A struck bell does not stop ringing all at once. It fades, and to any ear it fades smoothly. Under the hood, the technical reality is different: at the quantum scale a resonator’s vibrational energy drops in discrete steps, and a Stanford team has now caught single units of sound making that jump in real time.
The team, led by physicist Amir Safavi-Naeini, published the work in the journal Science. Quantum jumps, the sudden transitions from one energy state to another, have been part of theory since the early 1900s. Researchers demonstrated them in trapped ions in 1986 and in photons in 2007. Sound stayed out of reach. Earlier experiments had turned up evidence that it jumps too, but this is the first to follow individual phonons, the quantum units of sound, as they do it.
A phonon is not a particle in the way a photon is. It represents the coordinated motion of a large group of atoms.
A tuning fork that rings for hours
The resonator was made with chipmaking techniques and is small enough that many could share one chip. Its trick is longevity. It behaves like a microscopic tuning fork that vibrates for two milliseconds, which sounds unremarkable until you scale it up: a normal-sized tuning fork with the same relative staying power would ring for several hours.
That long ringdown is the detail that matters. It gave the researchers time for hundreds of measurements, enough to pin down the moment the vibration vanished and the sound dropped from an energy state of 1 to 0.
Taking those measurements without wrecking the thing being measured was the harder problem. Takuma Makihara and Erik Szakiel, the co-first authors, paired the resonator with a superconducting qubit, an electrical circuit that stores quantum information and here doubles as the detector. The qubit checks, over and over through the two milliseconds, whether the phonon sits at 1 or at 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 it might be good for
Safavi-Naeini, an associate professor of applied physics, called it a prerequisite. “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,” he said.
Error correction is the obvious target. In many quantum computing designs a quantum jump signals an error, yet spotting when one happens has been difficult. His group is also working with Michael Roukes’ team at Caltech to see whether the resonator-and-qubit pair could detect and identify proteins inside cells.
Szakiel, a current doctoral student in the lab, pointed to more ordinary hardware, since sound is built into smartphones and many other devices. “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.
Funding came 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 US Department of Defense. Safavi-Naeini and co-author David Schuster are both Amazon Scholars.
Sources
- Researchers observe first real-time quantum jump in sound
- For the first time, scientists watch sound jump between quantum states
