A precision test of the isotropy of the speed of light using rotating   cryogenic optical cavities
  A test of Lorentz invariance for electromagnetic waves was performed by comparing the resonance frequencies of two stable optical resonators as a function of orientation in space. The crystalline resonators were operated at 3.4 K in a cryostat employing a pulse-tube refrigerator. A new analysis yields the Robertson-Mansouri-Sexl theory parameter combination $\beta-\delta-1/2=(-0.6\pm 2.1\pm1.2)\cdot10^{-10}$ and one parameter of the Standard Model Extension theory, $(\tilde{\kappa}_{e-})^{ZZ} = (-2.9\pm2.2)\cdot 10^{-14}$. 
S. Schiller, P. Antonini, M. Okhapkin