Simple quantum states of light
3.1 The electromagnetic oscillator
In Chapter 2 we arrived at a relatively simple description of light in the quantum world: light consists of a superposition of modes. The mode functions Ak(r, t) contain the classical, wave-like aspects of light, the fact that light is an electromagnetic wave; the Ak(r, t) obey the Coulomb gauge (2.4), the wave equation (2.5) and the boundary conditions. The quantum degrees of freedom are the mode amplitudes k; their expectation values and fluctuations depend on the quantum state of light. For a single mode, the electromagnetic field is proportional to the mode amplitude; ...