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.