A Scalable Quantum Computer Chip

A scalable quantum computer chip for atomic qubits has been built for the first time by researchers at the University of Michigan (Christopher Monroe, crmonroe@umich.edu), offering hopes for making a practical quantum computer using conventional semiconductor manufacturing technology.

Exploiting the strange rules of the atomic world, quantum computers could potentially break top-secret codes and perform certain kinds of searches much more quickly than conventional computers. The building blocks of quantum computers are called "qubits," or quantum bits, made of such objects as atoms or photons. Connecting multiple qubits via an electrostatic (or other suitable) interaction could then result in a quantum computer, similar to how wiring together individual transistors can make a traditional computer.

Unlike a conventional computer's bits, which can have values of either 0 or 1, a qubit can possess a value of 0 and 1 simultaneously, analogous to a light switch that's on and off at the same time. For their qubit the Michigan group chose an individual cadmium ion, held in free space by a number of electrodes inside a postage-stamp-sized gallium arsenide semiconductor chip. There additional electric fields are able to manipulate the position of the ion, and laser beams could control the qubit value in the ion.

Ions pose an advantage over other potential qubits, such as photons and electron dots, in that they are easier to isolate and shield from external disturbances (noise) that can disrupt their operation. An integrated semiconductor chip is a markedly different environment for ion qubits, which were previously held in hand-made ion traps that could not be easily scaled up or mass produced.

The researchers have not yet demonstrated a quantum computer based on this design, as it only consists of a single qubit. Making a quantum computer would require scaling up a single chip so that it contains enough electrodes to trap many ions simultaneously (Stick et al., Nature Physics, January 2006).

This has been a busy month for announcements in the ion computing realm: groups at the National Institute of Standards and Technology in Boulder, Colo. (Leibfried et al., Nature, December 1), and at Innsbruck University in Austria (Häffner et al., Nature, December 1) independently reported entangling up to eight ions, while the Michigan group used another setup to perform a simple version of a quantum search known as Grover's algorithm. (Brickman et al., Physical Review A, November 2005).

Stick et al., Nature Physics, January 2006.
Nature Physics is a new journal that began publication in October 2005.