New quantum-based communication and measurement systems that use single and correlated photons is an active area of research. The current tools to calibrate the components in these systems are inadequate for these emerging applications. For accurate calibrations, a detector capable of determining the number of photons in a single pulse of light is needed. We have demonstrated a new detector that is capable of determining the photon number state in quantum interferometry. This single photon counting device is based on the superconducting transition-edge sensor (TES) technology currently being developed for photon-counting from gamma rays down into the near-infrared. By exploiting the sharp superconducting-to-normal resistive transition of tungsten at 100mK, these TES single photon counters give an output current pulse that is proportional to the cumulative energy in an absorption event. This proportional pulse-height enables the determination of the energy absorbed by the TES and the direct conversion of sensor pulse-height into photon number. I will discuss our results of using this new type of detector in quantum information applications and our progress towards developing detectors with quantum efficiencies approaching 100%.

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