  The conversion between microwave photons and optical photons with quantum
coherence is important for quantum communication and computation. In this
paper, we report a proposal using an ensemble of atoms coupled to microwave and
optical resonators. Input photons to one resonator are converted into output
photons in the other resonator without active operation. Usually the conversion
is only optimized at certain frequency. In our proposal, we find that the
efficiency is almost a constant and can be close to 100% in a large interval of
frequency, i.e. a high-bandwidth conversion can be realized with our proposal.
