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

© 2015 American Physical Society. ©2015 American Physical Society. 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 20.96GHz, 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 1.7GHz 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.