High-dynamic range quantum sensing of magnons with a superconducting qubit in a local magnetic field

Hybrid quantum systems (HQS) of superconducting (SC) quantum circuits and magnons—collective excitations of spin waves in magnetic materials—offer potential applications in non-reciprocal devices, frequency transduction, and quantum-enhanced sensing. Here, we present a HQS that combines a SC qubit with a magnetic material to probe magnon excitations with a dynamic range of a couple thousand excitations. We overcome the inevitable difficulty in combining superconductors and magnetic materials with a practical setup that applies a local magnetic field only to the magnetic material. We employ a YIG (yttrium iron garnet) sphere, a well-studied low damping ferromagnetic material, coupled to a SC qubit to test our hardware and quantum sensing concept. We use fast qubit control and weak dispersive coupling between the transmon qubit and YIG sphere to resolve magnon population and decay over a wide dynamic range. Our results demonstrate the utility of SC qubits as high-sensitivity, large-dynamic range detectors that can resolve time-dynamics. Our hardware illustrates a robust and practical way for integrating SC qubits with magnetic materials.