LOWER-BOUND ON PHASE NOISE OF CLASSICAL STATES

An uncertainty relation for the number and phase noise of a single-mod e field state is derived. This is then used to find a lower bound for the phase noise of a classical state. Any state that violates this con dition is nonclassical. An example of such a nonclassical state is pre sented. Because a nonclassical state can have considerably less phase noise than a classical state with the same average photon number, nonc lassical states can play a role in the measurement of small phase shif ts.