Researchers at Stanford University report the first direct observation of quantum jumps in sound, achieving a milestone in the century-long study of quantum phenomena.
The phenomenon of quantum jumps describes abrupt transitions of energy states, a concept with roots that extend back to the beginning of the last century. Although the first demonstrations occurred four decades ago with experiments involving ions, and again two decades later in experiments with photons, quantum jumps had never been observed in sound—until now.
Quantum Leap
In new research published in the journal Science by Amir Safavi-Naeini, a Stanford Associate Professor of Applied Physics, and colleagues, the team reports the first known recording of the phenomenon.
Safavi-Naeini says the breakthrough could help advance quantum technologies that use mechanical vibrations, or sound, to store, manipulate, or detect quantum information.
But what, precisely, does the team’s recent observation of a single phonon—that is, a discrete quantum of mechanical vibration, or sound—jumping from one energy state to another tell us about the quantum nature of sound that past experiments hadn’t yet revealed?
“Previous experiments showed that mechanical resonators have quantized energy levels, but here we can actually watch an individual mechanical system move between those levels in real time,” Safavi-Naeini told The Debrief in an email.
“We repeatedly ask whether the resonator contains one phonon or zero phonons,” Safavi-Naeini said. “For a while, the measurements say mostly ‘one,’ and then at some random moment they abruptly switch to ‘zero.’ That is the quantum jump which hadn’t been demonstrated before.”
Microscopic Resonators
For the team’s research, Safavi-Naeini and his colleagues used a microscopic mechanical resonator, which can maintain its vibration for roughly two milliseconds, allowing researchers enough time to repeatedly measure its quantum state.
Achieving such a long-lived mechanical quantum state didn’t come without its challenges, according to Safavi-Naeini.
“The hard part was making a vibrating object that remains extremely well isolated while also coupling it strongly enough to a detector that we can measure it quickly,” Safavi-Naeini told The Debrief. “Those two requirements tend to conflict.”
Safavi-Naeini says that although the mechanical vibration their resonator produces is short-lived, this still allowed the researchers to perform close to 170 checks within that seemingly short window. “This became possible through years of work on low-loss nanomechanical resonators, superconducting-qubit measurement, and the necessary fabrication techniques to combine them,” Safavi-Naeini says.
Future Applications
One of the most promising aspects of the team’s recent work is how advancing our understanding of quantum jumps in sound could eventually contribute to a range of innovations, which include quantum error correction and extremely sensitive sensors. However, taking the team’s work from being a proof-of-principle experiment to becoming a practical quantum technology will require additional work.
“Currently, each measurement preserves the phonon number about 99% of the time, but because we make many measurements, that small disturbance adds up,” Safavi-Naeini told The Debrief. “[The] next step is a mechanical quantum memory using two resonators coupled to the same qubit, where repeated measurements could tell us when an error has occurred without destroying the stored quantum information.”
Safavi-Naeini also says that since mechanical resonators are naturally sensitive to very small forces and other physical signals, there may be additional applications with remote sensing technologies as well.
“We’re trying to detect single proteins with these devices,” Safavi-Naeini said.
Fundamentally, such applications could allow the integration of multiple technologies at a relatively small scale. If researchers can further improve their ability to monitor mechanical quantum states without disturbing them, the technology could eventually contribute to quantum memories, error correction, and extraordinarily sensitive sensors. Safavi-Naeini’s team is already exploring one striking possibility: using the devices to detect individual proteins.
Thanks to the team’s work, sound may soon offer researchers a new way to explore—and ultimately harness—the effects of the quantum world for use in a range of practical technologies.
The team’s recent paper, “Quantum jumps of sound,” appeared in the journal Science.
Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. A longtime reporter on science, defense, and technology with a focus on space and astronomy, he can be reached at micah@thedebrief.org. Follow him on X @MicahHanks, and at micahhanks.com.