A
Fock state (also known as a number state), in
quantum mechanics, is any state of the
Fock space with a well-defined number of
particles in each state. The name is for
V. A. Fock.
If we limit to a single mode for simplicity (doing so we formally describe a mere harmonic oscillator), a Fock state is of the type 
with n an integer value. This means that there are n quanta of excitation in the mode. 
corresponds to the ground state (no excitation). It is different from 0, which is the null vector.
Fock states form the most convenient basis of the Fock space. They are defined to obey the following relations in the bosonic algebra
with a (resp. 
) the annihilation (resp. creation) bose operator. Similar relations hold for fermionic algebra.
