Local quantum coherence and superfluidity
We consider a model of bosons on a regular lattice with a kinetic energy due to hopping among sites and a potential energy due to strong on site interaction. A superfluid phase is expected when the ground state of the local energy is doubly degenerate. We consider a new scheme of simmetry breaking associated to the superfluid phase in which the order parameter is the statistical average of the quantum coherence operator associated to the superposition of the degenerate local ground states. In the strong coupling limit a systematic expansion of the free energy can be performed in terms of the hopping amplitude at constant order parameter. Within such an expansion we obtain a self-consistent equation for the order parameter. The first order approximation gives, in the case of degeneracy between single occupied and empty state, the same result of the standard mean field approximation for the ``hard core bosons''. This new approach to the superfluid phase is shown to have a natural application to the implementation of quantum computation on a superfluid.