Given these definitions of Markovianity, it is possible to construct a metric on the space of density matrices in order to quantitatively measure the relative distinguishability between two quantum states. This metric is a generalisation of the classical Kolmogorov distance, and a simplification of the more general Helstrom metric. For a pair of states, ρ 1 and ρ 2 , the trace distance metric of distinguishability is defined as :