Reduced fluctuations for bosons in a double well

We review two recent results on the ground state properties of bosonic systems trapped by a double well external potential. In the limit of large population and large separation between the wells we prove that fluctuations in the number of particles occupying each single-well low energy mode occur at a reduced scale with respect to $\sqrt{N}$, the latter being the typical prediction of the central limit theorem. This signals the breakdown of the independent and uncorrelated particles picture of standard Bose-Einstein condensation and the emergence of an interaction-driven correlated phase in the ground state.