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Hybrid systems combining magnons and superconducting quantum circuits have attracted increasing interest in recent years. Magnonic crystals (MCs) are one of the building blocks of room-temperature magnonics and can be used to create devices with an engineered band structure. These devices, exhibiting tunable frequency selectivity and the ability to store travelling excitations in the microwave regime, may form the basis of a set of new tools to be used in the context of quantum information processing. In order to ascertain the feasibility of such plans, MCs must be demonstrated to work at the low temperatures required for microwave-frequency quantum experiments. We report the first measurements of the transmission of microwave signals through an MC at 20 mK and observe a magnonic bandgap in both continuous-wave and pulsed excitation experiments. The spin-wave damping at low temperatures in our yttrium iron garnet MC is higher than expected, indicating that further work is necessary before the full potential of quantum experiments using magnonic crystals can be realised., Comment: 5 pages, 4 figures

We report measurements of a superconducting coplanar waveguide resonator (CPWR) coupled to a sphere of yttrium-iron garnet. The non-uniform CPWR field allows us to excite various magnon modes in the sphere. Mode frequencies and relative coupling strengths are consistent with theory. Strong coupling is observed to several modes even with, on average, less than one excitation present in the CPWR. The time response to square pulses shows oscillations at the mode splitting frequency. These results indicate the feasibility of combining magnonic and planar superconducting quantum devices., Comment: 5 pages, 4 figures

In this work, we experimentally investigate the propagation of pulsed magnetostatic surface spin-wave (magnon) signals in an yttrium iron garnet (YIG) waveguide at millikelvin temperatures. Our measurements are performed in a dilution refrigerator at microwave frequencies. The excellent signal-to-noise ratio afforded by the low-temperature environment allows the propagation of the pulses to be observed in detail. The work gives insight both into low-temperature magnon dynamics in YIG and the potential application of systems of propagating magnons to solid-state quantum information processing., Comment: Updated to bring in line with journal version, including an added a segment on GGG affecting spin-waves as seen from a shift in the FMR frequency