The desire for quantum-generated cryptographic key for broadband encryption services has motivated the development of high-transmission-rate single-photon quantum key distribution （QKD） systems. The maximum operational transmission rate of a QKD system is ultimately limited by the timing resolution of the single-photon detectors and recent advances have enabled the demonstration of QKD systems operating at transmission rates well in to the GHz regime. We have demonstrated quantum generated one-time-pad encryption of a streaming video signal with high transmission rate QKD systems in both free-space and fiber. We present an overview of our high-speed QKD architecture that allows continuous operation of the QKD link， including error correction and privacy amplification， and increases the key-production rate by maximizing the transmission rate and minimizing the temporal gating on the single-photon channel. We also address count-rate concerns that arise at transmission rates that are orders of magnitude higher than the maximum count rate of the single-photon detectors. Keywords： Single-photon detection， quantum key distribution， avalanche photodiode

　　1. INTRODUCTION Quantum key distribution systems can produce cryptographic key whose security can be verified without placing conditional bounds on an eavesdropper’s technological capabilities ［1］。 To achieve this QKD systems are designed in such a way that the actions of an eavesdropper cause detectable changes in the system. Specifically， by using single photons randomly prepared in non-orthogonal states it is possible to ensure that an eavesdropper’s attempts to measure the state of the photons necessarily induce effects that are discernable to the link operators. This allows the link operators to place an upper bound on the amount of information that could have been attained by the eavesdropper. If this bound is sufficiently low， privacy amplification algorithms are then used to reduce the eavesdropper’s knowledge to an arbitrarily low level ［2］， resulting in a verifiably secure key that can be used for encryption. The requirement to operate the QKD link at the single-photon level imposes significant limitations on the system. In particular， both the link losses and the signal-to-noise ratio on the single-photon channel place a finite bound on the range over which secure key can be produced ［3］。 To date， sophisticated demonstrations of QKD have achieved secure-key production over 200 km in fiber ［4］， and 144 km in free-space ［5］， though the key production rates in both of these demonstrations were relatively low （~10 bits/sec）。 A number of groups have recognized the fact that below the security bound it is possible to increase the key production rate by increasing the transmission rate on the single-photon channel， and this has motivated the demonstration of QKD systems with transmission rates well in to the gigahertz regime ［6-8］。 We present a technique that supports continuous operation at gigahertz transmission rates， limited only by the timing resolution of the single-photon detectors. We also address important operational concerns associated with transmission rates that are orders of magnitude higher than the dead time of the constituent single-photon detectors. As described below， a QKD system requires both a single-photon， or quantum， channel and an associated classical channel to produce key. We implement clock-recovery over the classical channel to synchronize the transmitter’s and receiver’s data clocks with relative jitter less than 50 ps， thus creating a contiguous series of temporal gates in which we can transmit on the quantum channel. With a classical channel data rate of 1.25 Gbits/s， detection events on the quantum channel are gated in 800 ps windows， and we can transmit at rates as high as 1.25 GHz. This approach is limited only by the ability of the single-photon detectors to resolve events intended for a particular gate， allowing us to choose a repetition rate that maximizes the bandwidth of the quantum channel. We have demonstrated a system that operates at a repetition rate of 625 MHz， and we identify a practical solution for higher speeds.