A morphism between two mathematical objects of the same type (e.g. groups, algebras, representations, categories, etc.), which preserves the defining structures of the objects, is one of the most important notions in mathematics. However, how to define such a morphism between two QFT’s (or quantum phases) had never been considered in physics until arXiv:1502.01690. In this talk, I will give a review of this notion and discuss its applications in the study of topological orders and more general quantum liquids. I will also clarify its relation with “topological symmetry” or “SymTFT”.
