Physicists count sound particles with quantum microphone

Stanford physicists count sound particles with quantum microphone
Artist's impression of an array of nanomechanical resonators designed to generate and trap sound particles, or phonons. The mechanical motions of the trapped phonons are sensed by a qubit detector, which shifts its frequency depending on the number of phonons in a resonator. Different phonon numbers are visible as distinct peaks in the qubit spectrum, which are shown schematically behind the resonators. Credit: Wentao Jiang

Stanford physicists have developed a "quantum microphone" so sensitive that it can measure individual particles of sound, called phonons.

The device, which is detailed July 24 in the journal Nature, could eventually lead to smaller, more efficient computers that operate by manipulating sound rather than light.

"We expect this device to allow new types of quantum sensors, transducers and for future quantum machines," said study leader Amir Safavi-Naeini, an assistant professor of applied physics at Stanford's School of Humanities and Sciences.

Quantum of motion

First proposed by Albert Einstein in 1907, phonons are packets of vibrational energy emitted by jittery atoms. These indivisible packets, or quanta, of motion manifest as sound or heat, depending on their frequencies.

Like photons, which are the quantum carriers of light, phonons are quantized, meaning their vibrational energies are restricted to discrete values—similar to how a staircase is composed of distinct steps.

"Sound has this granularity that we don't normally experience," Safavi-Naeini said. "Sound, at the quantum level, crackles."

The energy of a mechanical system can be represented as different "Fock" states—0, 1, 2, and so on—based on the number of phonons it generates. For example, a "1 Fock state" consist of one phonon of a particular energy, a "2 Fock state" consists of two phonons with the same energy, and so on. Higher phonon states correspond to louder sounds.

Until now, scientists have been unable to measure phonon states in engineered structures directly because the energy differences between states—in the staircase analogy, the spacing between steps—is vanishingly small. "One phonon corresponds to an energy ten trillion trillion times smaller than the energy required to keep a lightbulb on for one second," said graduate student Patricio Arrangoiz-Arriola, a co-first author of the study.

To address this issue, the Stanford team engineered the world's most sensitive microphone—one that exploits quantum principles to eavesdrop on the whispers of atoms.

In an ordinary microphone, incoming sound waves jiggle an internal membrane, and this physical displacement is converted into a measurable voltage. This approach doesn't work for detecting individual phonons because, according to the Heisenberg uncertainty principle, a quantum object's position can't be precisely known without changing it.

"If you tried to measure the number of phonons with a regular microphone, the act of measurement injects energy into the system that masks the very energy that you're trying to measure," Safavi-Naeini said.

Instead, the physicists devised a way to measure Fock states—and thus, the number of phonons—in sound waves directly. "Quantum mechanics tells us that position and momentum can't be known precisely—but it says no such thing about energy," Safavi-Naeini said. "Energy can be known with infinite precision."

Singing qubits

The quantum microphone the group developed consists of a series of supercooled nanomechanical resonators, so small that they are visible only through an electron microscope. The resonators are coupled to a superconducting circuit that contains electron pairs that move around without resistance. The circuit forms a quantum bit, or qubit, that can exist in two states at once and has a natural frequency, which can be read electronically. When the mechanical resonators vibrate like a drumhead, they generate phonons in different states.

"The resonators are formed from periodic structures that act like mirrors for sound. By introducing a defect into these artificial lattices, we can trap the phonons in the middle of the structures," Arrangoiz-Arriola said.

Like unruly inmates, the trapped phonons rattle the walls of their prisons, and these mechanical motions are conveyed to the qubit by ultra-thin wires. "The qubit's sensitivity to displacement is especially strong when the frequencies of the qubit and the resonators are nearly the same," said joint first-author Alex Wollack, also a graduate student at Stanford.

However, by detuning the system so that the qubit and the resonators vibrate at very different frequencies, the researchers weakened this mechanical connection and triggered a type of quantum interaction, known as a dispersive interaction, that directly links the qubit to the phonons.

This bond causes the frequency of the qubit to shift in proportion to the number of phonons in the resonators. By measuring the qubit's changes in tune, the researchers could determine the quantized energy levels of the vibrating resonators—effectively resolving the phonons themselves.

"Different levels appear as distinct peaks in the qubit spectrum," Safavi-Naeini said. "These peaks correspond to Fock states of 0, 1, 2 and so on. These multiple peaks had never been seen before."

Mechanical quantum mechanical

Mastering the ability to precisely generate and detect phonons could help pave the way for new kinds of quantum devices that are able to store and retrieve information encoded as particles of sound or that can convert seamlessly between optical and mechanical signals.

Such devices could conceivably be made more compact and efficient than quantum machines that use photons, since phonons are easier to manipulate and have wavelengths that are thousands of times smaller than light particles.

"Right now, people are using photons to encode these states. We want to use phonons, which brings with it a lot of advantages," Safavi-Naeini said. "Our device is an important step toward making a 'mechanical quantum mechanical' computer."


Explore further

Coupling qubits to sound in a multimode cavity

More information: Patricio Arrangoiz-Arriola et al. Resolving the energy levels of a nanomechanical oscillator, Nature (2019). DOI: 10.1038/s41586-019-1386-x
Journal information: Nature

Citation: Physicists count sound particles with quantum microphone (2019, July 27) retrieved 23 August 2019 from https://phys.org/news/2019-07-physicists-particles-quantum-microphone.html
This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only.
7447 shares

Feedback to editors

User comments

Jul 27, 2019
These Phonon Rock Stars

A phonon is a collective excitation in a periodic elastic arrangement of atoms
The concept of phonons
Introduced in 1932
By Soviet physicist Igor Tamm
Analogous to the word photon
According to Wikipedia
But
Not so this phys.org
Foreth
First proposed by Albert Einstein in 1907
Phonons are packets of vibrational energy
Emitted by jittery atoms
As sound or heat, depending on their frequencies

We have these two Phonon Rock Stars
Igor Tamm in 1932 in the red corner
Albert Einstein in 1907 in the blue corner
Competing for this prize

Jul 27, 2019
Sounds fantastic.

Jul 27, 2019
My physics is somewhat behind me, but in view of this comment: "'Quantum mechanics tells us that position and momentum can't be known precisely—but it says no such thing about energy," Safavi-Naeini said. "Energy can be known with infinite precision.'",

what happened to
ΔEΔt≥ℏ2?

Jul 27, 2019
Does anyone know what paper/talk/comment Einstein supposedly made in 1907 regarding phonons or what became to be know as phonons? Wikipedia says: "The concept of phonons was introduced in 1932 by Soviet physicist Igor Tamm."

Jul 27, 2019
Does anyone know what paper/talk/comment Einstein supposedly made in 1907 regarding phonons or what became to be know as phonons? Wikipedia says: "The concept of phonons was introduced in 1932 by Soviet physicist Igor Tamm."
If your puzzlement is based on what granville wrote, be advised that his/her comments are a hybrid of free-verse and science.

Jul 27, 2019
My physics is somewhat behind me, but in view of this comment: "'Quantum mechanics tells us that position and momentum can't be known precisely—but it says no such thing about energy," Safavi-Naeini said. "Energy can be known with infinite precision.'",
what happened to
 ΔEΔt≥ℏ2?
Even the "...position and momentum can't be known precisely..." I find on the ambiguous side. The precision of one measure is inversely proportional to that of the other, but one or the other can be determined with some precision. An analogy would lie fairly in adjoining walls of a (square) building. To the extent you can fully see one side, the other is highly obscured.

Jul 27, 2019
What's the odds, that the ear does this?

Jul 27, 2019
@Ophelia, you should get some idea what they're talking about from this Wikipedia article: https://en.wikipe...in_solid

As for the time-energy uncertainty principle, yes, I think that's correct. Obviously someone had a brain fart.

Jul 27, 2019
"wavelengths that are thousands of times smaller than light particles"

No way. Neat. I thought "light" supposedly covered every wavelength. Guess not.

Jul 27, 2019
Pretty sure you can get an exact measurement on a photon but the process destroys the photon, so the conjugates of Heisenberg's UP in that case do not actually fit the application. Fully destructive measurements really do not relate to the HUP at all, but the HUP can apply to nondestructive ways of looking at photons.

Jul 27, 2019
The HUP starts to become irrelevant as the mass involved increases, so I do not suppose putting
HUP constraints on a phonon is necessarily meaningful.

Jul 27, 2019
When it comes to virtual particles of the vacuum you can view the HUP as setting a limit on the energies and lifetimes of the virtual particles. Otherwise a decent template for using the HUP seems to be that you can measure both conjugate variables, if doing so one at a time, e.g. alternately, to any desired precision, however the precision applied to one conjugate variable measurement introduces uncertainty to the previous precise measure of the other conjugate variable, and this process of alternate interfering measurements can be repeated indefinitely. Of course this cannot apply to a completely destructive measurement of either conjugate variable. I have no experience with this stuff so I would not claim to be an expert, I have merely read some things on the uncertainty principle.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more