Adding or subtracting single quanta of sound

Date:
January 25, 2021
Source:
Imperial College London
Summary:
Researchers perform experiments that can add or subtract a single
quantum of sound -- with surprising results when applied to noisy
sound fields.



FULL STORY ========================================================================== Researchers perform experiments that can add or subtract a single quantum
of sound -- with surprising results when applied to noisy sound fields.


========================================================================== Quantum mechanics tells us that physical objects can have both wave and particle properties. For instance, a single particle -- or quantum -- of
light is known as a photon, and, in a similar fashion, a single quantum
of sound is known as a phonon, which can be thought of as the smallest
unit of sound energy.

A team of researchers spanning Imperial College London, University of
Oxford, the Niels Bohr Institute, University of Bath, and the Australian National University have performed an experiment that can add or subtract
a single phonon to a high-frequency sound field using interactions with
laser light.

The team's findings 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. The
details of their research are published today in the journal Physical
Review Letters.

To add or subtract a single quantum of sound, the team experimentally
implement a technique proposed in 2013 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 team 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. Thus, at any one time,
the exact number of sound quanta present is unknown but on average there
will be n phonons initially.

What happens now when you add or subtract a single phonon? At first
thought, you may expect this would simply change the average to n +
1 or n -- 1, respectively, however the actual outcome defies this
intuition. Indeed, quite counterintuitively, when you subtract a single
phonon, 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. 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," describes Michael
Vanner, Principal Investigator of the Quantum Measurement Lab at Imperial College London.

It's important to note that this result certainly does not violate energy conservation and comes about due to the statistics of thermal phonons.

The team's results, combined with their recent experiment that reported
strong coupling between light and sound in a microresonator, open a new
path for quantum science and technology with sound waves.


========================================================================== Story Source: Materials provided by Imperial_College_London. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. G. Enzian, J. J. Price, L. Freisem, J. Nunn, J. Janousek,
B. C. Buchler, P. K. Lam, M. R. Vanner. Single-
Phonon Addition and Subtraction to a Mechanical Thermal
State. Physical Review Letters, 2021; 126 (3) DOI:
10.1103/PhysRevLett.126.033601 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/01/210125112308.htm

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