  We introduce a probability current analysis of excitation energy transfer
between states of an open quantum system. Expressing the energy transfer
through currents of excitation probability between the states in a site
representation enables us to gain key insights into the energy transfer
dynamics. It allows to, i) identify the pathways of energy transport in large
networks of sites and to quantify their relative weights, ii) quantify the
respective contributions of unitary dynamics, dephasing, and
relaxation/dissipation processes to the energy transfer, and iii) quantify the
contribution of coherence to the energy transfer. Our analysis is general and
can be applied to a broad range of open quantum system descriptions (with
coupling to non-Markovian environments) in a straightforward manner.
