We evaluate the NbN single-photon detector (SSPD) for the purpose of integration into a fiber-based quantum key distribution (QKD) network. We first review free-space system measurements to characterize the SSPD in terms of counting rate and timing jitter n demonstrate its utility in fiber-based systems in two such systems. The first utilizes fiber-coupled SSPDs placed in a cryogen-free refrigerator capable of reaching mK temperatures, and the SSPDs are evaluated in terms of system quantum efficiency (SQE) and dark counts over a broad temperature range. The second system utilizes fiber-coupled SSPDs assembled on an insert placed in a standard helium dewar with each fiber permanently glued to a device. The SSPDs, evaluated in terms of SQE, dark counts, and timing resolution, show that the system provides relatively high fiber-detector coupling efficiency, good timing resolution, and can integrate easily into the DARPA network.; We also investigate the SSPD's limitations by analyzing a model which takes into account the SSPD detection mechanism and device inductance to predict its response time. We then optimize the SSPD meander geometry in designing devices terms of area, stripe width, fill factor, and thickness using detailed inductance simulations. We will also present a novel low inductance SSPD design and model its photoresponse.; With these designs and measurement results, we will show that the SSPD outperforms its superconducting and semiconducting counterparts for quantum cryptography systems with high clock rates. Thus, the SSPD, with its combination of high QE, and low timing jitter at telecommunications wavelengths, as well as low dark counts, make it a natural choice for the DARPA network and quantum cryptography systems in general.
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