MAINTAINING COHERENCE IN QUANTUM COMPUTERS

The effects of the inevitable coupling to external degrees of freedom of a quantum computer are examined. It is found that for quantum calculations (in which the maintenance of coherence over a large number of states is important), not only must the coupling be small, but the time taken in the quantum calculation must be less than the thermal time scale Latin small letter h with stroke/kBT. For longer times the condition on the strength of the coupling to the external world becomes much more stringent. © 1995 The American Physical Society.