In a Stanford Lab, Sound Takes Its First Recorded Quantum Jump

Physicists at Stanford University have watched a single quantum of sound abruptly change energy states in real time, the first direct observation of a "quantum jump" involving sound rather than light or matter. The result, published in the journal Science on September 17, closes a gap in experimental physics that has stood since quantum jumps were first proposed more than a century ago.

The team, led by Stanford associate professor of applied physics Amir Safavi-Naeini, built a microscopic mechanical resonator, a sliver of material engineered to vibrate at a single, well-defined frequency, and coupled it to a superconducting qubit sensitive enough to register the resonator's energy one quantum unit at a time. Those units are called phonons, the acoustic equivalent of the photon. By continuously monitoring the resonator, the researchers caught the exact instant a phonon disappeared, the vibrational energy dropping without warning from one discrete level to the next.

The numbers

Quantum jumps had previously been recorded in only two other physical systems: trapped ions in 1986 and photons trapped in a microwave cavity in 2007, work that later contributed to a Nobel Prize in physics. Sound had remained the holdout, mainly because mechanical resonators tend to lose their vibrational energy to their surroundings too quickly for a jump to be caught mid-flight.

Stanford's device oscillated coherently for roughly 2 milliseconds, an interval that sounds negligible but is, by the standards of quantum acoustics, extraordinarily long. Scaled up to the size of an ordinary tuning fork, a vibration lasting that long relative to its frequency would correspond to a fork ringing audibly for several hours rather than fading within seconds. That extended lifetime gave the team's superconducting qubit enough time to interrogate the resonator hundreds of times within a single 2-millisecond window, frequently enough to pinpoint the moment a phonon dropped from its first excited state to its ground state rather than merely inferring, after the fact, that a change had occurred.

How researchers got here

The experiment sits at the intersection of two research traditions that have converged only in the past decade: superconducting quantum circuits, the technology behind much of today's quantum computing hardware, and micro- and nanomechanical engineering, the discipline that builds tiny vibrating structures for sensors and timing devices. Safavi-Naeini's lab has spent years developing techniques to fabricate resonators pure and well-isolated enough to preserve quantum states for meaningful stretches of time, then wiring them to superconducting qubits without disturbing either system, a pairing physicists call circuit quantum acoustodynamics.

Co-first author Takuma Makihara, a doctoral researcher in Safavi-Naeini's group, said the fabrication problem was as demanding as the physics.

"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."

Quantum jumps themselves have a long history in physics. Niels Bohr proposed in the early twentieth century that electrons change energy levels discontinuously rather than gradually, an idea that unsettled even some of quantum theory's founders. Direct experimental evidence did not arrive until 1986, when physicists watched individual trapped ions flicker between energy states, and it took until 2007 for the same feat to be repeated with photons confined inside a microwave cavity. Extending the observation to phonons required solving problems in materials science, cryogenics and quantum measurement simultaneously, according to the Stanford team and outside coverage of the work by outlets including ScienceDaily and the trade publication The Quantum Insider.

Who stands to benefit

The immediate audience for the result is the quantum computing community. Many proposed quantum processors, including some built from superconducting circuits, are vulnerable to a class of errors that show up as unexpected quantum jumps in the hardware itself. Detecting those jumps as they happen, rather than discovering their consequences only once a calculation has failed, is considered a prerequisite for practical quantum error correction. A resonator-qubit pairing that can register a single phonon's transition, as the Stanford device does, offers a testbed for exploring how such jumps might be tracked and corrected for in real time.

A second, less obvious audience sits in biology. Safavi-Naeini's group is collaborating with Caltech physicist Michael Roukes on adapting the same resonator-qubit architecture to detect individual proteins moving inside living cells, using the mechanical sensitivity that phonon-counting requires. A third, more distant audience is the consumer electronics industry: sound is already the operating principle behind filters, timing crystals and sensors inside every smartphone, and finer control over how those devices lose or retain vibrational energy could eventually translate into more efficient hardware, though that application remains speculative.

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 U.S. Department of Defense and Canada's Natural Sciences and Engineering Research Council, according to the paper's acknowledgments, reflecting the mixed academic-industrial-defense funding base that has become typical of quantum hardware research.

What people are saying

Safavi-Naeini described the result as a foothold for a broader research program rather than an endpoint. Co-author Erik Szakiel, also a doctoral researcher in the lab, framed the achievement in terms of the control it demonstrates over vibration itself.

"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."

The paper's co-authors also include David Schuster, a Stanford professor of applied physics whose group works on superconducting qubit design; Shannon Harvey, a scientist at the SLAC National Accelerator Laboratory; Mihir Pendharkar, a research scientist at Stanford's Edward L. Ginzton Laboratory; and doctoral researchers Oliver Hitchcock, Matthew Maksymowych, Kaveh Pezeshki and Rachel Gruenke-Freudenstein. The work was announced jointly by Stanford's School of Humanities and Sciences and distributed through the American Association for the Advancement of Science's EurekAlert news service, alongside the paper's formal publication in Science.

What happens next

The Stanford group's near-term plans center on using the same platform to study how phonon jumps might be predicted or steered rather than simply observed, a step that would be necessary before the technique could contribute to working error-correction schemes in quantum processors. Longer term, the collaboration with Roukes' Caltech lab aims to push the resonator-qubit approach toward single-molecule sensitivity inside biological samples, an application that would depend on resonators becoming smaller, more sensitive and easier to integrate with existing microscopy setups.

None of those follow-on applications is imminent. The Science paper itself is a proof of principle: a demonstration that a phenomenon long assumed to apply to sound, by analogy with ions and photons, can actually be seen happening. Whether that observation leads anywhere useful in quantum computing or cellular biology will depend on years of further engineering, the same trajectory that photonic and trapped-ion quantum jumps followed after their own first observations decades ago.