The quantum Josephson Hamiltonian of two weakly linked Bose-Einstein condensates is written in an overcomplete phase representation, thus avoiding the problem of defining a Hermitian phase operator. We discuss the limit of validity of the standard, non-rigorous Mathieu equation, due to the onset of a higher order $cos 2 phi$ term in the Josephson potential, and also to the overcompleteness of the representation (the phase $phi$ being the relative phase between the two condensates). We thereby unify the Boson Hubbard and Quantum Phase models.
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