Frequency fluctuations of ferromagnetic resonances at millikelvin temperatures

Unwanted fluctuations over time, in short, noise, are usually detrimental to device performance, especially for quantum coherent circuits. Recent efforts have demonstrated routes to utilizing magnon systems for quantum technologies by interfacing single magnons with superconducting qubits. However, the coupling of several components often introduces additional noise to the system, hence degrading its coherence. Researching the system's temporal behavior can help us to identify the underlying noise sources, which is a vital step toward improving coherence times and hybrid device performance. Yet, the frequency noise of the ferromagnetic resonance (FMR) has so far been unexplored at mK-temperatures. Here, we investigate such FMR frequency fluctuations of a yttrium-iron-garnet (YIG) sphere and find that these fluctuations are independent of temperature and drive power. This suggests that the measured frequency noise in YIG is dominated by undetermined noise sources, whose properties are not consistent with the conventional model of two-level systems, despite their effect on the sample linewidth. Moreover, the functional form of the FMR frequency noise power spectral density (PSD) cannot be described by a simple power law. By employing time-series analysis, we find a closed function for the PSD that fits the observations. Our results underline the necessity of coherence improvements to magnon systems for useful applications in quantum magnonics. Published under an exclusive license by AIP Publishing.