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Friday, January 23, 2009

A Quantum Memory Leap

Credit: John Hersey

In recent years, physicists have devised numerous ways to use the oddities of quantum mechanics to transmit and process information.

Now a team of researchers has announced an important step toward using this quantum information: the ghostly transfer of the quantum state of a single ion to another one a meter away. Since ions can store a quantum state for many seconds, this scheme for "quantum teleportation" could buy enough time for manipulations that allow long-distance communications that are immune to eavesdropping, or for computations that exploit the quantum mechanics to perform blazing fast calculations.

To transfer a little quantum information from one atomic-sized system to another, the two systems must start out in the quantum condition called entanglement. Entangled systems always give corresponding answers, like two coins that, although individually unpredictable, always come up one heads and one tails. Physicists have teleported the state between entangled photons of light, but unfortunately, they can't store the quantum information for very long. Recently, other researchers have teleported the much longer-lived quantum state of individual ions, but only when they were trapped very close together.

To transfer persistent quantum information over longer distances, Chris Monroe and his group at the University of Maryland teamed with Luming Duan at the University of Michigan to trap and cool two individual ytterbium ions.The team encoded quantum information by mixing two states that differ only by the angular momentum of the nucleus. Unlike the "0" or "1" value of a bit in ordinary computing, the researchers can create an arbitrary mixture of the two nuclear states, known as a qubit, by subjecting the ions to microwaves. Once formed, an ion retains this mixture for several seconds--long enough to perform calculations that act on both values simultaneously.

Extending a technique that Monroe's team demonstrated in 2007, the researchers exposed both ions to an ultrashort pulse of light, knocking each to a higher-energy state. Each ion then returned to its original state by emitting a photon. Measuring the color of this photon would have left the ion in one of the two nuclear states. But instead, the researchers just tested whether the two photons were different colors. Since they didn't determine which color came from which ion, seeing this result left the ions in an entangled state that included both possibilities.

Beam me up: Each of the two cylindrical chambers (left and right) holds a single atom. The black tubes in the foreground are used to image each atom. Optical fibers that channel single photons from each atom are opposite the tubes, on the left side of the picture, covered in black paper. The photons interfere inside the big black rectangular box at the left.
Credit: Curt Suplee

To perform teleportation, the researchers prepared the left-hand ion in an arbitrary quantum state, and then repeatedly zapped the ions with laser pulses until they saw the pairs of opposite-color photons that heralded the entangled state. They quickly measured which nuclear state the left-hand ion was in, in the process destroying its quantum mixture. But the entanglement causes a closely related mixture to appear in the right-hand ion. The researchers turned this back into a teleported version of the original state by manipulating it in one of two ways, depending on which state they measured for the left-hand ion.

"This is the first realization of quantum teleportation between two remote atoms," observes Myungshik Kim of Queen's University Belfast, in Northern Ireland, who was not involved in the work. "It's a quite clever technique."

One problem is that it takes almost 100 million laser pulses--about 10 minutes--to get a single entangled pair. To be useful for further experiments, this number needs to be improved about 1,000-fold, mainly by collecting more of the emitted photons. The scheme for teleporting between distant ions could enable quantum repeaters that allow long-distance transmission of quantum information, Monroe notes. In addition, he says that it is well suited for an increasingly studied approach to quantum computation that starts with a large number of entangled qubits.