Magnon linewidth in hybrid quantum systems at milliKelvin temperatures

The dynamic properties of collective spin excitations (magnons) have been studied for decades. Only recently there has been progress towards studying the quantum properties of magnons at microwave powers where the average number of probe photons is less than one. Strongly coupled hybrid systems of microwave photons in superconducting quantum circuits and magnons may be suitable for application in quantum information processing.
We report on limiting effects of the intrinsic magnon linewidth, representing the coherence time of a quantum memory, at milliKelvin temperatures. Power dependent spectroscopic measurements on a hybrid system consisting of a YIG sphere and a 3D microwave cavity at temperatures of 50 mK to 1.5 K have been performed. The magnon frequency is swept across the cavity resonance by an external magnetic field. Using input-output formalism, we extracted the linewidths and coupling within the system. At lowest temperatures, the observed magnon linewidth is dominated by losses to near-resonance two-level systems (TLSs). Increasing the power from quantum excitation to multi-photon excitations, the TLSs get saturated and their contribution to the linewidth vanishes. At higher temperatures, the power influence decreases due to the presence of thermally saturated TLSs.