Improved Laser Test of the Isotropy of Space

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 Δll=(1.5±2.5)×10^-15, with the expected P₂(cosθ) signature. This null result represents a 4000-fold improvement on the best previous measurement of Jaseja et al.