Hi all, I am reading this Zeilinger paper Look at fig 1. There are four detectors D1 D2 D3 and D4. When one of these detectors click, what does it mean? what is the state it detects? The author gives a formula in the first page. P13 = (1 + cos x)/2. how can we get it and what is the state D1 tensor D3? Thanks.