Physical Methods Applied to Donaldson Theory

I would like to explain some applications of quantum field theory methods to Donaldson theory. But first, perhaps, I should explain what Donaldson theory is. We start with an oriented four-manifold M,and a compact gauge group, say G = SU (2) We pick a principal G bundle P over M, and let A denote a connection on P The space of such connections will be called A.