Determination of a Small Photon Number by Statistical Amplifying Transients

We introduce a "statistical microscope" which allows the accurate determination of a small number of infrared photons inside a laser cavity by optical (rather than electron) multiplication. For this purpose, we induce a loss switch in a CO 2 laser and then measure the statistical distribution of passage times through a threshold value within the linear regime. Our approach is model independent, since the measured cumulants provide both average values and associated error bars. Furthermore, our method discriminates the initial photon number from noise contributions along the amplifying path, thus it does not depend on the statistical features of the amplifying process.