Stanford watches a microscopic tuning fork lose its last phonon

Two milliseconds of ringing is what made the experiment possible. The Stanford resonator holds its vibration for periods of two milliseconds ^[11], which Discover reports is equivalent to a normal-sized vibration lasting hours ^[13]. The long ring time means the device can take hundreds of readings and pinpoint the exact moment the energy state changes ^[14]. Read "hundreds" as two hundred and each reading has roughly ten microseconds to work in ^[3].

"A normal tuning fork sounds like it gradually gets quieter and quieter," said Amir Safavi-Naeini, associate professor of applied physics at Stanford ^[8]. "The quantum version is more like an extremely tiny tuning fork where, if you could directly measure its energy, the last unit of vibration stays there for some random amount of time and then disappears all at once." ^[12]

A phonon is the coordinated movement of many atoms ^[7], so the thing making the jump here is a mechanical object with many degrees of freedom. The harder problem was reading it without disturbing the phonon ^[16]. Takuma Makihara and Erik Szakiel, the paper's co-first authors, established a process that couples the resonator to a superconducting qubit, which acts as the detector and identifies the moment the phonon count goes from one to zero ^[17]. "Physically, the resonator goes from having one quantum of vibrational energy, one phonon, to having zero," said Safavi-Naeini ^[15].

Bohr put jumps into theory in the 1910s, according to the American Physical Society ^[4]. Direct observation waited until 1986 in trapped ions ^[5] and 2007 in photons ^[6]: twenty-one years between those two ^[1], and eighteen more before the Stanford group had its first jump data in the spring of 2025 ^[2].

The group did not publish that data. "But you had to squint to see the jumps," Safavi-Naeini told Discover of those first measurements ^[9], and the team refined the technology to reduce noise until the result would "pop out" ^[10]. "When they showed me the results from the new chip, it was clear that we had the result; I was extremely excited," he said ^[18].

The readout still perturbs what it measures. "Our measurement is very good but not perfectly non-destructive," said Safavi-Naeini ^[20]. Whether the same phonon can be interrogated twice depends on how large that disturbance is, and Discover's account puts no number on it.

Discover reports that jumps had already been recorded in ions and in photons, and that earlier phonon experiments found evidence of such leaps without observing one directly ^[2]. The researchers said the ability to detect phonon jumps could one day lead to advances in quantum computing, quantum sensing, and everyday technologies that involve sound ^[19].