Narrowband PPKTP source for entangled polarization photons

We present our efforts to create a narrow-bandwidth source of entangled photons with a high spectral brightness to facilitate their interaction with single trapped atoms. A pump beam with l=405 nm, focused into a type II PPKTP crystal, is down-converted to obtain signal and idler beams; their frequency can be tuned via the temperature of the crystal. By changing the beam waist inside the crystal, we determine the optimal focusing parameters for our source. We observe that weak focusing results in large pair to single ratio (efficiency).