I've looked at the DCQE paper by Kim, et al (arXiv:quant-ph/9903047) but I was not able to follow the math leading to the calculation of the patterns R_01, R_02, R_03, and R_04. Specifically, I was hoping to understand the source of the pi phase shift between R_01 and R_02. Is there a property of the idler photon (polarization, ...) that's different when it enters detector 1 as opposed to detector 2? Is there an "intuitive" explanation of how this relates to a phase shift in the signal photon pattern? Also, does anyone happen to know of a reference that explains the math of this to a reader like me that is not familiar with what these E+, A(*,*), ... functions are? Thanks!