  We put forward a quantum-optical model for a thermal diode based on heat
transfer between two thermal baths through a pair of interacting qubits. We
find that if the qubits are coupled by a Raman field that induces an
anisotropic interaction, heat flow can become non-reciprocal and undergoes
rectification even if the baths have equal dissipation rates and/or the qubits
are resonant. The heat flow rectification is explained by four-wave mixing and
Raman transitions between dressed states of the interacting qubits and are
governed by a global master equation. The anisotropic two-qubit interaction is
the key for this present simple quantum thermal diode, whose resonant operation
allows for high-efficiency rectification of large heat currents. Effects of
spatial overlap of the baths are addressed. We also discuss the possible
realizations of the model system in various platforms including optomechanical
systems, systems of trapped ions, and circuit QED.
