Bose-Einstein Condensation as a Quantum Phase Transition in an Optical   Lattice
  One of the most remarkable recent developments in the study of ultracold Bose gases is the observation of a reversible transition from a Bose Einstein condensate to a state composed of localized atoms as the strength of a periodic, optical trapping potential is varied. In \cite{ALSSY} a model of this phenomenon has been analyzed rigorously. The gas is a hard core lattice gas and the optical lattice is modeled by a periodic potential of strength $\lambda$. For small $\lambda$ and temperature Bose-Einstein condensation (BEC) is proved to occur, while at large $\lambda$ BEC disappears, even in the ground state, which is a Mott-insulator state with a characteristic gap. The inter-particle interaction is essential for this effect. This contribution gives a pedagogical survey of these results. 
M. Aizenman, E.H. Lieb, R. Seiringer, J.P. Solovej and J. Yngvason