The strange properties of the quantum world should allow a quantum computer to outperform any existing computer. While classical computers process binary digits (bits) of information, quantum processors use quantum bits, or qubits, encoded in the quantum states of particles such as atoms, photons and electrons. Since such particles can be in several states at once, qubits would allow huge numbers of computations to be carried out simultaneously.
But the practical obstacles are formidable, not least because the quantum properties such devices will exploit are extremely sensitive to disturbance from the outside world. Unless kept completely isolated they leak their quantum information- a phenomenon called "decoherence".
Last week, Robert Clark of the University of New South Wales in Sydney unveiled a silicon-chip qubit, complete with a readout mechanism, that seems to solve the problem. "This was thought to be impossible just a few years ago," Clark told the conference.
The UNSW device is based on a blueprint drawn up in 1998 by Bruce Kane of the University of Maryland. Kane envisaged using two phosphorus atoms embedded in a silicon crystal (New Scientist, 13 January 2001, p 14). The quantum states of phosphorus atoms are particularly long-lived, so the hope is that they will provide qubits that are resistant to decoherence. A large array of such chips would then allow useful quantum processing.
Building Kane's device calls for the atoms to be implanted extremely precisely, and researchers had doubted whether this would be possible. But working with a team at the University of Melbourne, Clark and his colleagues implanted phosphorus atoms within a silicon chip by focusing a high-energy beam of phosphorus ions onto it.
They have now verified the atoms' position within the chip, and shown that their quantum state can be read using sensitive single-electron transistors. The only deviation from Kane's blueprint is that the chip uses states of charge rather than spin to process information.
Clark believes that his device can be scaled up to make arrays of qubits, and hopes to be carrying out processing tasks by 2007. One of his goals is to run Grover's algorithm, a quantum database search that's much faster than is possible with standard computers.
Also at the meeting, Rainer Blatt of Innsbruck University in Austria announced that his team has carried out a quantum computation using a single trapped calcium ion. It is the first calculation made on a system proven to be in a quantum state.
The Innsbruck researchers used their calcium ion to execute a quantum procedure called the Deutsch-Josza algorithm, which involves working out whether an imaginary coin is the same or different on each side. A quantum computer can check both sides at once, so can answer the problem at least twice as fast as a classical computer.
The team has also made progress in using photons to carry quantum information between calcium qubits. This is an important step in linking individual qubits to form larger arrays.
The advances made in the field belie the difficulty of manipulating quantum information, according to Peter Knight, a quantum information researcher at Imperial College, London. But he believes there is now cause for optimism about developing a truly useful quantum processor. "There's been tremendously rapid progress in the last year. I was hugely impressed at how things have developed," he says.
New Scientist issue: 30th November 2002
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