This paper reports the current status of the DARPA Quantum Network, which became fully operational in BBN's laboratory in October 2003, and has been continuously running in 6 nodes operating through telecommunications fiber between Harvard University, Boston University, and BBN since June 2004. The DARPA Quantum Network is the world's first quantum cryptography network, and perhaps also the first QKD systems providing continuous operation across a metropolitan area. Four more nodes are now being added to bring the total to 10 QKD nodes. This network supports a variety of QKD technologies, including phase-modulated lasers through fiber, entanglement through fiber, and freespace QKD. We provide a basic introduction and rational for this network, discuss the February 2005 status of the various QKD hardware suites and software systems in the network, and describe our operational experience with the DARPA Quantum Network to date. We conclude with a discussion of our ongoing work.
We describe a technique of parameter estimation and control in a phase-encoded quantum key distribution that uses continuous control of receiver-interferometer differential path length to maintain alignment with the transmitter. In this fiber-based system, a small number of training frames are sent over the quantum channel allowing the receiver to compensate for drift in the transmitter and receiver interferometers due to slow changes in temperature. The minimum mean-square error estimation method used to infer the state of the system incorporates the prior knowledge of the fiber dynamics recursively. The optimal linear-quadratic regulator feedback design is described and combined with the estimator to obtain the stochastic linear regulator of the path-length error.
We describe a phase-encoded quantum key distribution system the uses continuous control of receiver-interferometer path length to maintain alignment with the transmitter. In this fiber-based system, a small number of training frames are sent over the quantum channel that allow the receiver to compensate for drift in the transmitter and receiver interferometers due to slow changes in temperature. The system is self-starting after disruption and can maintain a quantum bit error rate of less than 7% for phase drift rates of 0.5 deg/sec. The control system design is described and measured system data is compared with simulations.