Design for a parametrically controllable interface between superconducting circuits and spin ensembles

A practical hybrid quantum interface requires fast, efficient, and tunable coupling between its components. Solid state spin ensembles are compelling candidates for quantum information storage and transduction due to their long coherence times and access to microwave and optical transitions. However, controllably and efficiently interfacing these ensembles with other quantum information platforms, such as superconducting circuits, remains a considerable challenge. We present a design for parametrically pumped coupling between a three-wave Josephson junction mixing element and a spin ensemble, mediated by a lumped-element 3D microwave cavity. We predict strong pump-induced coupling between the mixer and the spin ensemble. We also present initial coupling measurements for this quantum hybrid interface using ¹⁷¹Yb ions implanted in a Y₂SiO₅ host crystal. This spin system’s high coherence microwave transitions at zero magnetic field attract particular interest for hybrid interfaces with superconducting qubits. Our findings provide a path for highly coherent, spin-based quantum memories for superconducting quantum circuits.