a cavity. Sound had been the conspicuous gap in that lineage: mechanical vibrations are usually far too noisy, and their surrounding environment too warm and jittery, to preserve a delicate quantum state long enough to watch anything happen to it in real time.

Closing that gap required cooling the resonator to temperatures near absolute zero and isolating it from vibration, electrical noise and stray heat well enough that a single phonon could survive milliseconds rather than nanoseconds. "Vibrating objects can exhibit quantum behavior, which is the prerequisite for many operations needed by quantum computing and sensing," Safavi-Naeini said, according to a summary of the findings distributed by ScienceDaily.

"What this study shows will allow us to move forward with developing new quantum technologies with sound."

Who stands to benefit

The immediate audience for the work is the community of physicists and engineers building quantum computers, many of whom rely on superconducting qubits that are themselves vulnerable to the kind of uncontrolled jumps the Stanford team just learned to watch. Detecting a quantum jump the instant it occurs, rather than after a calculation has already gone wrong, is a prerequisite for real-time quantum error correction, the process by which future quantum computers are expected to catch and fix mistakes faster than they accumulate. A mechanical resonator that behaves this predictably also gives quantum-hardware designers a new, comparatively simple component to add to that toolkit alongside superconducting circuits and trapped ions.

A second, more immediate beneficiary is biosensing. According to the release distributed through EurekAlert, Safavi-Naeini's group is collaborating with the Caltech physicist Michael Roukes on using mechanical resonators of this kind to detect and identify individual proteins inside living cells, a task that requires exactly the sensitivity to minute changes in vibration that this experiment demonstrated. Manufacturers of acoustic-wave filters, the components that help separate radio frequencies inside smartphones, are a further, longer-term audience, since the same techniques used to control phonons at the quantum level could eventually refine those far more ordinary devices.

• First direct, real-time observation of a quantum jump in a mechanical (sound) system
• Resonator preserved its quantum state for about two milliseconds, with hundreds of measurements taken during that window
• Completes a sequence of quantum-jump observations that began with trapped ions in 1986 and photons in 2007
• Potential applications include real-time quantum error correction and ultra-sensitive protein detection with Caltech's Michael Roukes

What happens next

The Stanford group's near-term plan is to extend the technique from a single resonator and a single phonon to systems that track multiple vibrational modes at once, a step toward using mechanical elements as working parts of a quantum processor rather than as laboratory curiosities. The collaboration with Roukes' Caltech lab is expected to move from concept toward prototype sensors capable of weighing and identifying individual biomolecules by the faint vibrations they induce. Funding for the research, which came from Amazon Web Services, the Air Force Office of Scientific Research, the Office of Naval Research, the National Science Foundation and Canada's Natural Sciences and Engineering Research Council, points to interest from both computing and defense-adjacent research programs in mechanical qubits as a complement to the superconducting and photonic hardware that dominates today's quantum computers. No timeline has been given for when phonon-based components might appear in a working quantum computer or commercial sensor.