  We perform a numerical and analytical investigation of microwave-to-optical
conversion based on four-wave mixing in Rydberg atoms. Our work demonstrates
that both all-resonant and off-resonant frequency-mixing configurations achieve
near-unit photon conversion efficiencies. We review the conditions that can
lead to the presence of two possible dark states. We find that for both
configurations, one of the dark states can be detrimental at high microwave
powers, and show that an additional limitation to all-resonant frequency mixing
is microwave-induced fluorescence. Finally, we confirm that the off-resonant
configuration is more appropriate as it allows for efficient photon conversion
on a wider range of input microwave intensities with reduced total power of the
auxiliary fields.
