Invertible Braided Tensor Categories

We prove that a finite braided tensor category A is invertible in the Morita 4-category BrTens of braided tensor categories if and only if it is nondegenerate. This includes the case of semisimple modular tensor categories, but also nonsemisimple examples such as categories of representations of the small quantum group at good roots of unity. Via the cobordism hypothesis, we obtain new invertible 4-dimensional framed topological field theories, which we regard as a nonsemisimple framed version of the Crane-Yetter-Kauffman invariants, after the Freed-Teleman and Walker constructions in the semisimple case. More generally, we characterize invertibility for E-1 and E-2-algebras in an arbitrary symmetric monoidal infinity-category, and we conjecture a similar characterization of invertible E-n-algebras for any n. Finally, we propose the Picard group of BrTens as a generalization of the Witt group of nondegenerate braided fusion categories, and pose a number of open questions about it.Y