How do you build a device that can hold a quantum sound?

The mechanical resonator at the heart of the experiment was built with chip fabrication techniques. It acts somewhat like a microscopic tuning fork, and its tiny size means many such resonators could potentially sit on a single chip to carry out complicated tasks.

The critical feature was endurance. The resonator vibrates for two milliseconds — an eternity at quantum scales. If a normal-sized tuning fork had the same relative ability to sustain vibrations, it would keep ringing for several hours.

That unusually long "ringdown time" gave the researchers enough time to collect their hundreds of measurements. The repeated readings let them pinpoint the exact moment when the vibration disappeared and the sound jumped from energy state 1 to 0.

How did they measure without destroying the signal?

The team also had to solve a long-standing problem in quantum engineering: how to look inside a quantum system without disrupting the delicate state being measured.

Co-first authors Takuma Makihara and Erik Szakiel developed a method for coupling the mechanical resonator to a superconducting qubit — an electrical circuit capable of storing quantum information, which in this setup doubled as the detector. The qubit repeatedly checked the resonator during its two milliseconds of vibration, determining whether the phonon sat in state 1 or state 0.

"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.

What could this be used for?

The researchers describe the work as an early but important step toward technologies that use sound as a quantum platform. Several applications stand out:

• Quantum error correction. Quantum computers could eventually solve certain complex problems beyond the reach of conventional machines, but their quantum states are extremely fragile, and errors can arise before a calculation finishes. In many systems, a quantum jump signals that an error has occurred. Detecting those jumps has been difficult, so monitoring them in sound could provide a new tool for identifying and correcting quantum errors.
• Ultra-sensitive biological sensors. The combination of the mechanical resonator and qubit could 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 detect and identify proteins inside cells.
• Better everyday devices. Sound already 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.

"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.

What are the caveats?

The findings are preliminary in the sense that practical devices remain far off. The study demonstrates a capability — real-time tracking of phonon jumps in one resonator type — rather than a finished technology. Turning that capability into working quantum error correction or protein-detecting sensors will require substantially more development.

The paper's co-authors include David Schuster, the Joan Reinhart Professor and professor of applied physics; Shannon Harvey of SLAC National Accelerator Laboratory; Mihir Pendharkar of the Edward L. Ginzton Laboratory; Rachel Gruenke-Freudenstein; and Oliver Hitchcock, Matthew Maksymowych, and Kaveh Pezeshki, doctoral scholars in applied physics. The research received support from 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. Safavi-Naeini and Schuster are both Amazon Scholars.