Technology 3 min read

Physicists Develop Quantum Microphone to Measure Sound Particles

Image courtesy of Shutterstuck

Image courtesy of Shutterstuck

Physicists at Stanford University have developed a quantum microphone that’s so sensitive it can measure phonons – individual sound particles. Thanks to this technology, researchers can one day develop more efficient quantum computers that depend on sound, rather than light.

It could also enable the invention of new types of quantum storage devices, sensors, and transducers.

Back in 1907, Albert Einstein proposed that jitters atoms emit packets of vibrational energy called phonons. And depending on their frequency, these indivisible packets of motion may be expressed as either sound or heat.

Like the quantum carrier of light photons, phonons are also quantized. That means the vibrational energies are limited to discrete values. Think of it as a staircase that’s composed of distinct steps.

Scientists represent the value of mechanical systems as “Fock” states, which could range from 0 to any finite number, based on the number of phonons the system generates.

For example, 1 Fock state is said to consist of one phonon energy, while 2 Fock state equals two phonons, with the same energy – it continues like that. As the phonon state increases, so do the sound’s loudness.

Before now, scientists have never successfully measured phonon states in engineered structure. That’s because the energy differences between states are vanishingly small.

A co-first author of the study and graduate student at Stanford, Patricio Arrangoiz-Arriola said:

“One phonon corresponds to energy ten trillion times smaller than the energy required to keep a lightbulb on for one second.”

As such, the researchers at Stanford had to engineer the world’s most sensitive microphone that could exploit quantum principles to register the sound of particles.

Creating a Quantum Microphone

To create the quantum microphone, the researchers coupled a series supercooled nanomechanical resonators to a superconducting circuit. The circuit forms a quantum bit – or qubit which exist in two states at once and had a natural frequency that researchers could read electronically.

As the mechanical resonators vibrate, they generate phonons in different states.

Arrangoiz-Arriola explained:

“The resonators are formed from periodic structures that act as mirrors for sound. By introducing a defect into these artificial lattices, we can trap the phonons in the middle of the structures.”

That was what precisely what the Stanford team did. The qubit then picks up the mechanical motion generated by the trapped phonons via ultra-thin wires.

The researchers tweaked the system to cause shifting in the qubit’s frequency in proportion to the number of phonons in the resonator. Then, they measured the qubit’s changes in tune to determine the quantized energy levels of the vibrating resonators – and by extension, the phonons.

Study leader and an assistant professor of applied physics at Stanford’s School of Humanities and Sciences, Amir Safavi-Naeini noted:

“Different phonon energy levels appear as distinct peaks in the qubit spectrum. These peaks correspond to Fock states of 0, 1, 2 and so on. These multiple peaks had never been seen before.”

Implications of the Study

The ability to generate and detect phonos could pave the way for new kind of compact quantum devices. For example, we could get a tool that can convert between optical and mechanical signals seamlessly.

Since phonons are easier to manipulate than light particles, such a device would be more compact and efficient than photon-based quantum machines.

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.”

Read More: Toward High-Temp Superconductors for Faster, More Efficient Quantum Computers

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Sumbo Bello

Sumbo Bello is a creative writer who enjoys creating data-driven content for news sites. In his spare time, he plays basketball and listens to Coldplay.

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