Photons and phonons combine for quantum solution

Collective vibrations in silicon crystals could provide memory for quantum computers, writes Michael Lucy.

A conceptual illustration showing the crystal (middle), a phonon (top) and the quantum probability distribution (bottom).
A conceptual illustration showing the crystal (middle), a phonon (top) and the quantum probability distribution (bottom).
Moritz Forsch / TU Delft

Vibrations in crystals can obey the laws of quantum mechanics and might be used to store information in quantum computers, according to new research published in the journal Science.

The new development, achieved by Sungkun Hong at the University of Vienna in Austria and colleagues, revolves around the idea of the phonon. Not to be confused with a photon (a quantum of light), a phonon is a collective movement of particles vibrating together.

For an analogue at the macroscopic scale, think of a single wave moving through the ocean – after the wave passes, the water molecules that made it go back to where they began, but the wave itself moves on.

At the microscopic, quantum level, a phonon behaves even more like a particle, to the point where it is often referred to as a “quasiparticle”. Physicists think phonons may provide a useful bridge between quantum and classical worlds, and the interactions between light and vibrations are a very active area of research.

Hong’s team analysed phonons using an optomechanical crystal – described as a “microfabricated silicon nanobeam” – that is designed to vibrate in a specific way when struck by a photon.

The researchers fired photons at the device, creating phonons within it. They then fired photons of a different frequency, which were reflected back after interacting with those phonons.

They then analysed the reflected photons via a process called Hanbury Brown and Twiss interferometry, which lets them gain information about the quantum state of the phonons. Using this technique, they proved that a single phonon in the crystal obeys the laws of quantum mechanics rather than classical physics.

Because of this quantum property, and the ability manipulate them with light, phonons in crystals could be “an ideal candidate for storage of quantum information”, the authors say.

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