Pair creation in inhomogeneous fields from worldline instantons
  We show how to do semiclassical nonperturbative computations within the worldline approach to quantum field theory using ``worldline instantons''.   These worldline instantons are classical solutions to the Euclidean worldline loop equations of motion, and are closed spacetime loops parametrized by the proper-time. Specifically, we compute the imaginary part of the one loop effective action in scalar and spinor QED using worldline instantons, for a wide class of inhomogeneous electric field backgrounds. 
Gerald V. Dunne (Univ. of Connecticut), Christian Schubert (UMSNH)