A quantum computer is one that would behave quite differently from a normal computer. Instead of being built from `bits' that take definite values (0 or 1) it would be constructed from `qubits' which can exist in quantum superpositions of states. In the simplest case the qubits have two states, |0> and |1>, and one could write the state of the qubit as
where a and b are arbitrary complex numbers. A computation would be performed by It is now known that a quantum computer, if one could be built, could perform certain tasks much, much faster than any `classical' computer. These tasks include the factoring of integers - which is of immediate practical importance since modern public-key encryption schemes rely on the fact that factoring of large integers is essentially impossible. A quantum computer would also be particularly effective at simulating the behaviour of other (quantum) systems, or at performing certain types of database search.
There is therefore great interest in whether or not a quantum computer can, in fact, be constructed. Several quite different physical systems have been proposed; these include almost-isolated ions in an ion trap and nuclei in magnetic resonance experiments. However many people believe that the most feasible systems for practical quantum computations are solid-state nanostructures.
The crucial question determining whether such a quantum computer could be built is how quickly the wavefunctions of the qubits lose their quantum-mechanical coherence. The origins of this `decoherence' are relatively well understood for relatively isolated systems such as ions in an ion trap, but much more poorly understood for condensed-phase systems in intimate contact with their environment. Our aim is to understand this decoherence, the mechanisms which mediate it condensed-matter systems, and the possibilities of controlling it.