Quantum sensing of magnon excitations with a superconducting qubit*

Magnons are quanta of collective spin excitations, such as in magnetostatic modes and spin waves, ubiquitously found in magnetic materials. Conventional experimental techniques like magnetic resonance and spintronics can only access large-amplitude "classical" signals in those modes. However, the recent progress in quantum magnonics has enabled us to investigate the properties of magnons at the single-magnon limit [1]. In this presentation, we discuss how we can control and detect single magnons in a macroscopic-scale ferromagnetic crystal. We strongly couple a superconducting qubit, an artificial two-level system realized in a superconducting circuit, to a magnon via a virtual photon excitation in a microwave cavity. The interaction allows us to generate entanglement between the qubit and the magnon excitation, and the qubit readout results in detecting a magnon [2,3].