The ability to store and retrieve a so-called qubit state in an atomic quantum memory node could help bring "quantum repeaters," devices that let quantum information transmit long distances by optical fiber.

In the lab, researchers excite (at 200 Hz) a cloud of rubidium atoms stored in a magneto-optical trap cooled to temperatures near absolute zero, a process that generates a single photon about once every 5 sec. Because the photon is in resonance with the atoms from which it was created, it carries specific quantum information about the excitation state of the atoms.

The liberated photon next travels along a 100-m-long optical fiber to a second cloud of trapped rubidium atoms. An intense laser beam controls the velocity of the photon until it is inside the second cloud, at which time the beam is switched off. The photon halts inside the dense ensemble of atoms where it sits for about 10 sec. The control beam is then switched on and the photon escapes the atomic cloud. Researchers verify the quantum information encoded on the exiting photon matches that which was carried into the second cloud.

Success of the experiments depends on careful control of potentially interfering magnetic fields. Stray magnetic fields are a problem because they can make atoms spin out of phase and lose their information. The team hopes to eventually add additional nodes to the rudimentary quantum network and encode useful information into photons.

Most recently the team demonstrated entanglement between two atomic qubits separated by a distance of 5.5 m. Such entanglements could find use in quantum cryptography, say researchers, though practical applications remain a long way off. Funding for the research comes from NASA, the Office of Naval Research Young Investigator Program, National Science Foundation, Research Corp., Alfred P. Sloan Foundation, and Cullen-Peck Chair.