  Optical networks that distribute entanglement among quantum technologies will
form a powerful backbone for quantum science but are yet to interface with
leading quantum hardware such as superconducting qubits. Consequently, these
systems remain isolated because microwave links at room temperature are noisy
and lossy. Building connectivity requires interfaces that map quantum
information between microwave and optical fields. While preliminary
microwave-to-optical (M2O) transducers have been realized, developing
efficient, low-noise devices that match superconducting qubit frequencies
(gigahertz) and bandwidths (10 kHz - 1 MHz) remains a challenge. Here we
demonstrate a proof-of-concept on-chip M2O transducer using
$^{171}\mathrm{Yb}^{3+}$-ions in yttrium orthovanadate (YVO) coupled to a
nanophotonic waveguide and a microwave transmission line. The device's
miniaturization, material, and zero-magnetic-field operation are important
advances for rare-earth ion magneto-optical devices. Further integration with
high quality factor microwave and optical resonators will enable efficient
transduction and create opportunities toward multi-platform quantum networks.
