Measuring magnon propagation in magnonic crystals at millikelvin temperatures

Magnon systems are increasingly widely recognized as a potential basis for solid-state quantum information processing. Propagating magnons are readily excited over the same range of microwave frequencies as are used in established quantum circuit technologies, and couple readily to electromagnetic fields. These facts, in combination with the relatively slow speeds and highly tunable dispersion of the excitations, make them a particularly interesting proposition in the context of quantum devices. Here, we present the first experimental study of microwave-frequency magnonic crystals (magnon systems with an artificially engineered bandgap) at millikelvin temperatures. Our magnonic crystals were prepared by etching grooves into a magnon waveguide made from a film of the magnetic insulator, yttrium iron garnet. The high signal-to-noise afforded by our low-temperature measurement environment makes it possible to make detailed observations of the dispersion of externally excited propagating magnon modes within the crystals. Our results lead us to suggest a range of device applications of dispersion-engineered magnonic systems in the context of microwave-circuit based quantum information processing.