Photon inner product and the Gauss linking number
  It is shown that there is an interesting interplay between self-duality, loop representation and knots invariants in the quantum theory of Maxwell fields in Minkowski space-time. Specifically, in the loop representation based on self-dual connections, the measure that dictates the inner product can be expressed as the Gauss linking number of thickened loops. 
Abhay Ashtekar, Alejandro Corichi (Penn State)