We provide a direct comparison between the InGaAs avalanche photodiode (APD) and the NbN superconducting single photon detector (SSPD) for applications in fiber-based quantum cryptography. The quantum efficiency and dark count rate were measured for each detector, and used to calculate the quantum bit error rate (QBER) and shared key rate for a QKD link. The results indicate that, despite low quantum efficiency, the speed of the SSPD makes it a superior detector for quantum information applications. Finally, we present results of an initial integration of an SSPD into a receiver node of the DARPA quantum network to perform quantum key distribution. Encoding quantum information on single photons has enabled the realization of a number of unique communication schemes such as the implementation of the BB84 protocol for quantum key distribution and quantum dense coding. Furthermore, the implementation of these schemes using photons at telecommunication wavelengths has permitted their extension to fiber optic networks, drastically increasing the practicality of quantum information processing. To date, the state of the art in commercially available single photon detection at telecommunication wavelengths (1550 nm) has been the thermo-electrically cooled InGaAs avalanche photodiode (APD). The cooled InGaAs APD is generally operated at T = —60℃ in the reverse-biased Geiger mode. Though these detectors have been successfully implemented in operating QKD links in optical fiber, the InGaAs APD suffers from afterpulsing effects that limit the key exchange rates. New SPD (single photon detector) technologies based on superconducting materials are emerging that hold tremendous promise for drastically increasing the photon counting rate and eliminating dark noise. The SSPD consists of a NbN thin film (t ～ 5 nm) patterned into a narrow (w ～ 100 nm) microstrip meander that is ～ 500 μm in length. The detector is cooled to T < 4K in a closed cycle refrigeration unit and a bias current I_b is supplied to the device just below the superconducting critical current (I_c). When a photon is incident upon the microstrip superconductivity is broken in the region local to photon absorption and a normal state region is formed in the device. The current passing through the device causes a measurable voltage pulse in the detector that can be amplified and shaped to act as an input for traditional logic circuitry. Due to the unique electronic bandstructure in a superconducting material the intrinsic response time of the device is on the order of picoseconds, so that the repetition rate may be increased above 1 GHz without being susceptible to afterpulsing effects that plague the InGaAs APD. To make a direct comparison between the InGaAs APD and the SSPD both were placed in a SPD characterization testbed to measure quantum efficiency and dark count rate at a variety of bias conditions. The QBER for a QKD link is calculated from the measured parameters and then compared to requirements for a provably secure QKD link. Following detector characterization, one of the nodes of the DARPA Quantum Network operated by BBN Technologies utilized an SSPD in the receiver to transmit key material. The demonstration provides an in situ comparison between the InGaAs APD, normally operated in the BBN link, and the SSPD.
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