  Linking superconducting quantum devices to optical fibers via
microwave-optical quantum transducers may enable large scale quantum networks.
For this application, transducers based on the Pockels electro-optic (EO)
effect are promising for their direct conversion mechanism, high bandwidth, and
potential for low-noise operation. However, previously demonstrated EO
transducers require large optical pump power to overcome weak EO coupling and
reach high efficiency. Here, we create an EO transducer in thin-film lithium
niobate, leveraging the low optical loss and strong EO coupling in this
platform. We demonstrate a transduction efficiency of up to $2.7\times10^{-5}$,
and a pump-power normalized efficiency of $1.9\times10^{-6}/\mathrm{\mu W}$.
The transduction efficiency can be improved by further reducing the microwave
resonator's piezoelectric coupling to acoustic modes, increasing the optical
resonator quality factor to previously demonstrated levels, and changing the
electrode geometry for enhanced EO coupling. We expect that with further
development, EO transducers in thin-film lithium niobate can achieve near-unity
efficiency with low optical pump power.
