  Photonic cavities are gathering a large interest to enhance the energy
transfer between two dipoles, with far-reaching consequences for applications
in photovoltaics, lighting sources and molecular biosensing. However,
experimental difficulties in controlling the dipoles' positions, orientations
and spectra have limited the earlier work in the visible part of the spectrum,
and have led to inconsistent results. Here, we directly map the energy transfer
of microwaves between two dipoles inside a resonant half-wavelength cavity with
ultrahigh control in space and frequency. Our approach extends F\"orster
resonance energy transfer (FRET) theory to microwave frequencies, and bridges
the gap between the descriptions of FRET using quantum electrodynamics and
microwave engineering. Beyond the conceptual interest, we show how this
approach can be used to optimize the design of photonic cavities to enhance
dipole-dipole interactions and FRET.
