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Photons Entangled over the Danube

Photonics Spectra
Sep 2003
To demonstrate the viability of free-space configurations as an alternative to optical fiber for quantum key distribution systems, scientists at Universität Wien in Vienna, Austria, have performed a real-world experiment in which they transmitted entangled photons through the atmosphere to receivers separated by 600 m across the Danube River. They presented their findings in the Aug. 1 issue of Science.

The team produced polarization-entangled photons by type II spontaneous parametric down-conversion in a nonlinear crystal pumped by an 18-mW laser diode operating at 405 nm. The 810-nm photons were then separated and routed to receiver stations equipped with polarizers and single-photon-counting avalanche photodiodes that were located on rooftops 150 and 500 m from the source. The average fidelity of the system was 87 ±3 percent, and an analysis of the Bell parameter indicated that the setup violated a Bell inequality by more than four standard deviations, proving that the receivers shared an entangled quantum state.

The researchers estimate that a cryptographic scheme employing the system would offer a key generation rate of tens of bits per second and a bit error rate of approximately 8.4 percent. They note that the 12-dB attenuation in each of the links is equivalent to that in a state-of-the-art optical link between a satellite-based transmitter orbiting at 600 km and a terrestrial receiver station.


GLOSSARY
optical fiber
A thin filament of drawn or extruded glass or plastic having a central core and a cladding of lower index material to promote total internal reflection (TIR). It may be used singly to transmit pulsed optical signals (communications fiber) or in bundles to transmit light or images.  
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