Figure 5: Hong–Ou–Mandel peak and NOON interference.

(a) Schematic shows HOM peak experiment arrangement. Red indicates on-chip spatial mode beamsplitter, and blue indicates fibre beamsplitter. Pairs of photons are simultaneously coupled into the fundamental mode (input port 1) and higher-order mode (input port 2) of the on-chip spatial mode beamsplitter. To determine the fundamental mode HOM peak, the photons from the fundamental mode (output port 1) are sent to a fibre beamsplitter, and coincidences are measured using detectors and coincidence logic. The same is done for the higher-order HOM peak using output port 2. (b) Coincidence rate of the fundamental mode output arm after the fibre beamsplitter. There is a peak in coincidences due to HOM bunching. (c) Coincidence rate of the higher-order mode output arm after the fibre beamsplitter. (d) Schematic shows NOON interference experiment arrangement. Red indicates the on-chip spatial mode beamsplitter. Pairs of photons are simultaneously coupled into the fundamental mode (input port 1) and higher-order mode (input port 2) of the on-chip spatial mode beamsplitter. The two photons are sent to the spatial mode phase shifter based on an integrated microheater that applies a phase shift (φ) between the fundamental and higher-order modes. Finally, coincidences are measured using detectors and coincidence logic. (e) Classical Mach–Zehnder interference is shown as a function of heater power that applies the phase shift. (f) NOON interference is shown as a function of heater power. The period of the quantum interference is half that of the classical interference. The error bars indicate standard error of measurement for all plots.