In cavity QED, one often consider only a single mode of the radiation field along the ê direction. This is commonly referred to as the single mode approximation in cavity QED, with the frequency ω c = πc/L (c is the speed of the light, and ω c represents the single mode frequency of the cavity), and the corresponding photonic creation and annihilation operators â † and â (where we have dropped the label of k for a single mode.)