Stanford catches single phonons jumping to zero inside a two-millisecond ringdown

A phonon, the quantum unit of sound, is the coordinated movement of a large group of atoms ^[7]. Measuring one runs into a standing problem in quantum engineering: how to get a signal out of a quantum system without disturbing its fragile state ^[12]. Here the superconducting qubit checks repeatedly during the two milliseconds of vibration whether the phonon is in energy state 1 or 0 ^[14].

Hundreds of those checks fit inside the ringdown, and they are what locate the moment the vibrational energy fell from 1 to 0 ^[10]. Divide two milliseconds by 200 readings and each one has 10 microseconds; divide by 1,000 and each has 2 ^[11]. Earlier experiments had found evidence that phonons jump, and this is the first record of individual phonons making the transition as it happened ^[6].

Building a very long-lived vibrating object and a working electrical detector on the same device was the fabrication problem. Makihara, a recent Stanford doctoral graduate, said the team "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," and had to do that "without ruining either subsystem" ^[15].

Safavi-Naeini framed the result in terms of what the platform now permits. "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," said Safavi-Naeini, associate professor of applied physics in the Stanford School of Humanities and Sciences ^[16]. He also said the work "will allow us to move forward with developing new quantum technologies with sound" ^[17].

The phys.org account calls the study a foundational first step ^[22], and it does not report a readout fidelity, the number of jumps recorded, or any comparison with photonic or trapped-ion hardware ^[23]. Those comparison classes have long head starts. Trapped ions showed quantum jumps in 1986 and photons in 2007, 21 years later ^[3][4][5], against a theory that dates to the early 1900s ^[2].

The nearer application is sensing, where the small size and high sensitivity of the resonator and qubit together are the draw ^[20]. Safavi-Naeini's group is already working with the physicist Michael Roukes and his team at Caltech to use the platform to detect and identify proteins within cells ^[21].