  Quantum states encoded in microwave photons or qubits can be effectively
manipulated, whereas optical photons can be coherently transferred via optical
fibre and waveguide. The reversible conversion of quantum states between
microwave and optical photons will hence enable the distribution of quantum
information over long distance and significantly improve the scalability of
hybrid quantum systems. Owning to technological advances, mechanical resonators
couple to quantum devices in distinctly different spectral range with tunable
coupling, and can serve as a powerful interface to connect those devices. In
this review, we summarize recent theory and experimental progress in the
coherent conversion between microwave and optical fields via
optoelectromechanical transducers. The challenges and perspectives in achieving
single-photon-level quantum state conversion will also be discussed.
