Stanford Team Captures First Direct Observation of Quantum Sound Jumps

For the first time, scientists at Stanford University have directly recorded quantum jumps of sound within a mechanical resonator. The discovery marks a significant milestone in quantum physics, building on over a century of research into how particles transition between energy states.

Quantum jumps, defined as sudden shifts from one energy level to another, were initially demonstrated in trapped ions in 1986 and later with photons in 2007. However, observing these transitions in sound has proven far more difficult. The new findings, led by Stanford physicist Amir Safavi-Naeini, are published in the journal Science.

“What this study shows will allow us to move forward with developing new quantum technologies with sound,” said Safavi-Naeini, an associate professor of applied physics at the Stanford School of Humanities and Sciences. “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.”

In the macroscopic world, vibrations tend to fade gradually, such as a bell slowly losing its ring. At the quantum scale, however, vibrational energy changes in discrete steps. The quantum equivalent of a photon is a phonon, representing the coordinated motion of numerous atoms. While previous experiments suggested sound could undergo these transitions, this study goes further by tracking individual phonons in real time.

The experiment utilized a microscopic resonator fabricated using chip fabrication techniques, allowing for the potential integration of multiple devices on a single chip for complex tasks. A key feature of the device was its exceptional longevity in vibration. Acting like a microscopic tuning fork, the resonator sustained vibrations for two milliseconds. To put this in perspective, if a standard tuning fork possessed the same relative ability to sustain sound, it would ring for several hours.

This prolonged “ringdown time” enabled the researchers to gather hundreds of measurements, identifying the precise moment the vibration ceased and the sound jumped from an energy state of 1 to 0.

A central challenge in quantum engineering is measuring internal system states without disrupting the delicate conditions being observed. Co-first authors Takuma Makihara and Erik Szakiel addressed this by developing a method to couple the mechanical resonator to a superconducting qubit, which functioned as the detector.

“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,” Makihara, a recent Stanford doctoral graduate, explained. The qubit repeatedly checked the resonator during its two-millisecond window, determining the phonon’s energy state and pinpointing the exact moment of the quantum jump.

The team views this as a foundational step toward quantum technologies that leverage sound. One major application is quantum error correction. While quantum computers promise to solve complex problems beyond the reach of conventional machines, their fragile states are prone to errors. In many systems, a quantum jump signals an error, but detecting these events has been challenging. The ability to monitor sound jumps could provide a critical tool for identifying and correcting such errors.

Additionally, the combination of the mechanical resonator and qubit could serve as a highly sensitive measurement platform. Safavi-Naeini is collaborating with Caltech physicist Michael Roukes to explore whether the system can detect and identify proteins inside cells.

The implications may extend beyond specialized quantum research. Sound is integral to smartphones and other electronics, and finer control over vibrations could enhance future devices. “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 current doctoral student in Safavi-Naeini’s lab.

Safavi-Naeini is also affiliated with Stanford Q-FARM and Bio-X. Additional co-authors include David Schuster, Shannon Harvey, Mihir Pendharkar, Rachel Gruenke-Freudenstein, Oliver Hitchcock, Matthew Maksymowych, and Kaveh Pezeshki. The research was supported by Amazon Web Services, the Air Force Office of Scientific Research, the Office of Naval Research, the National Science Foundation, the Natural Sciences and Engineering Research Council of Canada, and the U.S. Department of Defense. Both Safavi-Naeini and Schuster are Amazon Scholars.