High-capacity cooling of superconducting circuits with superfluid helium for microwave-to-optical quantum transduction*

In the past decade, hybrid superconducting platforms such as electro-optical, and electro-optomechanical systems have been extensively studied for building the quantum interface between superconducting quantum systems and photonic distribution networks. In these transducers, a strong optical drive is typically required to parametrically enhance the interaction between microwave and optical fields. However, the absorption of optical drive photons also introduces undesired noise and frequency shift of the superconducting resonator, undermining the performance of quantum signal transduction. To tackle this challenge, we achieved enhanced cooling capacity by submerging a cavity electro-optical transducer in superfluid helium, leveraging its tremendous heat transport characteristics. We demonstrated that the heating effect through optical photon absorption of the device substrate and packaging is largely suppressed, allowing a 30-dB improvement in the microwave-to-optical transduction throughput. On the other hand, the direct quasiparticle generation from photon absorption by the superconductor is unchanged. This study lays out a pathway to building a more efficient superconducting-photonic interface for future quantum networks.