Quantum Operation - Definition

Definition

Recall that a density operator is a non-negative operator on a Hilbert space with unit trace.

Mathematically, a quantum operation is a linear map Φ between spaces of trace class operators on Hilbert spaces H and G such that

• If S is a density operator, Tr(Φ(S)) ≤ 1.
• Φ is completely positive, that is for any natural number n, and any square matrix of size n whose entries are trace-class operators

and which is non-negative, then

is also non-negative. In other words, Φ is completely positive if is positive for all n, where denotes the identity map on the C*-algebra of matrices.

Note that by the first condition quantum operations may not preserve the normalization property of statistical ensembles. In probabilistic terms, quantum operations may be sub-Markovian. In order that a quantum operation preserve the set of density matrices, we need the additional assumption that it is trace-preserving.