  Microwave-to-optics transduction is emerging as a vital technology for
scaling quantum computers and quantum networks. To establish useful
entanglement links between qubit processing units, several key conditions have
to be simultaneously met: the transducer must add less than a single quantum of
input referred noise and operate with high-efficiency, as well as large
bandwidth and high repetition rate. Here we present a new design for an
integrated transducer based on a planar superconducting resonator coupled to a
silicon photonic cavity through a mechanical oscillator made of lithium niobate
on silicon. We experimentally demonstrate its unique performance and potential
for simultaneously realizing all of the above conditions, measuring added noise
that is limited to a few photons, transduction efficiencies as high as 0.9%,
with a bandwidth of 14.8 MHz and a repetition rate of up to 100 kHz. Our device
couples directly to a 50-Ohm transmission line and can easily be scaled to a
large number of transducers on a single chip, paving the way for distributed
quantum computing.
