The experimental set-up shown in Fig. 1 was implemented with a c.w. He-Cd laser operating at 442 nm. The output power was about 200 mW. It was used to pump two 1 cm long LiIO3 crystals. The distance between crystals was around 2 cm and the distance between crystal 1 and both detectors A and B was nearly 1.5 m. Crystals were cut for collinear degenerate down-conversion, that is to say the optical axis at 37.3 degrees relatively to the input/output faces. In order to make degenerate beams emerge from the crystal at angles different from zero, it was necessary to tilt it slightly relatively to the vertical direction. In our case, the direction of propagation of the twin beams were nearly 7 degrees with the pump beam direction. The output angle of the beams produced in crystal 2 were slightly bigger, in order to achieve superposition at the detectors with the beams originated at crystal 1.
The detectors were avalanche photodiodes inside photon counting modules (SPCM-AQ / EG&G). The incoming light passes through a small vertical slit (about 0.5 mm) and an AR coated lens with 25.4 mm focal length, before reaching the active detection area of about 0.2 mm. The modules are mounted on X-Y translation stages, so that the transverse detection plane can be scanned with up to 5 mm resolution. The output pulses were sent to counters (SR-400 and SR-620 / SRS), where single rates were counted and the coincidence logic was implemented. Counters were controlled by a microcomputer which was used to save data.