Quantum sensing of magnons with a superconducting qubit

Magnons are quanta of collective spin excitations, such as in magnetostatic modes and spin waves, ubiquitously found in magnetic materials. While conventional experimental techniques in magnetic resonance and spintronics only deal with large amplitude signals, the recent progress in quantum magnonics has enabled us to investigate the single magnon limit [1]. In this talk, we demonstrate how to control and detect single magnons in a macroscopic-scale ferromagnetic crystal. We strongly couple a superconducting transmon qubit, an artificial two-level system realized in a superconducting circuit, to a magnon in a millimeter-sized single-crystalline sphere of yttrium-gallium garnet (YIG) via a virtual excitation of a photon in a copper microwave cavity. In the strong dispersive regime, the interaction results in a magnon-number-resolved qubit spectrum, through which we can generate entanglement between the qubit and the magnon mode by applying a qubit π-rotation conditioned on the magnon number. Subsequent readout of the qubit leads to the detection of a magnon [2]. In another scheme, we apply the so-called T₂^* relaxometry and measure the tiny steady-state time-averaged population of magnons in the YIG sphere through Ramsey interference of the qubit. Counterintuitively, the sensitivity is enhanced by the dissipation of magnons [3]. We will discuss the current limitations and possible improvements of the techniques.