". . . a coherent state remains unchanged by the detection (or annihilation) of field excitation or, say, a particle." — Wikipedia's entry on "Coherent States" I do not understand this. Is it saying that if a system in a coherent state, like a quantum harmonic oscillator, emits a particle, like a photon, it remains in the same state? How can any system lose energy yet remain in the same state? I know that a coherent state is not a particle number (energy) eigenstate, but like any state it has an energy expection value. Shouldn't this value change when energy is emitted? Thanks.