STANFORD, September 19 — A Stanford University-led team has reported the first direct observation of quantum jumps of sound in a mechanical resonator, a result published in the journal Nature on Wednesday that closes an arc of scientific exploration begun more than a century ago. The experiment used a tiny vibrating drumhead — a mechanical oscillator roughly a micrometer in size — cooled to its quantum ground state and coupled to a superconducting qubit.
By monitoring the qubit, the researchers watched the resonator jump between discrete energy levels: the phonon equivalent of the quantum jumps of light that Niels Bohr proposed for electrons in atoms in 1913. Sound quanta, called phonons, are the mechanical analog of photons.
Observing single phonon jumps has been far harder than watching light, because mechanical resonators are massive compared with atoms and easily disturbed by heat.
A century in the making
The work builds on more than a century of quantum theory. Max Planck introduced the quantum of energy in 1900. Albert Einstein explained the photoelectric effect in 1905.
Bohr’s 1913 model of the hydrogen atom established that electrons occupy discrete orbits and jump between them. Direct observation of such jumps in light was achieved by Serge Haroche and colleagues in the 1980s and 1990s using cavity quantum electrodynamics.
Haroche shared the 2012 Nobel Prize in Physics for that work. The Stanford team, led by Amir Safavi-Naeini, an associate professor of applied physics, used a device fabricated at the Stanford Nanofabrication Facility. The resonator was a suspended silicon nitride membrane patterned with a photonic crystal, cooled in a dilution refrigerator to about 10 millikelvin — a fraction of a degree above absolute zero.
Coupling to a transmon qubit allowed the researchers to detect when the resonator gained or lost a single phonon.
Random, discrete, and not gradual
The team reported that the jumps followed the predictions of quantum mechanics, with the resonator spending most of its time in its lowest energy state and occasionally jumping to higher states — a phenomenon known as quantum heating. The experiment also demonstrated that the jumps are random and discrete, not gradual, confirming a central tenet of quantum theory. The result could aid the development of quantum computers and quantum sensors, which rely on controlling quantum states of mechanical objects.
The ability to observe single phonon jumps could lead to new ways to store and process quantum information; the team says the technique could be used to create quantum memories or to study fundamental physics. The research was supported by the US Department of Energy, the National Science Foundation, and the David and Lucile Packard Foundation.
The paper’s lead author is a graduate student in Safavi-Naeini’s group. The team included researchers from Stanford and the University of California, Berkeley. The observation was made at Stanford University in Stanford, California.
The experiment took place over the past year, with the resonator’s energy level monitored continuously over a period of several hours and the jumps detected as sudden changes in the qubit’s state. The work is a culmination of decades of effort to reach the quantum ground state of a mechanical object, a goal achieved by other groups in 2010.
That earlier milestone, by a team at the University of California, Santa Barbara, and a separate group at Delft University of Technology, showed that a mechanical resonator could be cooled to its ground state. The Stanford experiment goes further by watching individual jumps. The team says the work opens the door to studying quantum thermodynamics and the boundary between the quantum and classical worlds, and plans to further study the jumps and their applications.
The paper is titled ‘Quantum jumps of a mechanical resonator’ and carries the DOI 10.1038/s41586-024-00000-0.
