Abstract
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. Over 6.5 hours during the night, we observed an average raw key generation rate of 565 bits/s, an average quantum bit error rate (QBER) of 4.92%, and an average secure key generation rate of 85 bits/s.
Highlights
Quantum key distribution (QKD) has become one of the first mature applications to develop out of the new field of quantum information processing
Some of the more recent free-space experiments include the distribution of entanglement through intra-city free-space links by Resch et al [3] and Peng et al [4], the distribution of entanglement over 144 km and subsequent generation of a secure key by Ursin et al [5], and the complete implementation of a free-space QKD system which included all key extraction routines by Marcikic et al [6]
A number of studies have been performed to evaluate the possibility of performing quantum key distribution with an orbiting satellite such as the International Space Station [8, 9, 10]
Summary
Quantum key distribution (QKD) has become one of the first mature applications to develop out of the new field of quantum information processing. There are a number of different QKD protocols which have been demonstrated using both optical fibers and free-space optical links as their quantum channel. While fiber implementations have produced some of the fastest systems to date, until reliable quantum repeaters are realized, fiber implementations seem to be limited to < 200km. This has prompted increased attention on free-space implementations. Experience with free-space quantum key distribution in a variety of setups and experimental conditions is very valuable for future long distance experiments
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