Single-photon level study of microwave properties of lithium niobate at millikelvin temperatures

The 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 Fe3+ spin ensemble in iron-doped and nondoped crystals. The S = 5/2 structure of the Fe3+ impurities demonstrates zero field splittings of 11.21 and 20.96 GHz, a significant asymmetry of the Zeeman lines, and additional lines with anomalous g factors of 1.37 and 3.95. Also, interactions between different transitions of the Fe3+ ion are observed. An additional ion impurity ensemble with a splitting of about 1.7 GHz is shown to couple to the dominating Fe3+ spins, and the effect on the Q factors of the microwave photon modes due to the Fe3+ ion ensemble is also demonstrated. Measurements down to less than one photon level are made, with a loss tangent of order 10(-5) determined.