Extremely long decay time optical cavity

We report on the resonant Fabry Perot cavity of the PVLAS (Polarization of the Vacuum with LASer) experiment operating at $\lambda = 1064$ nm with a record decay time of 2.7 ms, a factor more than two larger than any previously reported optical resonator. This corresponds to a coherence length of $8.1\cdot10^5$ m. The cavity length is 3.303 m, and the resulting finesse is 770 000