Stanford University researchers have achieved a historic milestone in quantum physics by directly observing the quantum jumps of sound in a mechanical resonator for the first time. This breakthrough successfully bridges a long-standing gap in quantum mechanics, extending a lineage of research that stretches back more than a century into the fundamental nature of energy states.
Quantum jumps—the sudden, discrete transitions of a system from one energy state to another—have been a foundational pillar of quantum theory since the early decades of the 1900s. While these abrupt changes have long been accepted theoretically, capturing them experimentally has proved to be an extraordinary scientific challenge. Researchers first demonstrated quantum jumps in trapped ions back in 1986, a milestone that was later mirrored with photons, the fundamental particles of light, in 2007. Sound, however, remained a stubborn target due to the complex nature of mechanical vibrations.
Now, a team led by Stanford physicist Amir Safavi-Naeini has recorded these acoustic jumps directly. Their findings, which represent a significant leap forward in experimental physics, were recently published in the prestigious journal Science.
"What this study shows will allow us to move forward with developing new quantum technologies with sound," said Safavi-Naeini, who serves as an associate professor of applied physics in 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."
Watching Sound Behave Quantum Mechanically
To understand the significance of the experiment, it helps to examine the fundamental units of energy across different mediums. Just as the smallest discrete unit of light is a photon, the quantum equivalent for sound is a phonon. A phonon represents the coordinated, collective motion of many atoms moving in unison.
In everyday life, the behavior of sound and vibration appears entirely continuous. When a bell is struck, the ringing sound fades smoothly into silence, gradually growing quieter until it disappears completely. However, at the quantum scale, the picture is radically different. Instead of fading out continuously, a microscopic resonator’s vibrational energy changes in distinct, discrete steps. This quantised behavior mirrors the phenomena previously observed in trapped ions and photons.
While earlier experiments across the physics community had produced tantalizing indirect evidence that sound could undergo these abrupt transitions, Safavi-Naeini’s team went a crucial step further. The new study succeeds in directly tracking individual phonons as they make quantum jumps in real time, providing an unprecedented window into the quantum mechanical properties of mechanical motion.
A Microscopic Resonator With an Unusually Long Ring
The physical heart of the experiment is a microscopic mechanical resonator meticulously constructed using advanced semiconductor chip fabrication techniques. Because of its tiny dimensions, the device opens the door to immense scalability, meaning that many such resonators could potentially be integrated onto a single silicon chip to carry out complex, multi-layered computational tasks in future quantum architectures.
A critical, make-or-break feature of the experiment involved the duration for which the device could continue vibrating once excited. Acting somewhat like a microscopic tuning fork, the custom-built resonator can sustain its vibrations for an impressive two milliseconds. To put that endurance into perspective, if a normal-sized tuning fork possessed the same relative ability to sustain vibrations, it would continue ringing continuously for several hours.
This unusually long "ringdown time" provided the researchers with a vital operational window, granting them enough time to collect hundreds of consecutive measurements. These repeated readings were essential for identifying the precise moment when the vibration finally dissipated, allowing researchers to catch the exact instant the sound jumped from an energy state of 1 down to 0.
Measuring a Fragile Quantum State
Executing the experiment required the Stanford team to solve a notoriously difficult, long-standing problem in the field of quantum engineering: how to measure what is happening inside a delicate quantum system without inadvertently disrupting or destroying the very state they are trying to observe.
Takuma Makihara and Erik Szakiel, who served as co-first authors on the new study, spearheaded the development of an innovative methodology for coupling the microscopic mechanical resonator directly to a superconducting qubit. A qubit is an advanced electrical circuit capable of storing and processing quantum information. In this specific experimental setup, the qubit also played the dual role of a high-precision 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," explained Makihara, a recent Stanford doctoral graduate who worked closely on the project.
During the process, the qubit repeatedly interrogates the mechanical resonator throughout its two milliseconds of active vibration. By checking the system continuously, the qubit determines whether the phonon resides in an energy state of 1 or 0. By executing these readouts repeatedly and rapidly, the research team can pinpoint precisely when the quantum jump occurs without collapsing the fragile wave function prematurely.
Toward Quantum Computing and Ultra-Sensitive Sensors
The research team views this successful demonstration as an early, yet vital, foundational step toward the development of entirely new classes of technologies that harness sound as a primary quantum platform.
One of the most promising potential applications lies in the realm of quantum error correction. While quantum computers hold the theoretical potential to solve complex computational problems that are entirely beyond the reach of even the most powerful conventional supercomputers, their underlying quantum states remain extraordinarily fragile. Environmental noise and internal decoherence mean that errors frequently arise and corrupt calculations before they can be completed.
In many modern quantum computing architectures, a sudden quantum jump can serve as a diagnostic signal that an error has occurred within the system. Because detecting those elusive jumps has historically proven exceptionally difficult, the newly demonstrated ability to monitor them reliably using sound could provide scientists and engineers with a powerful new tool for identifying, isolating, and correcting quantum errors on the fly.
Beyond computing, the potent combination of the mechanical resonator and the superconducting qubit could also evolve into a highly sensitive measurement platform. Safavi-Naeini’s research group is already collaborating with a team led by physicist Michael Roukes at the California Institute of Technology (Caltech). Together, the researchers are exploring whether the specialized system can be adapted to detect, measure, and identify individual protein molecules hidden inside biological cells, opening up novel avenues for nanoscale biosensing.
Better Control of Sound
The implications of this advance may eventually extend far beyond specialized, cryogenic quantum laboratories and into everyday consumer electronics. Sound plays a critical role in modern smartphones, wireless communication devices, and countless other everyday electronic systems. According to Erik Szakiel, a current doctoral student in Safavi-Naeini’s laboratory, achieving increasingly precise, atomic-scale control over mechanical vibrations could ultimately contribute to entirely new generations of commercial technologies.
"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," Szakiel noted.
Safavi-Naeini holds dual affiliations as a member of Stanford Q-FARM and Bio-X, interdisciplinary initiatives fostering collaborative research across the physical and biological sciences.
Additional Stanford co-authors contributing to the published study include David Schuster, the Joan Reinhart Professor and professor of applied physics in the School of Humanities and Sciences; Shannon Harvey, a scientist affiliated with the SLAC National Accelerator Laboratory; Mihir Pendharkar, a physical research scientist at the Edward L. Ginzton Laboratory; Rachel Gruenke-Freudenstein, a former applied physics doctoral scholar; and Oliver Hitchcock, Matthew Maksymowych, and Kaveh Pezeshki, all current doctoral scholars in applied physics at Stanford.
The research described in the study received substantial financial and institutional support from a diverse coalition of backers, including 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 United States Department of Defense. Furthermore, both study leaders, Safavi-Naeini and Schuster, currently serve as Amazon Scholars.
