This is an inside view of the two-channel photon and coincidence counter of the book‘s Figure 145. It is used in the photon entanglement experiments of Chapter 8.
This circuit processes the outputs from our single-photon counting modules through two two 74ACT74 dual D-type positive edge-triggered flip-flops that perform pulse stretching and coincidence counting. The counts are displayed through Veeder-Root A103-000 counter modules. A Veeder-Root A103-008 allows counting to happen for a preset amount of time.