http://arXiv.org/abs/quant-ph/0207045
shows the connection of proper time and return probabilities (view
pdf).
More special results following from discretization of the vacuum maxwell equations using the Lattice software are contained in wqpm1.pdf which is an updated version of http://arXiv.org/abs/quant-ph/0312188 .
A synthesis of quant-ph/0108121 and quant-ph/0207045 is
published in
IJMMS
2004:19 (2004) 1003-1023
.