AdS interpretation of two-point correlation function of QED
  We have considered the two-point correlation of QED in worldline formalism. In position space it has been written in terms of heat kernel. This leads to introducing the $K_1$ function, which is related with the bulk-to-boundary propagator of massless scalar field and to reveal bulk-to-boundary propagator in the expression of photon polarization operator. 
Sh. Mamedov