Physicists Develop "Quantum Microphone" that Can Measure Individual Sound Particles

Up to now, it has been impossible to measure phonon states.

Particles of sound known as phonons are packets of vibrational energy emitted by atoms. Depending on their frequencies, they manifest as sound or heat.

Just like photons, which are the quantum carriers of light, phonons are quantized. Their vibrational energies have discrete values.

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Impossible to measure phonon states

However, up to now, it has been impossible to measure phonon states. This is because the energy differences between states is incredibly small. That may all have changed.

Stanford physicists have developed a "quantum microphone" capable of measuring phonons. The device may see the introduction of quantum computers that operate by manipulating sound rather than light, making them smaller and more efficient.

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

The device has been referred to as the "world's most sensitive microphone" and consists of an invention capable of measuring Fock states, and therefore the number of phonons. Fock states represent the energy of a mechanical system.

These states are expressed according to the number of phonons they generate. For instance, a "1 Fock state" consists of one phonon of a particular energy. The higher the Fock state the louder the sound.

"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."

The physicists devised of a quantum microphone made from supercooled nanomechanical resonators coupled with a superconducting circuit that contains electron pairs that move around without resistance. The circuit forms a quantum bit which can be read electronically. 

New kinds of devices

This is exciting because mastering the ability to generate and detect phonons could lead to new kinds of devices that are able to store and retrieve information encoded as particles of sound instead of light. These devices would be more compact and efficient than quantum machines that use photons because phonons are easier to manipulate and have much smaller wavelengths than light particles.

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"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."

The device is detailed in a study in the journal Nature.

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