Classical model for bulk-ensemble NMR quantum computation

We present a classical model for bulk-ensemble NMR quantum computation: the quantum state of the NMR sample is described by a probability distribution over the orientations of classical tops, and quantum gates are described b y classical transition probabilities. All NMR quantum computing experiments performed so far with three quantum bits can be accounted for in this clas sical model. After a few entangling gates, the classical model suffers an e xponential decrease of the measured signal, whereas there is no correspondi ng decrease in the quantum description. We suggest that for small numbers o f quantum bits, the quantum nature of NMR quantum computation Lies in the a bility to avoid an exponential signal decrease. [S1050-2947(99)04612-0].