LONDON A team at Hitachi’s Cambridge Laboratory at the University of Cambridge in England has developed a silicon device for quantum computing: a quantum-dot charge “qubit”. This structure, based on Hitachi's many years of work on single-electron devices, is the first step in the development of a quantum computer based on conventional silicon technology, according to Hitachi Europe Ltd.
Quantum computers make use of quantum bits (qubits), which can exist for a limited time as a superposition of two quantum states a mixture of both 0 and 1 simultaneously. Qubits are also subject to the strange phenomenon of quantum entanglement. When two or more are entangled, they behave as one system, so that the state of one qubit depends directly on the state of the others.
This means that the potential processing power of a quantum information system increases exponentially with the number of qubits, rather than linearly, Hitachi said.
The Hitachi Cambridge Laboratory scientists have demonstrated the creation of a single qubit in a silicon circuit, made using standard fabrication techniques and which is the first step towards making a silicon quantum computer.
Qubits have been made before in gallium arsenide but these have had very short coherence times of a couple of nanoseconds. The Hitachi team has demonstrated an isolated double quantum-dot as a qubit built in silicon with a coherence time 100 times longer than shown in other solid-state implementations, Hitachi said.
The key to building a quantum computer is to produce enough qubits at so that they can interact within their coherence times; to be manipulated, formed into architectures and then to process data. The Cambridge team has performed all the basic operations ’; initialization, manipulation, and measurement using electrical gates for initialization and manipulation, and a single-electron transistor for measurement, Hitachi said.
As qubits can be combined in a variety of two-dimensional circuits, as in conventional microprocessors, there is the possibility of scaling-up from one device to a large quantum circuit, which is Hitachi Cambridge Laboratory's next project.
The qubit research was due to reported in Physical Review Letters in August 2005, and at the International Symposium on Foundations of Quantum Mechanics in the Light of New Technology, which was due to held in the Advanced Research Laboratory, Hitachi, Ltd., Hatoyama, Saitama, Japan, also in August 2005.