Fig. 4: Four-level quantum interferences.
From: Quantum state processing through controllable synthetic temporal photonic lattices
a,c, Spatial representation of four roundtrips of the DTQW in the coupled fibre-loop system under a roundtrip-wise controlled scheme (scheme 1) (a) and roundtrip- and synthetic position-wise controlled scheme (scheme 2) (c). Coincidence counts are measured at the central time bin coincidence window of the fibre loop’s interference output, and at zero photon time delay. b,d, Normalized coincidence counts of four-level entangled photons and classical light intensity (blue squares and grey circles, respectively) as a function of the relative phase difference (θ) between the time bins. Experimental measurements are fitted with theoretical models (solid blue line for entangled qudits and dashed grey line for classical light). From two-photon quantum interference measurements, raw visibility values of Vd=4 = 91.55% and V′d=4 = 89.61% were extracted for scheme 1 (b) and scheme 2 (d), respectively.
