  Encoding information onto optical fields is the backbone of modern
telecommunication networks. Optical fibers offer low loss transport and vast
bandwidth compared to electrical cables, and are currently also replacing
coaxial cables for short-range communications. Optical fibers also exhibit
significantly lower thermal conductivity, making optical interconnects
attractive for interfacing with superconducting circuits and devices. Yet
little is known about modulation at cryogenic temperatures. Here we demonstrate
a proof-of-principle experiment, showing that currently employed Ti-doped LiNbO
modulators maintain the Pockels coefficient at 3K---a base temperature for
classical microwave amplifier circuitry. We realize electro-optical read-out of
a superconducting electromechanical circuit to perform both coherent
spectroscopy, measuring optomechanically-induced transparency, and incoherent
thermometry, encoding the thermomechanical sidebands in an optical signal.
Although the achieved noise figures are high, approaches that match the
lower-bandwidth microwave signals, use integrated devices or materials with
higher EO coefficient, should achieve added noise similar to current HEMT
amplifiers, providing a route to parallel readout for emerging quantum or
classical computing platforms.
