Japanese boffins advance quantum computing

By John Leyden

Posted: 31/10/2003 at 16:21 GMT

A Japanese research team has for the first time successfully demonstrated one of the fundamental building blocks needed to construct a viable quantum computer.

The team from NEC and the RIKEN Institute of Physical and Chemical Research were able to establish a quantum logic gate (controlled NOT or CNOT) operation in a solid-state device consisting of two coupled quantum bit (qubits).

A CNOT gate is a basic building block to a quantum computer in much the same way that that AND / OR gates are building blocks in traditional semiconductor devices.

However, this is just one step in a highly complex development process. For one thing, researchers need to establish how to extend the quantum entanglement of the bits necessary for the operation of the gate beyond fractions of a second.

IDG reports that experts predict that quantum computers are at least ten years away.

Full details of the research results will be published in the October 30 issue of the British science journal Nature.

Quantum computing (whose basic concepts are enough to make your head spin) has the potential to revolutionise computing as never before. Over to NEC for an explanation of its researchers' work:

The quantum computer, when it is finally brought to fruition, will likely far surpass the capabilities of even the most modern of today's supercomputers, as it utilizes a "qubit," a quantum superposition of the "0" and "1" states, as the basic information unit.

Information processing in a quantum computer is conducted by a series of quantum gate operations to the qubit, and it has been theoretically proven that only two kinds of gate operations are sufficient for quantum computation, no matter how complicated the algorithms are and no matter how many qubits they require.

The two gates are the one-qubit rotation gate which controls the state of one qubit, and the controlled-NOT (C-NOT) gate which works as a conditional gate for two qubits.

In 1999, NEC succeeded in controlling the quantum state of one qubit made of small superconductors. This was the first demonstration of the one-qubit rotation gate in solid state devices.

Then in February of 2003, a joint NEC/RIKEN laboratory demonstrated the world's first quantum entanglement in a 2-qubit solid-state device.

However, to date no one has succeeded in realizing the above-mentioned two-qubit conditional gate.

Now NEC and RIKEN have proposed a new scheme to implement the C-NOT gate using a similar type of two-qubit circuit made of superconductors, and have experimentally demonstrated that it works properly as a logic gate for two qubits.

Having succeeded in realizing both the one-qubit rotation gate and the two-qubit conditional gate, the research laboratory will continue to strive toward achieving further integration of the qubit to demonstrate quantum algorithm, an effort which is expected to contribute significantly to the practical realisation of a quantum computer. ®

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