Figure 2

Experimental setup. (Inset) A β − BaB 2 O 4 (BBO) crystal pumped by a vertically polarized ultraviolet laser. A spherical mirror reflects the photon pairs, which are emitted due to a degenerate spontaneous parametric down-conversion, and the pump beam to a second pass through the crystal, after passing a QWP (or λ / 4 ) with its optical axis orienting at 45°. Choosing four points in the entanglement ring yields a photon pair entangled both in path and polarization, which enters the main setup shown in the main panel. The up path (green) stands for the first photon (labeled A ), while the bottom path (red) represents the second one (labeled B ); they are vertically separated around 5 mm. For each photon, they have two spatially separated path modes (left and right with around one meter from each other) and two polarization modes (horizontal and vertical); the former is taken as the meter and the latter as the system. For preparing the initial meter state, we use two neutral optical attenuators (ATTs) to precisely reduce the relative intensity of relevant modes. Half- and quarter-wave plates are used to manipulate the polarization of each path independently and glass compensators (COs) are used to offset the phase difference. The two paths of each photon are combined in one unpolarized beam splitter (BS). Glass plates (GPs) are used to introduce a controllable phase so different path states can be measured. The PBS postselects the final polarization state. After spectrum filters, the photons are collected by four single-mode fibers and guided to four avalanche single-photon detectors (D-1,2,3,4).