Hyperbolic structures on link complements, octahedral decompositions, and quantum $\mathfrak{sl}_2$

Abstract: Hyperbolic structures (equivalently, principal $\operatorname{PSL}2(\mathbb C)$-bundles with connection) on link complements can be described algebraically by using the octahedral decomposition, which assigns an ideal triangulation to any diagram of the link. The decomposition (like any ideal triangulation) gives a set of gluing equations in shape parameters whose solutions are hyperbolic structures. We show that these equations are closely related to a certain presentation of the Kac-de Concini quantum group $\mathcal{U}_q(\mathfrak{sl}_2)$ in terms of cluster algebras at $q = \xi$ a root of unity. Specifically, we identify ratios of the shape parameters of the octahedral decomposition with central characters of $\mathcal{U}\xi(\mathfrak{sl}_2)$. The quantum braiding on these characters is known to be closely related to $\operatorname{SL}_2(\mathbb C)$-bundles on link complements, and our work provides a geometric perspective on this construction.