To extend the distance of these links, researchers are looking beyond optical fibers as the medium to distribute quantum keys. Scientists have trekked to mountaintops--where the altitude minimizes atmospheric turbulence--to prove the feasibility of sending quantum keys through the air. One experiment in 2002 at Los Alamos National Laboratory created a 10-kilometer link. Another, performed that same year by QinetiQ, based in Farnborough, England, and Ludwig Maximilian University in Munich, stretched 23 kilometers between two mountaintops in the southern Alps. By optimizing this technology--using bigger telescopes for detection, better filters and antireflective coatings--it might be possible to build a system that could transmit and receive signals over more than 1,000 kilometers, sufficient to reach satellites in low earth orbit. A network of satellites would allow for worldwide coverage. The European Space Agency is in the early stages of putting together a plan for an earth-to-satellite experiment. (The European Union also launched an effort in April to develop quantum encryption over communications networks, an effort spurred in part by a desire to prevent eavesdropping by Echelon, a system that intercepts electronic messages for the intelligence services of the U.S., Britain and other nations.)

   
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