| Some of the most interesting intersection numbers on the moduli space of rank r bundles on a Riemann surface satisfy straightforward degeneration rules, and can therefore be integrated in a 2d topological quantum field theory. This TQFT is based on representations of the unitary group U(r) and is closely related to the quantum cohomology of a suitable Grassmannian. I will describe the theory and discuss this circle of ideas, which originated in a classic paper of Witten. |