Scientists in Boulder, CO have sent a Quantum Key Distribution (QKD) signal across 107 km. of fiber optic cable. This is the first long distance demonstration of this technology. Theoretically QKD encodes a single photon with a security key. However, in practice it has proven difficult to encode a single photon. If multiple photons are encoded a photon-number-splitting (PNS) attack by an eavesdropper is possible. The difficulty of encoding one photon was overcome by using a decoy-state protocol.

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