  There exists an increasing evidence supporting the picture of the Josephson
junction (JJ) as a "macroscopic quantum system". On the other hand the
interpretation of experimental data strongly depends on the assumed theoretical
model. We analyse the possible states of a Cooper pair box ("charge qubit") for
the two types of models : two-mode Bose-Hubbard model with its large $N$
aproximations and the many-body description within the mean-field approximation
(Gross-Pitaevski equation). While the first class of models supports the
picture of JJ being a quantum subsystem of a single degree of freedom, the
second approach yields an essentially classical structure of accessible quantum
states which, in particular, implies the absence of entanglement for two
coupled JJ's. The arguments in favor of the mean-field theory are presented and
different experimental tests including a new proposal are briefly discussed.
