A He-Ne ring laser has given the Earth-rotation-induced Sagnac frequency to a precision of 1 microHz, or 2 parts in 10(8), 2 x 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.
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Stedman et al. (1995) studied this question.
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