  We study the nonequilibrium steady state (NESS) in a quantum system in
contact with two heat baths at different temperatures. We use a
time-independent perturbative expansion with respect to the coupling with the
two heat baths to obtain the density matrix for the NESS. In particular, we
show an explicit representation of the density matrix for the reflection
symmetric and weakly nonequilibrium case. We also calculate the expectation
value of the energy current and show that the Kubo formula holds in this case.
