We propose a method for determining the spins of BPS states supported on line defects in 4d N = 2 theories of class S. Via the 2d-4d correspondence, this translates to the construction of quantum holonomies on a punctured Riemann surface C. Our approach combines the technology of spectral networks, which decomposes flat GL(K, C)-connections on C in terms of flat abelian connections on a K-fold cover of C, and the skein algebra in the 3-manifold C × [0, 1], which expresses the representation theory of the quantum group U q (gl K ). With any path on C, the quantum holonomy associates a positive Laurent polynomial in the quantized Fock-Goncharov coordinates of higher Teichmüller space. This confirms various positivity conjectures in physics and mathematics.arXiv:1603.05258v2 [hep-th]