The achieved sensitivity is a very encouraging result, and we have already identified several ways to improve it. Let us cite a few improvements that are planned for the near future: first we will optimize the mode matching and cavity alignment, which should allow us to improve our results by at least one order of magnitude. Second, we will place the cavity in a vacuum chamber in order to improve the thermo-acoustic insulation and to decrease the disturbances in the 100-Hz frequency range, where the detection takes place. We also plan to extend the servo-loop bandwidth by a factor 4 to 5 by replacing the Acousto- Optic Modulator by a broadband Electro-Optic Modulator, thus suppressing the delay which severly limits the servo-loop bandwidth. This should in turn increase the servo-loop gain at low frequencies by at least the same factor, provided it is not limited by the operational amplifiers maximum gain. All these developments should allow us to overcome the present state-of-the art by about one order of magnitude [12] with our present apparatus. Ultimately, we can reasonably gain 1 to 2 more orders of magnitude on the sensitivity by increasing the cavity finesse and the optical power. Of course, thermal effects on the mirror coatings usually limit the intracavity power. We can ultimately increase it somewhat more by using automatic alignment servos as is done on gravitational waves interferometers [1–5]. Now that our apparatus is functional, we will use it to perform some novel measurements. Indeed, it has definitely the required sensitivity to achieve the unprecedented detection of several magneto-electro-optic effects in gases. In addition to the directional anisotropy we have already mentioned, our apparatus also allows us to measure for the first time the Jones magneto-electric birefringence in gases [28, 29]. The authors thank the LCAR technical staff for support, J. Vigu´ e for fruitful discussions, M. Fouch´ e and M. Reigneau for contributing to a preliminary set-up. G. B. and C. R. are grateful to C. Rizzo for initiating this experiment and for initial support. Acknowledgments [1] See the LIGO collaboration website: http://www.ligo.org [3] See the GEO600 collaboration website: http://www.geo600.org [2] See the VIRGO collaboration website: http://www.virgo.infi.it 8