  We present a new control algorithm and system design for a network of quantum
repeaters, and outline the end-to-end protocol architecture. Such a network
will create long-distance quantum states, supporting quantum key distribution
as well as distributed quantum computation. Quantum repeaters improve the
reduction of quantum-communication throughput with distance from exponential to
polynomial. Because a quantum state cannot be copied, a quantum repeater is not
a signal amplifier, but rather executes algorithms for quantum teleportation in
conjunction with a specialized type of quantum error correction called
purification to raise the fidelity of the quantum states. We introduce our
banded purification scheme, which is especially effective when the fidelity of
coupled qubits is low, improving the prospects for experimental realization of
such systems. The resulting throughput is calculated via detailed simulations
of a long line composed of shorter hops. Our algorithmic improvements increase
throughput by a factor of up to fifty compared to earlier approaches, for a
broad range of physical characteristics.
