  Quantum transducers that can convert quantum signals from the microwave to
the optical domain are a crucial optical interface for quantum information
technology. Coherent microwave-to-optics conversions have been realized with
various physical platforms, but all of them are limited to low efficiencies of
less than 50\%, the threshold of the no-cloning quantum regime. Here we report
a coherent microwave-to-optics transduction using Rydberg atoms and
off-resonant scattering technique with an efficiency of $82\pm 2\%$ and a
bandwidth of about 1 MHz. The high conversion efficiency is maintained for
microwave photons range from thousands to about 50, suggesting that our
transduction is readily applicable to the single-photon level. Without
requiring cavities or aggressive cooling to quantum ground states, our results
would push atomic transducers closer to practical applications in quantum
technologies.
