The nonclassical distance of a state of the electromagnetic field is defined. This distance allows one to say how nonclassical a given state is and places bounds on the extent to which the behavior of that state can deviate from that of classical states. Methods are developed for estimating nonclassical distance and are applied to number states and squeezed states.

• Received 27 May 1986

DOI:https://doi.org/10.1103/PhysRevA.35.725