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Giant $g$-factors of Natural Impurities in Synthetic Quartz

We report the observation of $g$-factors of natural paramagnetic impurities in a pure synthetic quartz crystal at milli-Kelvin temperatures. Measurements are made by performing spectroscopy using multiple high-$Q$ Whispering Gallery Modes sustained in the crystal. Extreme sensitivity of the method at low temperatures allows the determination of natural residual impurities introduced during the crystal growth. We observe $g$-factors that significantly differ from integer multiples of the electron $g$-factor in vacuum, and with values of up to $7.6$, which reveals much stronger coupling between impurities and the crystal lattice than in previous studies. Both substitutional and interstitial ions are proposed as candidates for the observed interactions.