Long-Distance Free Space Distribution of Entangled Photons

We demonstrate the distribution of quantum entanglement via optical free-space links to independent receivers separated by 600 m, with no line of sight between each other. A Bell inequality between those receivers is violated by more than four standard deviations, confirming the quality of the entanglement. This outdoor experiment represents a step toward satellite-based distributed quantum entanglement.

One of the benefits of a free-space distribution of quantum entanglement is the possibility of bridging large distances by additional use of space infrastructure such as satellites. Free-space links will provide a unique means to study fundamental limits due to long-distance deterioration of quantum correlations because of decoherence. In contrast, free-space quantum channels may provide the means to build truly global quantum communication networks. In the specific example of quantum cryptography, quantum entanglement enables secure key generation between the parties sharing entanglement, whereas the violation of a Bell inequality can provide a security test of the protocol. Although we did not intend to establish a cryptographic protocol, we can estimate the results for a cryptographic scheme based on entangled photons. A raw key is established by the anti-correlated detection events, which are obtained when both receivers randomly choose the same basis. A cryptographic system based on our setup would have shown a total raw key generation rate of a few tens of bits per second and an estimated quantum bit error rate (QBER) of 8.4%. We believe that these numbers can be improved significantly through improvements in telescope design. Our link attenuation of 12 dB corresponds to a value that might be achievable with state-of-the-art space technology when establishing a free-space optical link between an Earth-based receiver telescope of 100 cm diameter and a satellite-based transmitter telescope of 20 cm diameter orbiting earth at a distance of 600 km. Typical losses in an actual satellite experiment might vary, depending on the link optics and on the performance of satellite pointing and tracking. Even then, our results represent an encouraging basis for future space experiments with entangled photons.

M. Aspelmeyer, H. Böhm, T. Gyatso, T. Jennewein, R. Kaltenbaek, M. Lindenthal, G. Molina-Terriza, A. Poppe, K. Resch, M. Taraba, R. Ursin, Ph. Walther and A. Zeilinger

Long-Distance Free-Space Distribution of Entangeld Photons
Science 301 (2003) 621-623