Fig. 2. Photon ID.

a, The interferometer set-up. Two consecutive cluster photons interfere on a non-polarizing beam splitter (NPBS). Two detectors on the output ports register their detection times, t₁ and t₂, respectively. A half-wave plate (λ/2) is used to select the photons’ relative polarization state between co- and cross-linear polarization. A pair of LCVRs and a linear polarizer (Pol.) project the photons’ polarization, and a transmission grating (TG) filters their energy. b, ID measurements (averaged over both rectilinear polarizations H and V) between two consecutive cluster photons for various externally applied magnetic fields. The solid lines represent the measured second-order correlation function g⁽²⁾(t₂ − t₁) using 50,000 two-sequential photon correlation events. In each period (13 ns apart) there are five correlation peaks³⁷. Only the central peak, which results from the interference of the two photons, is shown. The other four peaks form the measured background. As they are polarization independent, they can be straightforwardly subtracted from the measured interference. The measured ID is then defined as I_nd = 1 − A_co/A_cross, where A_co (A_cross) is the area under the co (cross)-polarized correlation function, represented by the coloured (black) solid lines.