  We present a hierarchical quantum master equation (HQME) approach, which
allows a numerically exact simulation of nonequilibrium transport in general
open quantum systems involving multiple bosonic and fermionic environments. The
performance of the method is demonstrated for a model of a nanosystem, which
involves interacting electronic and vibrational degrees of freedom and is
coupled to fermionic and bosonic baths. The results show the intricate
interplay of electronic and vibrational degrees of freedom in this
nonequilibrium transport scenario for both voltage and thermally driven
transport processes. Furthermore, the use of importance criteria to improve the
efficiency of the method is discussed.
