Many groups in industry and academia worldwide are actively developing quantum information processing (QIP) into a real information technology, with many approaches needing substantial input from materials science.

Quantum computing offers the possibility of solving certain types of problem that are impossible to solve with conventional computers, and represents a whole new way of processing information. While quantum cryptography and related communications technologies offer the reality of high security information transfer, albeit over relatively short distances at present.

In a classical computer, the basic unit of information is the ‘bit’, which can exist in one of two possible states, e.g. yes/no (or 0/1 in binary language). Quantum computers make use of quantum bits (qubits), which can exist in a superposition of both states, e.g. a mixture of both 0 and 1 simultaneously. Qubits are also subject to quantum entanglement. When two or more qubits are entangled, they behave as one system, so that the state of one qubit depends directly on the state of the others. Entanglement has the consequence that the potential processing power of a quantum information system increases exponentially with the number of qubits. The difficulty lies in maintaining this fragile quantum coherence against the many interactions that allow the information to leak away. A thorough understanding of the materials science and transport physics is needed to design devices that will work sufficiently well to be useful.

One approach to building a solid-state quantum computer is by exploiting the quantum states of artificial atoms and molecules built in semiconductor quantum-dot (QD) systems, for example, using an isolated Si double QD as a qubit. The key challenges in producing efficient quantum circuits are to have a system with a sufficiently high number of operations within the characteristic coherence time of the qubits, to control the coupling between qubits to form architectures, and to integrate the qubits with manipulation and measurement circuitry. All operations (initialization, manipulation, and measurement) have been achieved using electrical gates for initialization and manipulation, and a single-electron transistor for measurement. Such a system offers the possibility of scaling-up from one device to a large quantum circuit – a necessary criterion for making a useful quantum computer. This structure, based on years of work on single electronics, is the first step in the development of a quantum computer based on conventional Si technology. However, many problems remain. Even though Si technology represents one of the most intensively studied materials systems, studies have largely concentrated on those aspects of Si/SiO2/metal silicide structures in mainstream use. There are still many materials issues to be solved involving nonstandard materials combinations and geometries.

When it comes to other implementations, the need to understand the materials issues becomes more compelling, even for approaches that are not regarded as strictly ‘solid state’ (if the qubit itself is not in a solid), as there are usually solid components nearby that form a critical part of the experiment.

Although the principles behind quantum computing have been established and small model systems constructed, it still remains a considerable task to scale these up to practical, working computers. This is certainly worth doing, however, as it would enable certain types of computation that are currently, if not impossible using classical computers, impractical within a sensible timescale. A raft of potential applications includes bioinformatics, molecular modeling, codebreaking, and encryption. Quantum computers could also be used as simulators to solve quantum mechanics problems.

There are several challenges to be met before a practical quantum computer can be built. Some of the current issues include algorithm development, maintaining quantum entanglement, and hardware architecture.

Cryogenics may provide a means of achieving a working solid-state quantum computer using ultralow temperatures to preserve quantum coherence and remove any thermal processes that could interfere with computations. In the past, potential performance improvements have led to the suggestion of cooling conventional processors, but in all cases it has been cheaper and more convenient to use more processors or run them for longer. The difference here is that a working quantum computer could perform tasks that are for all practical purposes impossible by classical means – even putting all the world's supercomputers to work on the problem.

Although quantum computers are currently in the early stages of development, small model systems have already been demonstrated and the potential power and number of applications remains huge. Indeed, many more applications may become apparent once larger working models have been built and more algorithms have been tested. Although the origins of this discipline are in highly specialized theoretical physics, the potential applications may have a dramatic impact on everyone from physicists to pharmaceutical chemists, and materials science has a major role to play in this process.

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DOI: 10.1016/S1369-7021(07)70191-8