# Cavity optomechanics

Cavity optomechanics studies the interaction between light and mechanical oscillators. The oscillators can take many forms—such as a tiny drum, miniature string, or a cloud of atoms. Photons—like other particles—exert pressure and cause the mechanical oscillator to move when they reflect off it. This radiation pressure is, under normal circumstances, much weaker than any other force and difficult to observe.

This effect can be enhanced inside an optical resonator. First, one makes sure that the mechanical oscillator reflects a lot of light. This ensures that the radiation pressure is maximal—the more photons are reflected, the larger the pressure. Then, one takes a second mirror and places it facing the mechanical oscillator. When light can bounce many times between the two mirrors, each photon kicks the mechanical oscillator many times. And more kicks add up to a larger pressure.

But why do physicists go to such lengths to couple light and mechanical motion?

Using light, we can control the motion of the mechanical oscillator. We can cool the oscillator to its quantum ground state. Every oscillator shakes a little but we can use light to make it sit still. The oscillator can also serve as a memory for light—we can transfer information encoded in light onto the mechanical motion and back. Or we can use light to measure the oscillator’s motion very precisely. The most famous example of such a measurement are gravitational wave detectors. These devices can measure movements as small as a hundred thousandth of a proton!

With such a level of control, we can use mechanical oscillators for a many applications.
We can measure oscillator’s motion to determine external forces acting upon it. This is useful for measuring weak magnetic fields, torques, or small masses.

We can test the limits of quantum physics. If we control the motion using a quantum system (light), its behaviour should also follow the laws of quantum mechanics. But the macroscopic world (which includes mechanical oscillators) is classical. We can thus study where the boundary between the quantum and the classical lies and what it looks like.

We can also use mechanical oscillators as transducers—devices that convert signals from one form to another. Vibrations already serve this way in our smartphones where they filter and convert electrical signals. By coupling them to light, we can use similar effects in fibre-optic communications or, in the future, in quantum communication.