We construct a general measure for the degree of non-Markovian behavior in
open quantum systems. This measure is based on the trace distance which
quantifies the distinguishability of quantum states. It represents a functional
of the dynamical map describing the time evolution of physical states, and can
be interpreted in terms of the information flow between the open system and its
environment. The measure takes on nonzero values whenever there is a flow of
information from the environment back to the open system, which is the key
feature of non-Markovian dynamics.Comment: 4 pages, 2 figures, published versio