Bulk spin quantum computation: toward large-scale quantum computation

Quantum computation offers the potential for solving specific problems exponentially faster than the best classical machines, by virtue of its non-classical dynamics and information representation. Quantum mechanical bits (called ´qubits´) can be both 0 and 1 at the same time, in which case the qubit is said to be in a superposition state. A quantum gate with input 0 or 1 gives the same output as a classical gate, but if the input is 0 and 1 simultaneously, the quantum gate performs the two corresponding operations simultaneously. Similarly, a 2-qubit gate with an input state prepared as a superposition of the 4 combinations 00, 01, 10 and 11, performs four simultaneous computations. In general, an N-qubit gate could perform 2/sup N/ computations at the same time. So as we increase the size of the system, the computational capacity of the apparatus goes up exponentially, compared to linearly for a classical computer. Using commercial NMR-spectrometers, simple quantum circuits are experimentally implemented consisting of multiple cascaded logic gates.