Figure 4: Suppression law in a four-mode qFFT integrated chip.

(a) Complete set of 36 measured coincidence patterns (raw experimental data) for all input–output combinations in the four-mode chip. For each input–output combination, the measured coincidence pattern as a function of the time delay is shown (points: experimental data, lines: best-fit curves). Cyclic inputs (1,3) and (2,4) exhibit enhancement (green) and suppression (red) on cyclic and non-cyclic outputs, respectively. For all points, error bars are due to the Poissonian statistics of the events. In each subplot the measured visibility with corresponding error and the sample size are reported. For each visibility, the error is obtained through a Monte Carlo simulation by averaging over 3,000 simulated data sets. In each subplot the zero level coincides with the baseline, while a dashed line represents the number of coincidence events in the distinguishable limit. (b) HOM visibilities for all 36 input–output configurations. (left to right) Experimental measured visibilities (V_qFFT, obtained from raw experimental data), visibilities calculated from the reconstructed unitary (V_rec), and visibilities calculated from the theoretical unitary (V_F). (c) Representation of the reconstructed experimental transformation , and comparison with . Coloured disks represent the moduli of the reconstructed matrix elements (all equal to 4^−1/2 for ). Arrows represent the phases of the unitary matrix elements (green: reconstructed unitary, blue: Fourier matrix).