Categories of topological orders I.

We develop the mathematical theory of separable and unitary $n$-categories based on Gaiotto and Johnson-Freyd's theory of condensation completion. We use it to study the categories of topological orders by including gapless quantum phases and defects. In particular, we show that all the topological features of a potentially gapless quantum phase can be captured by its topological skeleton, and that the category of the topological skeletons of higher dimensional gapped/gapless quantum phases can be explicitly computed categorically from a simple coslice 1-category.