  I introduce an algorithm to detect one-way quantum information between two
interacting quantum systems, i.e. the direction and orientation of the
information transfer in arbitrary quantum dynamics. I then build an
information-theoretic quantifier of one-way information which satisfies a set
of desirable axioms. In particular, it correctly evaluates whether correlation
implies one-way quantum information, and when the latter is transferred between
uncorrelated systems. In the classical scenario, the quantity measures
information transfer between random variables. I also generalize the method to
identify and rank concurrent sources of quantum information flow in many-body
dynamics, enabling to reconstruct causal patterns in complex networks.
