Michelson-Morley with a Birefringent Cavity

We report on the progress of a birefringent cavity test of the isotropy of the speed of light. Previous experimental tests have constrained anisotropies in the speed of light at the level of parts in $10^{17}$ [1-2]. These experiments search for frame-dependent variations in the resonant frequencies of two orthogonally mounted optical cavities. Uncorrelated fluctuations in the cavity lengths are a significant challenge for such experiments. Our experiment uses a single dielectric-filled cavity, and measures the difference in the resonant frequency of two orthogonally polarized modes. Anisotropies in the speed of light will manifest as a frame-dependent strain on the dielectric [3-4], giving rise to a frame-dependent variation in the cavity birefringence. By making the length of each cavity mode identical, we expect that our experiment will be less sensitive to thermal cavity fluctuations. \\[4pt] [1] S. Herrmann, A. Senger, K. M\"ohle, M. Nagel, E.V. Kovalchuk and A. Peters, PRD {\bf 80,} 105011 (2009).\newline [2] Ch. Eisel, A. Yu. Nevsky, and S. Schiller, PRL {\bf 103,} 090401 (2009).\newline [3] H. M\"uller, PRD {\bf 71,} 045004 (2005).\newline [4] V.A. Kosteleck\'y and M. Mewes, PRD {\bf 80,} 015020 (2009).