Quantum computers are hypothetical machines that would exploit the superposition principle of quantum mechanics to perform an immense number of calculations in parallel. Certain calculations in particular, such as factorization of a very large number, could be done much faster on a quantum computer than on a conventional classical computer. At the APS March Meeting, several physicists discussed the latest efforts towards building a practical quantum computer, a device that uses quantum particles (such as ions or photons) to represent the 0s and 1s (the binary digits, or "bits") employed in computations. Unlike ordinary bits, quantum bits (or "qubits") can represent 0 and 1 simultaneously.

According to Richard Hughes of Los Alamos National Laboratory, the quantum mechanical concepts on which the technology is based were first applied to computation by Paul Benioff, Richard Feynman, and others in the early 1980's. The idea was based on the observation that ordinary, classical, computers follow definite histories in the course of any computation. If one could have a quantum mechanical machine capable of following many histories at the same time, and associate a different computation with each history, one would have a way of doing many calculations in parallel. The next major development came in 1994 when concrete quantum algorithms were found for solving such interesting problems as factorization. This transformed the subject from a curiosity to one with potential for real applications.

Many schemes have since been proposed for building a real quantum computer. Although primitive devices have already been built, a likely scheme for an advanced design would be to trap a string of ions with electromagnetic fields and then use laser beams to manipulate the ions' quantum states and carry out calculations, according to Peter Zoller of the University of Innsbruck, a theorist who co-authored this proposal in 1995. Depending on the computation to be performed, a precise sequence of laser pulses is applied to the ions, which are excited and de-excited in an intricate dance that can only be described in terms of coexisting histories.

Some experimental progress has been made towards realizing this scheme. Having cooled and trapped a string of 5 ions, Hughes and several LANL colleagues have demonstrated that they can point a laser beam at an ion without disturbing its neighbors (the closest of which are 20 microns away). Jeff Kimble of Caltech has been experimenting with an alternate approach in which the qubits are photons trapped between a pair of mirrors.

However, the performance of any real quantum machine will be limited by decoherence, which is a by-product of the same superposition principle that leads to the possibility of quantum computation in the first place. A classical computer can be halted and inspected in the midst of a computation. A quantum computation, by contrast, must be allowed to proceed to completion before any read out is performed. The problem is that it is near impossible to prevent accidental read-out.

Decoherence in the ion trap computer arises from two sources: spontaneous emission of radiation, and vibrations of the ions. The estimated theoretical decoherence in a proposed machine based on arrays of ions driven by precise sequences of laser pulses is surprisingly quite low, and of different origin depending on the size of the computer. Small-sized computers will most likely be limited by spontaneous emission in which the ions radiate light randomly into the surroundings, while larger computers suffer more from vibrations of the ions, which produce tiny electric fields that disrupt each others dynamical behavior.

"Although present-day technical difficulties in building quantum computers are enormously daunting, we know of no fundamental principle of physics standing in the way," Zoller concluded.

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