  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.
