We demonstrate the ability to calibrate a variable optical attenuator directly at the few photon level using a Superconducting Transition Edge Sensor (TES). Because of the inherent linearity of photon number resolving detection, no ex- ternal calibrations are required, including the energy of the laser pulses, which ranged from means of about 0.15 to 18 photons per pulse at the detector. To verify this method, calibrations were compared to an independent conventional calibration made at much higher photon uxes using analog detectors. Agreement between the methods is found to be within statistical uncertainties of 0.02 dB or less at 95% condence. Out few-photon measurement determined attenuations using the Poisson-In uenced K-Means Algorithm (PIKA) to extract mean numbers of photons per pulse along with the uncertainties of these means. The robustness of the method is highlighted by the agreement of the two calibrations even noting a drift in the light source of 20% occurred over the course of the experiment. In addition, we prove that an attenuated thermal distribution is also a thermal distribution.
Absolute Calibration , Optical Attenuator , Number-Resolving Detector