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Single Photon Level Study of Microwave Properties of Lithium Niobate at milli-Kelvin Temperatures

Properties of doped and natural impurities in Lithium Niobate single crystals are studied using the Whispering Gallery Mode method at low temperatures as a function of magnetic field. The study reveals considerable coupling of microwave photon modes to the Fe$^{3+}$ spin ensemble in iron-doped and non-doped crystals. The $S=5/2$ structure of the Fe$^{3+}$ impurities demonstrate Zero Field Splittings of $11.21$ and $20.96$ GHz, significant asymmetry of the Zeeman lines and additional lines with anomalous $\text{g}$-factors of $1.37$ and $3.95$. Also, interactions between different transitions of the Fe$^{3+}$ ion is observed. An additional ion impurity ensemble with a splitting of about $1.7$ GHz is shown to couple to the dominating Fe$^{3+}$ spins and the effect on $Q$-factors of microwave photon modes due to the Fe$^{3+}$ ion ensemble is also demonstrated. Measurements down to less than one photon level are made with a loss tangent of order $10^{-5}$ determined.