  Quantum transduction, the process of converting quantum signals from one form
of energy to another, is an important area of quantum science and technology.
The present perspective article reviews quantum transduction between microwave
and optical photons, an area that has recently seen a lot of activity and
progress because of its relevance for connecting superconducting quantum
processors over long distances, among other applications. Our review covers the
leading approaches to achieving such transduction, with an emphasis on those
based on atomic ensembles, opto-electromechanics, and electro-optics. We
briefly discuss relevant metrics from the point of view of different
applications, as well as challenges for the future.
