A Fock state is the simplest type of field state. for a single value of n. Since the trace of a density matrix is normalized, all the other diagonal elements must be zero. The off--diagonal elements are zero as well. This field state is very simple to formulate but difficult to fully understand. The Fock state is important not only as a theoretical tool but also experimentally. It has the following statistical properties 
Since the variance in the photon number is zero, the photon number is completely determined to be n. The photon number is directly related to the energy which classically implies that the amplitude of the magnetic and electric fields are completely determined. One version of the Heisenberg uncertainty principle states that: ; so, t is completely undetermined. The t is related to the period of the fields and thus the field's phase. A Fock state does not generate a field which is wave--like in the classical sense, for the field's frequency cannot be specified due to the field's completely random phase. Although the field's phase is completely random, spatially the electromagnetic field still has nodes inside the cavity.