Adding or subtracting a quantum of sound

Technology News | January 26, 2021

By Rich Pell

Materials & processes Test and Measurement Analog IoT Wireless Communications Electromechanical Optoelectronics

Similar to the photon, which is a single particle – or quantum – of light, the phonon can be thought of as the smallest unit of sound energy. The findings of the experiment, say the researchers, aid the development of future quantum technologies, such as hardware components in a future quantum internet, and help pave the way for tests of quantum mechanics on a more macroscopic scale.

To add or subtract a single quantum of sound, the researchers experimentally implemented a technique that exploits correlations between photons and phonons created inside a resonator. More specifically, laser light is injected into a crystalline microresonator that supports both the light and the high-frequency sound waves.

The two types of waves then couple to one another via an electromagnetic interaction that creates light at a new frequency. Then, to subtract a single phonon, the researchers detect a single photon that has been up-shifted in frequency.

“Detecting a single photon gives us an event-ready signal that we have subtracted a single phonon,” says lead author of the project Georg Enzian.

When the experiment is performed at a finite temperature, the sound field has random fluctuations from thermal noise. As a result, at any one time, the exact number of sound quanta present is unknown but on average there will be n phonons initially. When a single phonon is added or subtracted, it might be expected that this would simply change the average to n + 1 or n – 1, respectively.

However, say the researchers, the actual outcome defies this intuition. Instead, when a single phonon is subtracted, the average number of phonons actually goes up to 2n. This surprising result – where the mean number of quanta doubles – has been observed for all-optical photon-subtraction experiments and is observed for the first time outside of optics here.

“One way to think of the experiment is to imagine a claw machine that you often see in video arcades, except that you can’t see how many toys there are inside the machine,” says Michael Vanner, Principal Investigator of the Quantum Measurement Lab at Imperial College London, which was involved in the research. “Before you agree to play, you’ve been told that on average there are n toys inside but the exact number changes randomly each time you play. Then, immediately after a successful grab with the claw, the average number of toys actually goes up to 2n.”

This result, say the researchers, does not violate energy conservation and comes about due to the statistics of thermal phonons. The results, combined with the researchers’ recent experiment that reported strong coupling between light and sound in a microresonator, are thought to open a new path for optomechanical quantum science and technology with sound waves.

For more, see “Single-Phonon Addition and Subtraction to a Mechanical Thermal State.”