Dissipation-based quantum sensing of magnons*

Magnons are the quanta of collective spin excitations. We introduce a novel technique for quantum sensing of magnons by leveraging the quantum coherence of a superconducting qubit which interacts with a magnetostatic mode. This is enabled by the realization of a strong dispersive coupling between the uniformly precessing magnetostatic mode in a ferromagnetic sphere and a superconducting qubit in a hybrid system [1,2]. A finite magnon population induces additional dephasing in the qubit, and can thus be inferred by probing the qubit coherence via Ramsey interferometry. A magnon detection sensitivitiy of around 10^-3 magnons/√Hz is demonstrated, in good agreement with numerical simulations. The dissipation-based nature of our quantum sensor is confirmed by the dependence of the sensitivity on the detuning used in the Ramsey interferometry. The use of quantum sensing techniques in magnonics could find applications in fields such as magnon spintronics and magnetic field sensing.
[1] D. Lachance-Quirion et al., Science Advances 3, e1603150 (2017).
[2] D. Lachance-Quirion et al., arXiv:1910.09096.