Aug 17, 2019 | Updated: 07:24 AM EDT

Quantifying Sound Particles Through a Quantum Microphone

Aug 09, 2019 05:34 AM EDT

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Sound Particles
(Photo : Pixabay)

Imagine quantifying sound rather than light. This was done successfully by physicists from Stanford Univesrity who measured phonons, or individual sound particles, through a quantum microphone. 

The study was published in the journal Nature. It details the possibility of producing quantum computers that are smaller and more efficient that can be manipulated by sound rather than light. 

"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.

Albert Einstein proposed phonons in 1907. These packets of vibrational energy are emitted by moving atoms and are called as quanta, dependent on their frequencies, which manifest as sound or heat.

Phonons are quantized like photons. These means that their vibrational energies have discrete values. This is similar to a staircase with distinct steps. 

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

Different "Fock" states - 0,1,2,3, and so on -  represent the energy of a mechanical system. These are based on the generated number of phonons. One phonon of a particular energy comprise "1 Fock state" while two phonons with the same energy comprise "2 Fock state." The higher phonon state, the greater the sound levels. 

Previous methods were unable to quantify phonon states because of the minute energy differences. "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.

The Stanford scientists resolved this issue by designing the most sensitive microphone in the world that utilizes quantum principles to detect the sound 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," according to Eureka Alert

"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.

With this, the scientists utilized Fock states in measuring 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."

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