  Understanding thermal transport in nanoscale systems presents important
challenges to both theory and experiment. In particular, the concept of local
temperature at the nanoscale appears difficult to justify. Here, we propose a
novel theoretical approach where we replace the temperature gradient with
controllable external black-body radiations. The theory recovers known physical
results, for example the linear relation between the thermal current and the
temperatures difference of two black-bodies. Furthermore, our theory is not
limited to the linear regime and goes beyond accounting for non linear effects
and transient phenomena. In the strong coupling and large temperature
gradients, we show that there is a maximum energy current that the system can
sustain and we recover the Kramers' turnover. Since the present theory is
general and can be adapted to describe both electron and phonon dynamics, it
provides a first step towards a unified formalism for investigating thermal and
electronic transport.
