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Extremely sensitive readout of a stable "etalon of length" is achieved with laser frequency-locking techniques. Rotation of the entire electro-optical system maps any cosmic directional anisotropy of space into a corresponding frequency variation. We found a fractional length change l= (1. 52. 5) 10^-15, with the expected P₂ (cos) signature. This null result represents a 4000-fold improvement on the best previous measurement of Jaseja et al.
Brillet et al. (Mon,) studied this question.