Quantum invariants of 3-manifolds arising from non-semisimple categories

This survey article covers some of the results contained in the papers by Costantino, Geer and Patureau and by Blanchet, Costantino, Geer and Patureau. In the first one the authors construct two families of Reshetikhin–Turaev-type invariants of 3-manifolds, ${\mathrm N}_r$ and ${\mathrm N}^0_r$, using non-semisimple categories of representations of a quantum version of ${\mathfrak{sl}_2}$ at a $2r$-th root of unity with $r \geq 2$. The secondary invariants ${\mathrm N}^0_r$ conjecturally extend the original Reshetikhin–Turaev quantum ${\mathfrak{sl}_2}$ invariants. The authors also provide a machinery to produce invariants out of more general ribbon categories which can lack the semisimplicity condition. In the second paper a renormalized version of ${\mathrm N}_r$ for $r \neq 0 \; (\mathrm{mod} \; 4)$ is extended to a TQFT, and connections with classical invariants such as the Alexander polynomial and the Reidemeister torsion are found. In particular, it is shown that the use of richer categories pays off, as these non-semisimple invariants are strictly finer than the original semisimple ones: indeed they can be used to recover the classification of lens spaces, which Reshetikhin–Turaev invariants could not always distinguish.