Appears in collection : Spectral days / Journées méthodes spectrales

A quantum phase transition is commonly referred to as a point in a family of gapped Hamiltonians where the spectral gap closes. In the absence of a general perturbation theory for quantum spin systems in the thermodynamic limit, I will discuss necessary, and sufficient, conditions for a transition, and present explicit constructions of paths of uniformly gapped Hamiltonians in one dimension.