which describes a collection of n particles, one of which has quantum state ϕ ϕ 1 , another ϕ ϕ 2 and so on up to the n th particle, where each ϕ ϕ i is any state from the single particle Hilbert space H . Here juxtaposition (writing the single particle kets side by side, without the ⊗ ⊗ ) is symmetric (resp. antisymmetric) multiplication in the symmetric (antisymmetric) tensor algebra. The general state in a Fock space is a linear combination of product states. A state that cannot be written as a convex sum of product states is called an entangled state.