Will it ever be possible to perceive the grain of light, the particles of light? Suppose one single photon had been isolated in an experiment, a single grain of light trapped in a box with a peephole, stored in a room sealed from light.
—Thomas Hård af Segerstad 
This chapter is about light without matter in a box. Here you will get a quick glimpse of what cavity QED is all about and will be able to appreciate a number of interesting ideas. Having entered the quantum world in the preceding chapters, here we look for its classical limit introducing coherent states, the density matrix, and the P representation. Then we see how nature can be fooled with squeezed states, which have less noise than the standard quantum limit. Among other things, we will see how they can be detected and generated. The latter will lead us to a discussion on the interesting problem of what happens to the quantum radiation field when a cavity changes size suddenly.
One of the first things physicists do with a new theory is try to recover the old theory it surpasses as a limiting case. For quantum and classical mechanics in general, at the time of writing, this is still an open question. For the radiation field, however, we know at least that there are some quantum states that most closely approach the classical realm. In Section 4.1.1 we introduce these coherent states. Then we discuss how the idea of quantum state can be ...