where each n i takes the value 0 or 1 for fermionic particles and 0, 1, 2,... for bosonic particles. Note that trailing zeroes may be dropped without changing the state. Such a state is called a Fock state. When the | ψ ψ i ⟩ ⟩ are understood as the steady states of a free field, the Fock states describe an assembly of non-interacting particles in definite numbers. The most general Fock state is a linear superposition of pure states.