  Fiber optic communication is the backbone of our modern information society,
offering high bandwidth, low loss, weight, size and cost, as well as an
immunity to electromagnetic interference. Microwave photonics lends these
advantages to electronic sensing and communication systems, but - unlike the
field of nonlinear optics - electro-optic devices so far require classical
modulation fields whose variance is dominated by electronic or thermal noise
rather than quantum fluctuations. Here we present a cavity electro-optic
transceiver operating in a millikelvin environment with a mode occupancy as low
as 0.025 $\pm$ 0.005 noise photons. Our system is based on a lithium niobate
whispering gallery mode resonator, resonantly coupled to a superconducting
microwave cavity via the Pockels effect. For the highest continuous wave pump
power of 1.48 mW we demonstrate bidirectional single-sideband conversion of X
band microwave to C band telecom light with a total (internal) efficiency of
0.03 % (0.7 %) and an added output conversion noise of 5.5 photons. The high
bandwidth of 10.7 MHz combined with the observed very slow heating rate of 1.1
noise photons s$^{-1}$ puts quantum limited pulsed microwave-optics conversion
within reach. The presented device is versatile and compatible with
superconducting qubits, which might open the way for fast and deterministic
entanglement distribution between microwave and optical fields, for optically
mediated remote entanglement of superconducting qubits, and for new multiplexed
cryogenic circuit control and readout strategies.
