  We study quantum heat exchange in a multi-state impurity coupled to two
thermal reservoirs. Allowing for strong system-bath interactions, we show that
a steady-state heat exchange fluctuation theorem holds, though the dynamical
processes nonlinearly involve the two reservoirs. We accomplish a closed
expression for the cumulant generating function, and use it obtain the heat
current and its cumulants in a nonlinear thermal junction, the two-bath
spin-boson model.
