Broken lines and compatible pairs for rank 2 quantum cluster algebras

Abstract: There have been several combinatorial constructions of universally positive bases in cluster algebras, and these same combinatorial objects play a crucial role in the known proofs of the famous positivity conjecture for cluster algebras. The greedy basis was constructed in rank $2$ by Lee-Li-Zelevinsky using compatible pairs on Dyck paths. The theta basis, introduced by Gross-Hacking-Keel-Kontsevich, has elements expressed as a sum over broken lines on scattering diagrams. It was shown by Cheung-Gross-Muller-Musiker-Rupel-Stella-Williams that these bases coincide in rank $2$ via algebraic methods, and they posed the open problem of giving a combinatorial proof by constructing a (weighted) bijection between compatible pairs and broken lines. We construct a quantum-weighted bijection between compatible pairs and broken lines for the quantum type $A_2$ and the quantum Kronecker cluster algebras. By specializing the quantum parameter, this handles the problem of Cheung et al. for skew-symmetric cluster algebras of finite and affine type. For cluster monomials in skew-symmetric rank-$2$ cluster algebras, we construct a quantum-weighted bijection between positive compatible pairs (which comprise almost all compatible pairs) and broken lines of negative angular momentum.