Stanford researchers have directly observed quantum jumps of sound in a mechanical resonator for the first time, marking the latest milestone in a line of quantum physics research that stretches back more than a century.
Quantum jumps — sudden transitions from one energy state to another — have been part of quantum theory since the early 1900s. Researchers first demonstrated them in trapped ions in 1986, followed by photons, the fundamental particles of light, in 2007. Sound, however, remained a more difficult target. A team led by Stanford physicist Amir Safavi-Naeini has now recorded these jumps directly, with the findings published in Science.
“What this study shows will allow us to move forward with developing new quantum technologies with sound,” said Safavi-Naeini, 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
The smallest discrete unit of light is a photon. The quantum equivalent for sound is a phonon, which represents the coordinated motion of many atoms.
In everyday life, vibration seems to fade smoothly. A ringing bell, for example, gradually grows quieter until the sound disappears. At the quantum scale, the picture is very different. A resonator’s vibrational energy changes in distinct steps rather than continuously, much like the behavior previously observed in ions and photons.
Earlier experiments had produced evidence that sound could undergo these transitions. The new study goes further by directly tracking individual phonons as they make quantum jumps in real time.
A Microscopic Resonator With an Unusually Long Ring
The mechanical resonator used in the experiment was built with chip fabrication techniques. Its tiny size means that many such resonators could potentially be placed on a single chip to carry out complicated tasks.
A critical feature was how long the device could continue vibrating. Acting somewhat like a microscopic tuning fork, the resonator can vibrate for two milliseconds. If a normal-sized tuning fork had the same relative ability to sustain vibrations, it would continue ringing for several hours.
That unusually long “ringdown time” gave the researchers enough time to collect hundreds of measurements. Those repeated readings allowed them to identify the moment when the vibration disappeared, and the sound jumped from an energy state of 1 to 0.
Measuring a Fragile Quantum State
The experiment also required the researchers to solve a long-standing problem in quantum engineering: measuring what is happening inside a quantum system without disrupting the delicate state being measured.
Takuma Makihara and Erik Szakiel, the co-first authors on the study, developed a method for coupling the microscopic mechanical resonator to a superconducting qubit. A qubit is an electrical circuit capable of storing quantum information and, in this setup, also served 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,” said Makihara, a recent Stanford doctoral graduate.
The qubit repeatedly checks the mechanical resonator during its two milliseconds of vibration, determining whether the phonon is in an energy state of 1 or 0. By making these measurements again and again, the researchers can identify precisely when the quantum jump occurs.
Toward Quantum Computing and Ultra-Sensitive Sensors
The researchers see the work as an early but important step toward technologies that use sound as a quantum platform.
One potential application is quantum error correction. Quantum computers could eventually solve certain complex problems that are beyond the reach of conventional computers, but their quantum states are extremely fragile. Errors can arise before a calculation is completed.
In many quantum computing systems, a quantum jump can signal that an error has occurred. Detecting those jumps has been difficult, so the ability to monitor them in sound could provide an important new tool for identifying and correcting quantum errors.
The combination of the mechanical resonator and qubit could also become a highly sensitive measurement platform. Safavi-Naeini’s group is already working with physicist Michael Roukes’ team at Caltech to explore whether the system could be used to detect and identify proteins inside cells.
Better Control of Sound
The advance may eventually have applications beyond specialized quantum technologies. Sound plays an important role in smartphones and many other electronic devices, and increasingly precise control over vibrations could contribute to new generations of those technologies, according to Szakiel, a current doctoral student in Safavi-Naeini’s lab.
“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,” he said.
Safavi-Naeini is also a member of Stanford Q-FARM and Bio-X.
Additional Stanford co-authors include David Schuster, the Joan Reinhart Professor and professor of applied physics in H&S; Shannon Harvey, a scientist with SLAC National Accelerator Laboratory; Mihir Pendharkar, physical research scientist at the Edward L. Ginzton Laboratory; former applied physics doctoral scholar Rachel Gruenke-Freudenstein; and Oliver Hitchcock, Matthew Maksymowych, and Kaveh Pezeshki, doctoral 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.
Safavi-Naeini and Schuster are both Amazon Scholars.
