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.