Quantum phase transition and critical fluctuations of an attractive Bose gas in a double well potential

We consider a Bose gas with an attractive interaction in a symmetric double well potential. In the Hartree approximation, the ground state solution spontaneously breaks the symmetry of the trapping potential above certain value of the interaction strength. We demonstrate how the Landau-Ginzburg scheme of the second order phase transition emerges from the quantum model and show its link to the spontaneous symmetry breaking mentioned above. We identify the order parameter, the critical point and analyze quantum fluctuations around it.