Figure 3: Two-photon interference.

(a) Experimentally measured two-photon interference visibilities (red circles) are compared against the quantum (clear boxes) and classical (blue triangles) predictions. Regions I, II and III contain experiments using input modes cf, cd and df, respectively. The errors shown on the simulated quantum and classical visibilities were calculated by Monte–Carlo simulation (see Supplementary Methods); errors on the classical visibilities are smaller than the marker size because of the decrease in sensitivity of classical interference to the measured circuit parameters. (b,c) The residuals between the measurements and the calculated classical and quantum visibilities. (d,e) Example two-fold coincidence counts between output channels dg and fg when two photons are input into modes df and the optical delay of mode d is varied. The shaded area shows the uncertainty in the determined visibility.