Test of constancy of speed of light with rotating cryogenic optical resonators

A test of Lorentz invariance for electromagnetic waves was performed by comparing the resonance frequencies of two optical resonators as a function of orientation in space. In terms of the Robertson-Mansouri-Sexl theory, we obtain $\beta -\delta -1∕2=(+0.5\pm 3\pm 0.7)\times {10}^{-10}$, a tenfold improvement compared to the previous best results. We also set a first upper limit for a parameter of the standard model extension test theory, $\mid {\left({\tilde{\kappa }}_{e-}\right)}^{ZZ}\mid <2\times {10}^{-14}$.

Figure 1

The experimental setup. Two Nd:YAG lasers are frequency locked to two sapphire optical cavities located in a cryostat. The beams are fed to the resonators via optical fibers. The cavities’ modes can be observed by means of two charge-coupled device cameras. Acousto-optic modulators (AOM) stabilize the power of the beams fed to the resonators. All components shown are mounted on a rotating table.

Figure 2

Measured $2B\nu$ and $2C\nu$ amplitudes of spatial anisotropy for individual rotations and corresponding histograms. The fit error for each data point is less than $1\mathrm{Hz}$. Full line: the SME model plus systematic effect. Dashed line: the RMS model plus systematic effect. Mean values: $⟨2B⟩=2.8\mathrm{Hz}$, $⟨2C⟩=-3.3\mathrm{Hz}$.