Quantum Computing - Yes, no, or both?

Another, quite different, approach has been developed that also employs molecules to perform a computational function. Reactant molecules can be encoded with input information and then a computaion is performed through a chemical reaction, revealing its result in the product molecules. This has been implemented successfuly in a 'DNA computer' to solve several complex problems where the input information is encoded in DNA sequences and a reaction produces the output sequence. This reaction happens very quickly and can process a lot of information, however encoding the sequences and harvesting the result is an extremely labourious task that requires many stages of chemical analysis.

A much more exciting and ambitious idea is to use the actual quantum mechanical properties of a system to perform computational functions: to build a 'Quantum computer'. The use of quantum mechanical phenomena in this way leads to a fundamentally different logic to that employed by 'classical' computers – and promises to totally change the whole paradigm of information processing.

The basic principle of a quantum computer is that the fundamental entity (the bit in a classical computer) can not only exist in two definite states (1 or 0 - on or off) but also in a quantum superposition of them (i.e. both states at the same time) – this is the so-called quantum bit (or 'qubit'). When the system is observed (i.e. it interacts with the external world) it will 'collapse' into one of the two distinct states.

A collection of qubits, forming a register in a quantum computer, can exist in a superposition of all the possible states of the qubit register. For example, a register of eight binary qubits (a qubyte) has 2^8 = 256 distinct states. Performing a logical operation on the superposition of these states effectively performs the operation on all of the possible states simultaneously. This allows a quantum computer to perform many computations in parallel.

The properties of qubits and the quantum computers that could be formed from them seem very strange, and have led to the development of totally new algorithms and effectively a new form of mathematics. We have still not fully appreciated the capabilities of a working quantum computer and the problems it could be used to solve, but one example of its power could be in the modelling of large quantum mechanical systems such as biological molecules and nano-structures, which at present require huge computational power to run the models and calculate the properties of the simulated system. Simulations such as these are crucial to many areas of science, including chemistry, biology and medicine.

Although research into the development of a quantum computer is a very active area and many different potential technological solutions are now being investigated, there are significant practical difficulties in realising the goal. The greatest of the problems facing researchers is that the qubits in a register must be able to interact with each other, but also be totally isolated from everything else, otherwise the superposition would collapse (through a process called decoherence). Another problem is that each qubit must be addressed individually in order to exchange information with the computer, and since the qubits must usually be single atoms, electrons or photons managing an exchange of information becomes difficult due to the requirement of applying a small force to a precise location. Of the working quantum computers developed so far, demonstrations of quantum algorithms have only been possible using a very small number of qubits. According to David Deutsch, the pioneer of the concept of quantum computing, many hundreds of interacting qubits would be required for a 'useful' quantum computer that could tackle currently unsolvable problems.

It is very difficult to predict what will eventually replace the silicon microchip, and how our computers will work in 20 or 30 years time. But one thing is fore sure, we will never be satisfied with the computational power on offer and always be craving more.

For more information

Moore's Law
http://www.intel.com/technology/mooreslaw/index.htm

Computing with Single Molecules
http://www.picoinside.org

Introduction to Quantum Computing
http://www.cs.caltech.edu/~westside/quantum-intro.html