Improving coupling strengths and lifetimes in quantum magnonics*

Quantum magnonics is an architecture in which collective modes of spin excitations in magnetically-ordered systems interact coherently with superconducting qubits. One of the main milestones to be demonstrated in quantum magnonics is the creation and observation of macroscopic quantum states of magnons. This task is challenging due to (i) the second-order nature of the effective interaction between magnetostatic modes and superconducting qubits and (ii) the relatively high relaxation rate of magnons. The first challenge is tackled by using a three-dimensional lumped-element microwave cavity designed to optimally enhance the effective coupling strength up to a few tens of MHz. Furthermore, we have observed that two-level systems (TLSs) in yttrium iron garnet at millikelvin temperatures have lifetimes much longer than the magnon lifetime. The long lifetime of the TLSs could potentially enable us to perform quantum magnonics experiments in an out-of-equilibrium situation where magnon decay is partially suppressed. Both advances are important steps towards the observation of quantum states of magnons.