Figure 2
From: Experimental realization of equiangular three-state quantum key distribution
Experimental setup used for the experiment.
The light issued by a laser diode (LD) at 404.5 nm is collected into a polarization maintaining single mode fibre (PM-SMF) for spatial mode filtering. The polarization is adjusted by a PBS followed by a HWP and the beam is focused by a 200 mm lens (L) into the center of the interferometer. The interferometer is formed by a PBS, a dual-wavelength HWP (d-HWP) and a PPKTP crystal, where the down-conversion takes place. Photons exiting the interferometer are collected into single mode fibres (SMF), preceded by a long-pass filter (LPF) to filter out residual pump intensity. Fiber birefringence is compensated by two quarter-wave plates (QWP) and a half-wave plate (HWP). The receiver consists in an implementation of the POVM {Πi}, with output Ai and Bi corresponding to 
for, respectively, Alice and Bob. All the output signals are timed by a common timetagger.
