  Converting low-frequency electrical signals into much higher frequency
optical signals has enabled modern communications networks to leverage both the
strengths of microfabricated electrical circuits and optical fiber
transmission, allowing information networks to grow in size and complexity. A
microwave-to-optical converter in a quantum information network could provide
similar gains by linking quantum processors via low-loss optical fibers and
enabling a large-scale quantum network. However, no current technology can
convert low-frequency microwave signals into high-frequency optical signals
while preserving their fragile quantum state. For this demanding application, a
converter must provide a near-unitary transformation between different
frequencies; that is, the ideal transformation is reversible, coherent, and
lossless. Here we demonstrate a converter that reversibly, coherently, and
efficiently links the microwave and optical portions of the electromagnetic
spectrum. We use our converter to transfer classical signals between microwave
and optical light with conversion efficiencies of ~10%, and achieve performance
sufficient to transfer quantum states if the device were further precooled from
its current 4 kelvin operating temperature to below 40 millikelvin. The
converter uses a mechanically compliant membrane to interface optical light
with superconducting microwave circuitry, and this unique combination of
technologies may provide a way to link distant nodes of a quantum information
network.
