Entangled Quantum Key Distribution Over Two Free-Space Optical Links

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We report on the first real-time implementation of a quantum key distribution (QKD) system using entangled photon pairs that are sent over two free-space optical telescope links.

• The entangled photon pairs are produced with a type-II spontaneous parametric down-conversion source placed in a central, potentially untrusted, location.
• The two free-space links cover a distance of 435 m and 1,325 m respectively, producing a total separation of 1,575 m.
• The system relies on passive polarization analysis units, GPS timing receivers for synchronization, and custom written software to perform the complete QKD protocol including error correction and privacy amplification.