<p>Microwave-to-optical transduction of single photons will play an essential role in interconnecting future superconducting quantum devices. Various transducers have been developed that couple microwave and optical modes by utilizing nonlinear phenomena such as the Pockels effect and a combination of electromechanical, piezoelectric and optomechanical couplings. However, the limited strength of these nonlinearities necessitates the use of high-quality-factor resonators that can require sophisticated nanofabrication methods. Rare-earth-ion-doped crystals have high-quality atomic resonances that result in effective second-order nonlinearities that are many orders of magnitude stronger than those in conventional materials. Here we use ytterbium-171 ions doped in an YVO₄ crystal to implement an on-chip microwave-to-optical transducer. Without an engineered optical cavity, we achieve per-cent-level efficiencies with an added noise referred to the input as low as 1.24(9) photons. We demonstrate the interference of photons originating from two simultaneously operated transducers, enabled by the inherently matching frequencies of the atomic transitions. Our results establish rare-earth-ion-based devices as a competitive platform for transduction and pave the way towards the remote transducer-assisted entanglement of superconducting quantum machines.</p>