where { A m } are arbitrary operators and h is a positive semidefinite matrix. The latter is a strict requirement to ensure the dynamics is trace-preserving and completely positive. The number of A m operators is arbitrary, and they do not have to satisfy any special properties. But if the system is N -dimensional, it can be shown that the master equation can be fully described by a set of N 2 − − 1 operators, provided they form a basis for the space of operators.