“…The set of measurements needed to estimate O 2 corresponds to antibunching events shown in Fig. 1 , where g n for n = 1, 2, 3, 4,5,6,7,8,9,10,11,12,13,15,16,18,19,20,21,22,24,25,26,27,29,30,31,32,33,34,35,37,38,39,41,42,43,44,45,46,47, This makes the total number of required projections equal to 41. However, after closer examination, we see that all the required quantities can be measured in in 4-photon-pair interferometric configurations i n for n = 1, 2, 3, 4, 5, 6, 7, 8, 9 associated with the projective measurements in the following way: i 1 = g 10 g 34 , i 2 = g 1 g 38 , i 3 = g 13 g 18 , i 4 = g 24 g 29 , i 5 = g 26 g 30 , i 6 = g 43 , i 7 = g 44 , i 8 = g 45 , i 9 = g 46 , i 10 = g 47 , where 20 photon pairs are used and V operators are measured instead of singlet projections.…”