Scientists at Stanford have captured sound making sudden quantum jumps for the first time, an accomplishment that ushers in a new era for quantum computers and sensors capable of detecting the faintest biological signals. The work is described in Science.
Since the early 1900s, quantum theory has explained the sudden transitions between energy states known as quantum jumps. Researchers initially observed these in trapped ions in 1986, and later in photons in 2007. For sound, though, catching a quantum jump in the act has proven more difficult.
Things have shifted. Stanford physicist Amir Safavi-Naeini led the team whose new study directly followed individual phonons, the quantum unit of sound, as they jumped between energy states in real time.
The Phonon as Quantum Unit
The tiniest piece of light is the photon. Sound, in turn, has its own smallest unit, the phonon, which describes the joined motion of many atoms.
Everyday sound dies away gradually, like a ringing bell that grows quieter until the noise fades entirely. At the quantum level, though, the story changes. A resonator’s vibrational energy shifts in particular steps, showing the same kind of behavior that ions and photons display.
Some earlier experiments had indicated that sound could undergo these transitions. Now, this new study has gone further, actually watching individual phonons make each of the jumps directly.
A Microscopic Resonator That Rings Long
The experiment’s mechanical resonator was constructed using chip fabrication methods. Because of its small dimensions, multiple of these resonators can be placed on a single chip to handle intricate functions.
The length of time the device kept on vibrating was a standout feature. It functions as a tiny tuning fork, capable of oscillating for two milliseconds. If a regular-sized tuning fork operated with the same persistence, it would sound for several hours.
The extended “ringdown time” allowed the researchers to gather hundreds of readings, which in turn enabled them to pinpoint the instant when the vibration ceased and the sound leapt from an energy state of 1 to 0.
Measuring Without Ruining the State
The experiment tackled a difficult problem in the field of quantum engineering. The question was how to observe what is happening inside a quantum system without disturbing the sensitive state that is being measured.
Takuma Makihara and Erik Szakiel, who were co-first authors on the study, discovered a means of pairing the tiny mechanical resonator with a superconducting qubit. This electrical circuit is able to hold quantum information and served as the detector in this arrangement.
Makihara, a recent Stanford doctoral graduate, is the one who said “We 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 — without ruining either subsystem,”.
Over the two milliseconds of vibration, the qubit keeps checking the mechanical resonator to see if the phonon sits at an energy state of 1 or 0. Through repeated measurements, the researchers can pinpoint exactly when the quantum jump happens.
From Quantum Error Correction to Protein Detection
The researchers regard the findings as an initial yet significant move toward technologies that treat sound itself as a foundation for quantum work.
Quantum computers hold the promise of solving problems that conventional machines cannot handle, but their quantum states come apart easily. Errors can occur before a calculation is finished, which is where quantum error correction comes in.
When a quantum computer encounters an error, a quantum jump often signals the problem. Catching those jumps has proven difficult, which is why the capacity to monitor them in sound now offers a valuable new way to spot and fix quantum errors.
The mechanical resonator and qubit pair might develop into a highly sensitive detection platform, and the Safavi-Naeini’s group is already working with physicist Michael Roukes’ team at Caltech is testing whether the system can sense and distinguish proteins within cells.
Better Control of Sound
Beyond specialized quantum technologies, this kind of advance could eventually find more general uses. Sound carries great significance in smartphones and other electronic devices, and ever more exact command over vibrations could aid new generations of those technologies, according to Szakiel, a current doctoral student in Safavi-Naeini’s lab.
“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,” he said.
Stanford Q-FARM and Bio-X also claim Safavi-Naeini as a member. Among additional co-authors from the university, David Schuster, who holds the Joan Reinhart Professor chair and serves as a professor of applied physics in H&S, is listed. Shannon Harvey, a scientist with SLAC Nati, rounds out the group of authors from Stanford.
This research represents a recent step forward in the long history of quantum physics, which dates back more than a century. Before sound, scientists had witnessed quantum jumps in trapped ions and photons.
- Trapped ions showed quantum jumps in 1986.
- Photons showed jumps in 2007.
- Now, sound has joined the list.
The research adds sound to the list, doing so through direct tracking of individual leaps.
Source material: “For the first time, scientists watch sound jump between quantum states,” ScienceDaily.
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