This may be an important step in the construction of a future "quantum computer," in which quantum superpositions of inputs are processed simultaneously in a single device. Trapped atoms offer one of the only realistic approaches to precisely controlling the complex quantum systems underlying a quantum computer.

The demonstration is described in the April 2002 issue of Physical Review in an article, "Sympathetic Cooling of Trapped Cd+ Isotopes," by Boris B. Blinov, Louis Deslauriers, Patricia Lee, Martin J. Madsen, Russ Miller, and Christopher Monroe.

Partially based on these results, Monroe has proposed a new "Architecture for a Large-Scale Ion-Trap Quantum Computer," with co-authors David Kielpinski (MIT) and David Wineland (National Institute of Standards and Technology), in the June 13 issue of the journal Nature.

Interest in quantum computing has mushroomed in the last decade as its potential for efficiently solving difficult computing tasks, like factoring large numbers and searching large databases, has become evident. Encryption and its obverse, codebreaking, are just two of the applications envisioned for quantum computing if and when it becomes a practical technology.

Quantum computation has captured the imagination of the scientific community, recasting some of the most puzzling aspects of quantum physics---once pondered by Einstein, Schroedinger and others---in the context of advancing computer science. "Right now, there's a lot of black magic involved in understanding what makes a quantum computer tick and how to actually build one," Monroe said.

"Many physicists doubt we'll ever be able to do it, but I'm an optimist. We may not get there for decades, but given enough time and resources---and failing unexpected roadblocks like the failure of quantum mechanics---we should be able to design and build a useable quantum computer. It's a risky business, but the potential payoff is huge."

In their experiment, the Michigan researchers used electric fields to confine a crystal of exactly two Cd+ atoms of different isotopes. They were able to cool the single 112Cd+ atom to a chilly 0.001 degree Celsius above absolute zero through direct laser cooling of the neighboring 114Cd+ atom. Laser cooling of this "refrigerator atom" removes unwanted motion in the atom crystal without affecting the internal state of the other atom.

This is an important step toward scaling a trapped atom computer, where "qubits" of information are stored in the quantum states within the individual atoms.

The architecture proposed in the Nature article describes a "quantum charge-coupled device" (QCCD) consisting of a large number of interconnected atom traps. A combination of radiofrequency (RF) and quasistatic electric fields can be used to change the operating voltages of these traps, confining a few charged atoms in each trap or shuttling them from trap to trap, and the traps can be combined to form complex structures. The cooling of multiple species demonstrated at Michigan is a key component of this broader proposal.

"This is a realistic architecture for quantum computation that is scalable to large numbers of qubits," the authors conclude. "In contrast to other proposals, all quantum state manipulations necessary for our scheme have already been experimentally tested with small numbers of atoms, and the scaling up to large numbers of qubits looks straightforward."

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