silver, check out rule #3 for calculating probability amplitudes:
https://en.wikipedia.org/wiki/Probability_amplitude#The_laws_of_calculating_probabilities_of_events
It basically states that in order to calculate the amplitude of a photon going to 1 is a product of the amplitude of it going to A and then from A to 1. So we do need to remember what the amplitude was for a photon going toward A.
silver, check out rule #3 for calculating probability amplitudes:
https://en.wikipedia.org/wiki/Probability_amplitude#The_laws_of_calculating_probabilities_of_events
It basically states that in order to calculate the amplitude of a photon going to 1 is a product of the amplitude of it going to A and then from A to 1. So we do need to remember what the amplitude was for a photon going toward A.