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  • Cool Ions Promise Quantum Computing

Photonics Spectra
Aug 2002
Gary Boas

In theory, a computer that employs controlled quantum states could simultaneously process tremendous amounts of data, a great advantage for applications such as factoring large numbers and searching large databases. Although it still is unclear how one might design and build a quantum computer, researchers have reported an optical technique that may facilitate its construction and now have proposed an architecture.

A series of ion traps may lead to the development of a quantum computer. Lasers may prepare the ions by sympathetic cooling, such as in these images of 112Cd+ (left) and 114Cd+ (right) ions, illuminated by the "refrigerator" and the probe beam (A), by the probe beam (B) and by the refrigerator beam (C). Courtesy of Christopher Monroe.
A group at the University of Michigan in Ann Arbor noted that collections of cold-trapped ions offer a viable foundation for quantum computers. "In the trapped-ion quantum computer, 'quantum bits' of information are stored in particular internal electronic states of the atomic ions -- one qubit per ion," said Christopher Monroe, leader of the research team, which found that lasers can be employed to "cool" the atoms so that they fall to the bottom of an ion trap. The Coulomb repulsion of the ions, he explained, can then be exploited to make quantum logic gates for computation.

Ions tend to heat up as a result of interactions with the room-temperature environment, however, and further laser cooling is impossible as the quantum logic gates operate. To address this limitation, the researchers turned to sympathetic cooling, in which laser-cooled "refrigerator ions" act on the qubit ions with which they are coupled through the Coulomb interaction. Sympathetic cooling helps to reduce unwanted motion of the ions, but it leads to no adverse effects on internal qubit coherence.

In April, Monroe's team in Michigan reported the cooling of different isotopes. It trapped two cadmium ions and sympathetically cooled the 112Cd+ ion by directly cooling the 114Cd+ ion with a pair of continuous-wave, narrowband, tunable ultraviolet lasers. Now, Monroe and his colleagues David Kielpinski of Massachusetts Institute of Technology in Cambridge and David J. Wineland of the National Institute of Standards and Technology in Boulder, Colo., have presented a large-scale design for an ion-trap quantum computer.

Earlier proposals had investigated a large number of ions confined in a single trap, but the reliable control of even four ions in a trap has proved difficult. Instead, the researchers now suggest a quantum CCD architecture, in which the individual ions are shuttled back and forth between many interconnected traps. In this approach, the system could process large amounts of data, but because each of the traps would confine only a few ions, the logic gates would be stable and reliable.
"You can move ions around without measuring them (and destroying their quantum bit of information)," Monroe said. "But to do logic, they must be recooled to get rid of the random motion from transit. Here, sympathetic cooling once again comes to the rescue."

This is not the only architecture suggested for quantum computing. Physicists are considering approximately a dozen architectures, Monroe said, and most involve solid-state physics because the semiconductor fabrication infrastructure already exists. "The problem is most solid-state systems cannot store quantum information reliably, for the same reason that they are wonderful for storing classical information," he said. "The information carriers may be too close to the environment."

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