A Japanese research team from NEC and RIKEN has constructed a fundamental building block for a viable quantum computer.

The basic building blocks of a quantum computer are quantum bits or qubits using logic gates called CNOTs.

Among the startling properties of qubits is that they do not just hold either binary 1 or binary 0, but can hold a superposition of the two states simultaneously. As the number of qubits grows, so does the number of distinct states which can be represented by entangled qubits. Two qubits can hold four distinct states which can be processed simultaneously, three qubits can hold eight states, and so on in an exponential progression.

So a system with just 10 qubits could carry out 1,024 operations simultaneously as though it were a massively parallel processing system. A 40-qubit system could carry out one trillion simultaneous operations. A 100-qubit system could carry out one trillion trillion simultaneous operations.

That means calculations, such as working out the factors of prime numbers, which present problems for even the fastest supercomputers could be trivialized by a quantum computer. As an example Tsai estimated that using the Shor Algorithm to factor a 256-bit binary number, a task that would take 10 million years using something like IBM Corp.'s Blue Gene supercomputer, could be accomplished by a quantum computer in about 10 seconds.

According to NEC's Special Issue on Quantum Information Technology (July 2003) the team seems to be less optimistic about the practical use of the quantum computer and sees the problems we'll face in implementing such a computer.

As even more important thing is the issue of software. The design methodology of software must be completely rebuilt in the quantum computer system because the principle of the computation mechanism is entirely different from that of the conventional computer based on the so-called von Neuman model. The algorithms, languages, operating system software, and input/output system must be fundamentally reconsidered and newly developed, by which we mean abandoning all software assets accumulated in the past if we change to the quantum computer. Considering such things, the application field of the quantum computer may be very limited. Even so, however, its immeasurable power of computation will give full play to resolving the scientific and engineering problems we are now facing, and will face in the future.

If you are interested in how the logic in a quantum computer works then have a look at this article from last year by TRN.