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Laser-cavity locking at the $10^{-7}$ instability scale utilizing beam elipticity

Ultrastable lasers form the back bone of precision measurements in science and technology. Such lasers attain their stability through frequency locking to reference cavities. State-of-the-art locking performances to date had been achieved using frequency-modulation based methods, complemented with active drift cancellation systems. We demonstrate an all passive, modulation-free laser-cavity locking technique (squash locking) that utilizes changes in beam ellipticity for error signal generation, and a coherent polarization post-selection for noise resilience. By comparing two identically built proof-of-principle systems, we show a frequency locking instability of $5 \times 10^{-7}$ relative to the cavity linewidth at 10 s averaging. The results surpass the demonstrated performances of methods engineered over the last five decades, opening a new path for further advancing the precision and simplicity of laser frequency stabilization.