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The first element of a device that many believe holds the best hope for quantum 
information processing has been completed by Australian researchers, while an 
Austrian team has reported the first truly quantum calculation. The achievements go 
some way to dispelling the widely-held idea that doing anything useful with quantum 
computing is decades or even centuries away.

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 a 
discussion meeting at the Royal Society in London, UK.


Precise implant

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. 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 is much faster than is possible with 
standard computers.

Head and tails

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.