Quantum jumps are abrupt transitions from one energy state to another. They have been part of quantum theory since the early 1900s. Researchers first demonstrated them in trapped ions in 1986 and in photons, the fundamental particles of light, in 2007. Sound, however, has remained a more difficult target.

A team led by Stanford physicist Amir Safavi Naini directly recorded quantum jumps in sound and published its results in a forthcoming paper in Science.

“What this study shows will allow us to advance the development of new quantum technologies involving sound,” said Safavi Naini, associate professor of applied physics at Stanford University’s School of Humanities and Sciences. “We have seen that vibrating objects can exhibit quantum behavior, which is a prerequisite for many of the operations required for quantum computing and quantum sensing.”

How Sound Behaves at the Quantum Scale

The smallest unit of light is a photon. The quantum equivalent of sound is a phonon, which represents the coordinated motion of many atoms.

In everyday life, vibrations appear to disappear smoothly. A ringing bell, for example, gradually becomes quieter until the sound fades away. At the quantum scale, however, vibrational energy changes in discrete steps rather than continuously, much like the behavior previously observed in ions and photons.

Previous experiments had provided evidence that sound could undergo these changes. The new research goes further by directly tracking individual phonons as they make quantum jumps in real time.

A Microresonator with an Unusually Long Ring-Down Time

The mechanical resonator used in the experiment was built with chip-fabrication techniques. Its small size means that many such resonators could potentially be placed on a single chip to perform complex tasks.

The device’s key feature was its ability to keep vibrating for an unusually long time. The resonator works like a tuning fork on a microscopic scale and vibrates for 2 milliseconds. If a normal-sized tuning fork had the same relative vibration-sustaining capacity, it would continue ringing for several hours.

This unusually long “ring-down time” gave the researchers enough time to collect hundreds of measurements. The repeated readings allowed them to pinpoint when the vibrations disappeared and the sound jumped from energy state 1 to energy state 0.

Measuring Fragile Quantum States Without Destroying Them

The researchers also had to solve a long-standing problem in quantum engineering: how to measure what is happening inside a quantum system without disturbing its delicate state.

The study’s co-first authors, Takuma Makihara and Eric Zakiel, developed a method to couple tiny mechanical resonators to superconducting qubits. A qubit is an electrical circuit that can store quantum information and, in this experiment, also act as a detector.

“We had to continually develop new processes to create this very long-lived vibrating object and integrate it with our tiny electrical detector, the qubit, without messing up either subsystem,” said Makihara, a recent graduate of Stanford’s doctoral program.

During the 2-millisecond oscillation, the qubit repeatedly checks the mechanical resonator to determine whether the phonon’s energy state is 1 or 0. By repeating these measurements, the researchers can identify when quantum jumps occur.

What Quantum Sound Could Mean for Computing and Sensors

The researchers believe this work is an early but important step toward technologies that use sound as a quantum platform.

One potential application is quantum error correction. Quantum computers may eventually solve certain complex problems beyond the reach of classical computers, but their quantum states are extremely fragile. Errors can occur before a calculation is complete.

In many quantum computing systems, a quantum jump can signal that an error has occurred. These jumps have been difficult to detect, so the ability to monitor them with sound could provide an important new tool for identifying and correcting quantum errors.

The combination of mechanical resonators and qubits could also become a highly sensitive measurement platform. Safavi Naini’s group is already working with Caltech physicist Michael Rooks’ team to study whether the system can detect and identify proteins in cells.

Improving Control Over Sound and Vibrations

The breakthrough could eventually have applications beyond specialized quantum technologies. Sound plays an important role in smartphones and many other electronic devices, and more precise control of vibrations could contribute to a new generation of these technologies, said Zazakiel, currently a doctoral student in Safavi Naini’s lab.

“This shows that sound can be incredibly fine-tuned. This could mean that devices that use sound as an underlying technology could become much better,” he said.

Safavi-Naeini is also a member of Stanford Q-FARM and Bio-X.

Other Stanford co-authors include David Schuster, Joan Rinehart Professor and Professor of Applied Physics at H&S; Shannon Harvey, SLAC National Accelerator Laboratory scientist; Mihir Penderkar, physics research scientist at the Edward L. Ginston Institute; Rachel Gruenke-Freudenstein, former doctoral scholar in applied physics; and Oliver Hitchcock, Matthew Maksimowicz, Kave Pezeszki, PhD scholars in applied physics.

This research received support from Amazon Web Services Inc., 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.

Mr. Safavi Naini and Mr. Schuster are both Amazon Scholars.