Images of quantum representations of mapping class groups and Dupont-Guichardet-Wigner quasi-homomorphisms

Abstract

We prove that either the images of the mapping class groups by quantum representations are not isomorphic to higher rank lattices or else the kernels have a large number of normal generators. Further we show that the images of the mapping class groups have nontrivial 2-cohomology, at least for small levels. For this purpose we considered a series of quasi-homomorphisms on mapping class groups extending previous work of Barge and Ghys and of Gambaudo and Ghys. These quasi-homomorphisms are pull-backs of the Dupont-Guichardet-Wigner quasi-homomorphisms on pseudo-unitary groups along quantum representations.