‘Quantum microphone’ can count individual particles of sound

A new “quantum microphone” is so sensitive that it can measure individual particles of sound, called phonons, researchers report.

The device could eventually lead to smaller, more efficient quantum computers that operate by manipulating sound rather than light.

“We expect this device to allow new types of quantum sensors, transducers, and storage devices for future quantum machines,” says study leader Amir Safavi-Naeini, an assistant professor of applied physics at Stanford University.

Eavesdropping on atoms

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

“Sound has this granularity that we don’t normally experience… Sound, at the quantum level, crackles.”

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 says. “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” consists 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 havn’t been able 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,” says co-first author Patricio Arrangoiz-Arriola, a graduate student.

“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…”

To address this issue, the researchers 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 says.

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 says. “Energy can be known with infinite precision.”

How does the quantum microphone work?

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 researchers coupled the resonators 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 researchers 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 says.

The trapped phonons then rattle the walls of their prisons and ultra-thin wires convey these mechanical motions to the qubit.

“The qubit’s sensitivity to displacement is especially strong when the frequencies of the qubit and the resonators are nearly the same,” says co-first author Alex Wollack, also a graduate student.

By detuning the system so that the qubit and the resonators vibrate at very different frequencies, however, 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 phonon energy levels appear as distinct peaks in the qubit spectrum,” Safavi-Naeini says. “These peaks correspond to Fock states of 0, 1, 2, and so on. These multiple peaks had never been seen before.”

Better quantum computers?

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

Such devices could conceivably be 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 says. “Our device is an important step toward making a ‘mechanical quantum mechanical’ computer.”

The research appears in the journal Nature. Funding for the research came from the David and Lucile Packard Fellowship, the Stanford University Terman Fellowship, and the US Office of Naval Research.

Source: Stanford University