Optical counterpart of the Foucault pendulum
  The twin beam vortex interferometer with phase-conjugating mirror is analyzed in rotating reference frame.Circular motion of the interference pattern occurs due to exchange of the angular momenta between photons and interferometer. Using the concept of the $ideal$ phase-conjugating mirror it is shown that motion of the helical interference pattern of the interacting vortex photons with topological charge $\ell$ may be used for the detection of the slow rotations. The higher density of interference fringes may improve the sensitivity by factor containing $2 \ell$ compared to the conventional Michelson interferometry. 
A.Yu. Okulov
physics.optics nlin.PS quant-ph