Fig. 1: Experimental setup.
From: Experimental demonstration of coherence flow in PT- and anti-PT-symmetric systems
Blue area to the left: pairs of 808 nm single photons are generated by passing a 404 nm laser light through a type-I spontaneous parametric down-conversion and using a nonlinear-barium-borate (BBO) crystal. Orange area: after photons pass through the 3 nm interference filter (IF), one photon serves as a trigger and the other signal photon is prepared in an arbitrary linear polarization state. Gray area: two sets of beam displacers (BDs), together with half-wave plates (HWPs) and quarter-wave plates (QWPs), are used to construct the operators \({{{{{{{{\boldsymbol{U}}}}}}}}}_{{{{{{{{\mathcal{P}}}}}}}}{{{{{{{\mathcal{T}}}}}}}}}\) and \({{{{{{{{\boldsymbol{U}}}}}}}}}_{{{{{{{{\mathcal{APT}}}}}}}}}\). In the measurement part, the density matrix is constructed via quantum-state tomography. PBS: polarization beam splitter.
