Figure 2
From: Experimental demonstration of a quantum shutter closing two slits simultaneously
Schematic of the experimental setup.
The shutter photon mode 
is realized using the vertical polarization mode 
(blue line). The reflectance of PPBS S for horizontally and vertically polarized photons is 1/2 and 1, respectively. PMF: polarization maintaining fiber; SPCM: single-photon counting module; IF: interference filter; BBO: β-Barium borate; BD: beam dumper.
