Physical properties of a laser beam and the intracavity quantum state

We use the impossibility of sending information faster than light to show that it is impossible to distinguish between various types of laser cavity field state, such as a Fock state of uncertain energy and any superposition of Fock states of unknown free evolution time, by means of the physically measurable properties of the laser beam. Superpositions can include coherent states, squeezed states and cat states, indeed any pure intracavity state will produce the characteristic properties of the beam such as intrinsic phase coherence.