Why quantum computing is hard - and quantum cryptography is not provably secure

Despite high hopes for quantum computation in the 1990s, progress in the past decade has been slow; we still cannot perform computation with more than about three qubits and are no closer to solving problems of real interest than a decade ago. Separately, recent experiments in fluid mechanics have demonstrated the emergence of a full range of quantum phenomena from completely classical motion. We present two specific hypotheses. First, Kuramoto theory may give a basis for geometrical thinking about entanglement. Second, we consider a recent soliton model of the electron, in which the quantum-mechanical wave function is a phase modulation of a carrier wave. Both models are consistent with one another and with observation. Both models suggest how entanglement and decoherence may be related to device geometry. Both models predict that it will be difficult to maintain phase coherence of more than three qubits in the plane, or four qubits in a three-dimensional structure. The soliton model also shows that the experimental work which appeared to demonstrate a violation of Bell's inequalities might not actually do so; regardless of whether it is a correct description of the world, it exposes a flaw in the logic of the Bell tests. Thus the case for the security of EPR-based quantum cryptography has just not been made. We propose experiments in quantum computation to test this. Finally, we examine two possible interpretations of such soliton models: one is consistent with the transactional interpretation of quantum mechanics, while the other is an entirely classical model in which we do not have to abandon the idea of a single world where action is local and causal.