Defining quantum computer bits and pieces.

In principle, a computer obeying the laws of quantum mechanics can efficiently solve a number of complex problems that a conventional computer can't handle. This striking capability has stimulated a wide-ranging exploration of the possibility of building such a computer (SN: 1/14/95, p.30).

Three papers in the May 15 Physical Review Letters provide hints of how one might go about designing and assembling the logic units, or gates, needed for quantum computation. They also suggest ways of constructing such gates out of quantum dots, atomic beams, or trapped ions interacting with laser beams.

"These papers represent significant, though incremental steps in our understanding of quantum computation," says David P. DiVincenzo of the IBM Thomas J. Watson Research Center in Yorktown Heights, N.Y.

A decade ago, David Deutsch of the University of Oxford in England provided the first theoretical description of how a quantum computer might work. In the May 15 Physical Review Letters, Deutsch, Oxford colleagues Adriano Barenco and Artur Ekert, and Richard Jozsa of the University of Plymouth in England show how one can use a simple type of quantum logic gate as the main building block of an information-processing system.

The researchers focus on the quantum analog of the "controlled- NOT" gate. This gate works with two input bits, leaving the first bit unchanged and changing the second bit from 0 to 1 or 1 to 0 if the first bit is 1. Because it's possible to determine the values of the two input bits from the output values, such a gate is said to be reversible.

In their paper, Tycho Sleator of New York University and Harald Weinfurter of the University of Innsbruck in Austria also look at reversible quantum logic gates. They demonstrate that a two-bit quantum gate is sufficient for building any quantum logic network.

In other words, one can assemble a chain of two-level quantum systems, interacting two at a time, to achieve any computational result desired. Moreover, the identification of such a "universal" quantum gate "considerably simplifies the search for implementations of quantum computational networks," the researchers say.

Peter Zoller and J.I. Cirac of the University of Innsbruck describe how it may be possible to construct quantum gates out of a line of barely jiggling ions confined in a magnetic trap. In this case, the basic computational elements, or qubits, of the quantum computer are the ions themselves.

Normally, each ion in the row oscillates about an equilibrium position, and repulsive electric forces between adjacent ions induce a collective motion shared by all the ions (see diagram).

A laser beam shining on an ion causes a transition from one quantum state of the ion to another, which can alter the type of collective motion possible in the ion array. Thus, by selecting appropriate laser frequencies, one can control the interactions between individual ions via their shared movements.

"This paper is the most comprehensive and careful analysis to date about how close you can get to doing quantum operations," says IBM's Charles H. Bennett. But this doesn't mean that anyone is close to building even a rudimentary quantum computer. Many obstacles remain.