FIGURE 2. Coherence between the atomic ensembles L, R induced by a measurement event of the fields 1_L and 1_R at detector D_1a or D_1b.

Shown is the number of coincidences N_2a (triangles) and N_2b + N_2c (circles) recorded by the respective detectors D_2a,2b,2c for the fields 2_L and 2_R with the interferometer arrangement of Fig. 1b as a function of the relative phase . In a, N_2a,2b,2c are conditioned upon a detection event at D_1a with no count at D_1b, while in b, N_2a,2b,2c are conditioned upon an event at D_1b with no count at D_1a. At each setting of , data are acquired for 150 s with a detection window of width 190 ns. Although the interference fringes have comparable visibility, the different sizes arise from unequal quantum efficiencies for detectors D_2a and D_2b,2c (see Supplementary Information). The visibility values are obtained from an average of the visibilities of the red and black curves, respectively. Error bars reflect one standard deviation due to the finite number of counts.