  Environment-assisted quantum transport (ENAQT) is the possibility of an
external environment to enhance transport efficiency of quantum particles. This
idea has generated much excitement over recent years, especially due to the
experimentally-motivated possibility of ENAQT in photo-synthetic exciton
transfer complexes. Many theoretical calculations have shown ENAQT, but the
explanations for its origin differ, and a universal explanation has been
elusive. Here we demonstrate a universal origin for ENAQT in quantum networks
with a dephasing environment, based on a relation between exciton current and
occupation within a Markovian open quantum system approach. We show that ENAQT
appears due to two competing processes, namely the tendency of dephasing to
make the exciton population uniform, and the formation of an exciton density
gradient, defined by the source and the sink. Furthermore, we find a geometric
condition on the network for the appearance of ENAQT, relevant to natural and
artificial systems.
