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Physicists count sound particles with quantum microphone

  Stanford physicists count sound particles with a quantum microphone
Artistic representation of a series of nanomechanical resonators designed to generate and trap sound particles or phonons. The mechanical movements of the captured phonons are detected by a qubit detector, which shifts its frequency as a function of 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. Picture credits: Wentao Jiang

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

The device, which is described on July 24 in the journal Nature could eventually lead to smaller, more efficient quantum computers that use sound rather than light.

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

Quantum of Motion

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

Like photons, which are quantum carriers of light, phonons are quantized, which means that their vibrational energies are limited to discrete values ​​- similar to how a staircase is composed of different stages.

"Sound has this graininess that we normally do not experience," said Safavi-Naeini. "Sound at quantum level crackles."

The energy of a mechanical system can be represented as different "jib" states – 0, 1, 2, etc. – based on the number of phonons it generates. For example, a "1-jib state" consists of one phonon of a given energy, a "2 jib state" of two phonons of the same energy, and so on. Higher phoneme states correspond to loud noises.

Until now, scientists could not directly measure phonon states in constructed structures, because the energy differences between states – in the stairway analogy the distance between stages – are negligible. "A phonon equals an energy ten billion trillion times smaller than the energy required to keep a light bulb on for a second," co-lead author of the study, Patricio Arrangoiz-Arriola, said.

The Stanford team developed the world's most sensitive microphone – one that uses quantum principles to eavesdrop on the whisper of atoms.

In a normal microphone, incoming sound waves wobble with an inner membrane, and this physical displacement is transformed into a measurable voltage. This approach does not work for the detection of single phonons, because the position of a quantum object can not be determined exactly by the Heisenberg uncertainty principle without changing it.

"When you've tried to measure the number of phonons with a regular microphone, the measurement injects energy into the system that masks exactly the energy you want to measure," Safavi-Naeini said.

Instead, the physicists developed a method for measuring the jib states – and thus the number of states phonons – directly in sound waves. "Quantum mechanics tells us that position and momentum are not exactly known – but it does not say anything about energy," Safavi-Naeini said. "Energy can be known with infinite precision."

Singing Qubits

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

"The resonators consist of periodic structures that act like sound mirrors, and by introducing a defect into these artificial lattices, we can place the phonons in the middle of the structures," said Arrangoiz-Arriola.

Like defiant inmates, the captured phonons clatter on the walls of their prisons, and these mechanical movements are transmitted to the qubit by ultra-thin wires. "The sensitivity of the qubit to shifts is particularly high when the frequencies of the qubit and the resonators are almost equal," said joint lead author Alex Wollack, also a graduate student in Stanford The qubit and resonators vibrate at very different frequencies, weakening the researchers This mechanical connection triggers a kind of quantum interaction, known as the dispersive interaction, which connects the qubit directly to the phonons.

This binding causes the frequency of the qubit to shift in proportion to the number of phonons in the resonators. By measuring the mood changes of the qubit, the researchers were able to determine the quantized energy levels of the vibrating resonators – effectively dissolving the phonons themselves.

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

Mechanical Quantum Mechanics

Mastering the Ability to Precisely Create and Capture Phonons could help pave the way for novel quantum devices capable of storing information encoded as sound particles or seamlessly convert between optical and mechanical signals.

Such devices could potentially be made more compact and efficient than quantum machines that use photons because phonons are easier to manipulate and have wavelengths that are a thousand times smaller than light particles.

"People are currently using photons to encode these states, we want to use phonons, which entails many benefits," Safavi-Naeini said. "Our device is an important step in the direction of a 'mechanical quantum mechanical' computer."

Coupling of qubits to sound in a multimode resonator

Further information:
Patricio Arrangoiz-Arriola et al. Resolution of the Energy Levels of a Nanomechanical Oscillator, Nature (2019). DOI: 10.1038 / s41586-019-1386-x

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Stanford University

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Physicists count sound particles with quantum microphone (July 27, 2019)
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