Two physicists have demonstrated a way to reliably transfer quantum information from matter to light. The procedure may soon enable scientists to exploit the advantages of both matter and light in building systems for quantum communications.
For years, physicists have been excited about using the properties of quantum theory in computing and communications. In theory, a quantum computer could solve certain problems (such as cracking a code) much faster than a classical computer can; a communications system built upon quantum-mechanical particles would be functionally immune from eavesdropping.
But such systems depend on having information stored on quantum objects such as atoms and photons rather than classical ones such as hunks of silicon--and quantum objects are hard to handle. Quantum bits stored on particles of light travel well--they can zip for kilometers down a fiber-optic cable--but they are tricky to store. Quantum bits stored on matter "keep" for milliseconds or longer, but they're usually confined to a trap and can't be transmitted from place to place. Now scientists have found a way to bridge the two.
Alexei Kuzmich and Dmitri Matsukevich of the Georgia Institute of Technology in Atlanta have figured out how to store a quantum bit in a cloud of rubidium atoms and induce the cloud to inscribe that information, undamaged, upon a photon. By shooting a laser through two clouds of rubidium gas simultaneously, they force the clouds to emit a single photon that is quantum-mechanically entangled with both of the clouds. By manipulating the photon, the physicists can inscribe a quantum bit upon the two clouds, they report in the 22 October issue of Science. This bit can be stored in the cloud for a few hundred nanoseconds, then retrieved when a laser induces the clouds to emit another photon bearing the same quantum information.
That laser-driven retrieval transfers quantum information from matter to light, Kuzmich says. He and Matsukevich are now trying to hook up two of the matter-light devices to create a quantum repeater--an amplifier that reverses the inevitable loss of signal strength that occurs when light is sent down a long stretch of fiber-optic cable. Such repeaters would be essential for long-distance quantum communication.
"I firmly believe that these guys will do it," says Klaus Møller, a physicist at the University of Aarhus in Denmark. If they do, he says, quantum communications and large-scale quantum computation will be considerably closer than before.