A He-Ne ring laser has given the Earth-rotation-induced Sagnac frequency to a precision of 1 µHz, or 2 parts in 108, 2 × 10-21 of the laser frequency. Because the beat frequency in a ring laser originates in either timereversal or (with a more complicated polarization geometry) parity-violating effects, such systems have the potential of detecting ultrasmall symmetry violations.
© 1995 Optical Society of America
G. E. Stedman, M. T. Johnsson, Z. Li, C. H. Rowe, and H. R. Bilger, "T violation and microhertz resolution in a ring laser," Opt. Lett. 20, 324-326 (1995)