m that acts just like a much smaller number of pure qubits. The nuclear spin is beautifully isolated from the environment; its spin coherence can last for thousands ofseconds. By representing the effective computational qubits in such an ensemble, we get these very longcoherence times permitting thousands of logical operations before coherence is lost. Further, because thebits are represented in an ensemble, it is possible to continuously read out the quantum state (somthingthat is of course impossible for individual quantum degrees of freedom). Best of all, the most importantpart of the experimental apparatus is built by nature in the form of ordinary molecules. Implementing such a quantum computer requires the mature techniques of multiple pulse spin resonance.Using existing NMR spectrometers it will be straightforward to reach about 10 qubits, enough todemonstrate for the first time quantum superfast algorithms and quantum error correction, and to preparea range of unusual quantum states that have never been realized before (such as theGreenberger-Horne-Zeilinger states that maximally violate Bell's Theorem). The required instrumentationeven promises to scale down to the desktop, so that everyone could have a quantum co-processor.what makes quantum computers so different from their classical counterparts we begin the explanation by having a closer look at a basic chunk of information namely one bit. From a physical point of view a bit is a physical system which can be prepared in one of the two different states representing two logical values --- no or yes, false or true, or simply 0 or 1. For example, in digital computers, the voltage between the plates in a capacitor represents a bit of information: a charged capacitor denotes bit value 1 and an uncharged capacitor bit value 0. One bit of information can be also encoded using two different polarisations of light or two different electronic states of an atom. However, if we choose a...
