In an experiment, Stanford physicists demonstrated that tiny units of sound can suddenly change energy states, making a quantum jump. This is a long-anticipated breakthrough with the potential to advance a range of technologies.

A Stanford team has documented the first direct observation of quantum jumps of sound in a mechanical resonator, completing an arc of scientific exploration that started over a hundred years ago.

Quantum jumps—sudden transitions from one energy state to another—have been theorized since the early 1900s. Scientists first demonstrated these jumps in trapped ions in 1986 and later in photons, the fundamental particles of light, in 2007. Observing quantum jumps of sound had remained elusive, but a team led by Stanford physicist Amir Safavi-Naeini has recorded these phenomena, publishing the findings in the journal 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.”

While a quantum unit of light—its smallest possible discrete piece—is a singular photon, a quantum unit of sound, or “phonon,” represents the coordinated movement of a large group of atoms.

To the human senses, vibrating motion from sound, like that seen in a struck bell, appears to decline gradually rather than make an abrupt jump to stillness. At the quantum level, however, a resonator’s vibrational energy changes in discrete steps, or jumps, similar to the quantum behavior of ions and photons. Earlier experiments had found evidence of these jumps, but this study is the first to demonstrate individual phonons making quantum jumps in real time.
Size and timing

The mechanical resonator was fabricated using chipmaking techniques. It is so small that many resonators could be packed onto a chip to perform complex functions.

The length of time the resonator can “ring” is what made this breakthrough possible. Working sort of like a microscopic tuning fork, the resonator can vibrate for two milliseconds. For comparison, if the same capability were found in a regular-sized tuning fork, it would ring for several hours.

The long resonation or “ringdown time” of the resonator allowed for hundreds of readings to be taken to determine the moment the vibration was no longer present and the sound jumped—when it moved from an energy state of 1 to 0.