A quantum computer with fairly few components could therefore crack vast problems that a classical computer could not solve before the end of time. But there is a catch. This wipes out most of the information it contains. Solving the problem of decoherence is thus essential if a quantum computer is ever to be built. It is toward this end that Dr Gershenfeld and his colleague Isaac Chuang, of Los Alamos National Laboratory in New Mexico, believe they have taken a significant step.
Molecules contain atoms, and the nuclei of atoms act like tiny magnets.
The nuclei, cocooned within the molecule, make far more stable qubits than the other kinds tried so far. And there is, crucially, a way of manipulating the qubits and reading the result of the calculation. It involves putting the molecules in a magnetic field and hitting them with pulses of radio waves, to which they respond with signals that depend on how their spins are arranged. But these pulses do make the spins decohere. The researchers' answer is to use back-up copies, by having not one molecule, but a small cup containing about a hundred billion trillion of them.