Quantum Periods from TBA and Continued Fractions

Abstract

We consider quantum A-periods of local ${\mathbb P}^1 \times {\mathbb P}^1$, or equivalently the generating function of constant terms for powers of the Laurent polynomial $x + x^{-1} + y + y^{-1}$ where $x$ and $y$ are $q$-commuting variables. We derive a number of new representations for these rich combinatorial and algebraic objects. First, we find that the well-known TBA equation encodes not only the A-period itself but also higher diagonal modes of the corresponding noncommutative Laurent polynomials. Second, we obtain new formulas for the period in terms of continued fractions and related orthogonal polynomials, as well as a nontrivial explicit recursion allowing its efficient computation. Finally, we compute quantum corrections to the classical Picard-Fuchs equation, and observe that holonomicity of the classical period does not generalise to $q$-holonomicity of its quantum counterpart. Applications to random walks on the Cayley graph of the Heisenberg group, and related new approaches to more efficient generative AI systems, are outlined.