  We investigate the coherent energy transfer between two quantum systems
mediated by a quantum bus. In particular, we consider the energy transfer
process between two qubits, and how it can be influenced by using a third qubit
or photons in a resonant cavity as mediators. Inspecting different figures of
merit and considering both on and off-resonance configurations, we characterize
the energy transfer performances. We show that, while the qubit-mediated
transfer shows no advantages with respect to a direct coupling case, the
cavity-mediated one is progressively more and more efficient as function of the
number of photons stored in the cavity that acts as a quantum bus. The
speeding-up of the energy transfer time, due to a quantum mediator paves the
way for new architecture designs in quantum technologies and energy based
quantum logics.
