Optical cavity--not as painful as a trip to the dentist
Optical cavities are an arrangement of two mirrors facing each other. Typically, light is injected into the cavity along the symmetry axis of the system and then bounces back and forth, forming a standing wave between the mirrors. In this example, one of the mirrors is made to leak out a bit of the light. Because of the boundaries created by the mirrors, the light that is inside the cavity has well-defined characteristics. For instance, the cavity will only build up light that satisfies a resonance condition--the light's wavelength must be a half-integer multiple of the cavity length. This means that cavities can be used to create narrow frequency sources.
Read more to learn more about another way that cavities help quantum mechanics.
Atoms can also be trapped inside the cavity forming a hybrid atom-optical system that is used in quantum research. Cavities generally are described by so-called quality [Q] factors (finesse is also used), which basically characterizes the loss. High Q cavities can trap a single photon such that it will reflect over and over, interacting with an atom (or atoms) numerous times before exiting the cavity. This is called the "strong coupling regime."
Recently, physicists at the Max-Planck-Institut für Quantenoptik have used cavity reflection and how it modifies an atom's quantum state to extract photon properties, without actually destroying the photon. This is neat because usually photons are normally destroyed to get their information. An example of this is a CCD or camera--the photon is absorbed by the CCD in order to extract its information. See this Ars Technica article for more information and this Science article.