Stanford Watched Sound Make a Quantum Jump
Sound jumped. Physics finally watched.
In 1913, Niels Bohr proposed that electrons do not slide between energy levels. They vanish from one and appear instantly in another. The jump is not a transition. It is a discontinuity. That prediction reshaped physics, and for the next century it was confirmed in one particle after another. Trapped ions in 1986. Single photons in 2007. The one particle missing from the list was the phonon, the quantum unit of sound. Last week, Stanford physicists filled that gap. A team led by Associate Professor Amir Safavi-Naeini published the first direct observation of a real-time quantum jump in sound, in the journal Science. They watched a single phonon abruptly vanish from an energy state of 1 and reappear at 0. Not fade. Not drift. Vanish and reappear, just as Bohr described.
Credit: Pixabay/CC0.
Abstract rendering of a sound wave at the quantum scale, showing the burst-like discontinuities that characterise a phonon transitioning between energy levels.
A tuning fork that rings for several hours, scaled down to a chip
The device at the centre of the experiment is a microscopic mechanical resonator, small enough that many could fit on a fingernail, fabricated using the same chipmaking techniques used to build semiconductors. Think of it as a tuning fork, but at a scale where the rules of quantum mechanics take over from the rules of classical acoustics. At that scale, the resonator does not vibrate continuously and then slow down the way a struck bell does. Its vibrational energy is quantised: it can hold exactly zero phonons, or one phonon, or two, and nothing in between. The experiment targeted the transition between one and zero.
Two milliseconds, 294 measurements, one abrupt jump
The resonator vibrates for two milliseconds. Barely perceptible to a human, but in quantum terms it is a long time. Takuma Makihara, co-first author and a recent Stanford doctoral graduate, described what that required: developing new fabrication processes to keep the resonator vibrating long enough to measure, then coupling it to the qubit without breaking either. If the same vibrational quality were found in a room-sized tuning fork, it would ring for several hours. Coupled to the resonator was a superconducting qubit, acting as a detector. The qubit could read the phonon number hundreds of times without destroying the quantum state. The team ran 294 consecutive nondemolition measurements within that two-millisecond window. For many readings, the phonon was present. Then the pattern broke: the resonator jumped to zero. Abruptly, with no in-between.
"What this study shows will allow us to move forward with developing new quantum technologies with sound.
"Amir Safavi-Naeini · Associate Professor of Applied Physics · Stanford
Quantum jumps theorised
1913, Niels Bohr
First in trapped ions
1986
First in photons
2007
First in phonons (sound)
2026
Measurements in one 2ms window
294 consecutive QND readings
Quantum nondemolition fidelity
~99%
Resonator vibration lifetime
2ms (equivalent to hours at room scale)
What it opens
The most immediate application is quantum error correction. Quantum computers lose information when qubits decay unexpectedly. Catching a quantum jump the moment it happens is the first step toward correcting it before it propagates. Phonon-based systems, being mechanical rather than purely electronic, can be made compact and integrated easily with existing chip designs. Further out, Safavi-Naeini's group is collaborating with physicist Michael Roukes at Caltech to detect and identify individual proteins within living cells. A sensor that can resolve a single phonon can detect mass changes too small for any conventional instrument. The same principle applies to measuring force, acceleration, and strain.
The third application is the one with the largest existing market: \
smartphones. Sound is integral to how phones work, from microphones to oscillators. Erik Szakiel, co-first author and doctoral student in Safavi-Naeini's lab, put it plainly.
"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.
"Erik Szakiel · Doctoral student · Stanford Applied Physics