Quantum Computing - Yes, no, or both?

Our appetite for more computational speed seems to have become insatiable as we find new ways to use information processing to improve the quality of our lives. However, there are problems on the road ahead: the technology foundation on which all computers are built, semiconductor-based integrated circuits, will soon reach the physical limits of the speed at which they can perform operations. It seems the laws of nature are going to get in the way of the relentless increases in computational power that we now demand.

All modern computers are built around microprocessors that consist of many thousands of transistors (basically electronic switches) built into a small integrated circuit on a silicon chip. Essentially, the more transistors you can fit onto a chip, and the smaller they are, the faster and more powerful the processor. In 1965 Gordon Moore, one of the founders of Intel, predicted that the complexity of integrated circuits, i.e. the number of transistors built onto a single chip, would approximately double every two years, leading to an exponential growth in computational power. Even though this prediction was made over 40 years ago, when this technology was in its infancy, Moore's law has proven surprisingly accurate - however this cannot continue indefinitely. The latest generation of PC processors (and the one that you are probably using right now) have a smallest feature size of 65 nm (nanometers, or billionths of a meter) and a transistor density of hundreds of millions per square mm. Over the next few years, the smallest feature size is expected to drop further, and performance increase proportionally – however, both feature size and transistor density are now getting close to the atomic scale, which imposes a fundamental limit on how small we can go. When semiconductor devices reach the scale of a few nanometers, they will not function in the same way that they do at larger scales, since quantum mechanical effects will start to influence their properties. Essentially, transistors will no longer be capable of functioning as switches due to insulating materials being so thin that the flow of electrons cannot be prevented.

So what are the alternatives to the silicon chip? Whatever is eventually developed to replace semiconductor processors, the architecture will have to change significantly. One possibility that is now being seriously considered is to use individual molecules to form the basic components of a processor. A single small molecule can act as a transistor or even an entire logic gate, and could replace silicon transistors as the building blocks of a computational device. Single molecules would be able to perform an operation much faster than a semiconductor device and also be much smaller than the equivalent semiconductor device minimum possible size. This new field of 'molecular electronics' is growing rapidly and relies on many developments in chemistry as well as in physics and materials science. The outlook is promising, however there are many technical obstacles to overcome before an electronic processor composed of molecular components can be fully realised, such as how to connect the moelcules together and have complete control over the stucture of the device.