When optics researchers get together over coffee to discuss their favorite cavities, attention turns to microtoroids, tiny glass spheres, and photonic crystals with defects. These cavities all have a few things in common: they are all really good at storing light, and they are high-precision devices.

Physicists use a number, called Q, to characterize how well an optical cavity stores light: the higher the Q, the longer light stays confined in the cavity. You do not, it turns out, go blithely into your local fab and turn out a high-Q cavity. It takes a lot of practice and trial and error to create a high-Q optical cavity.