Theory and experiment of entanglement in a quasi-phase-matched two-crystal source

We report results regarding a source of polarization entangled photon pairs created by the process of spontaneous parametric downconversion in two orthogonally oriented, periodically poled, bulk KTiOPO{sub 4} crystals. The source emits light colinearly at the nondegenerate wavelengths of 810 and 1550 nm, and is optimized for a single-mode optical fiber collection and long-distance quantum communication. The configuration favors long crystals, which promote a high photon-pair production rate at a narrow bandwidth, together with a high pair probability in fibers. The quality of entanglement is limited by chromatic dispersion, which we analyze by determining the output state. We find that such a decoherence effect is strongly material dependent, providing for long crystals an upper bound on the visibility of the coincidence fringes of 41% for KTiOPO{sub 4}, and zero for LiNbO{sub 3}. The best obtained raw visibility, when canceling decoherence with an extra piece of crystal, was 91{+-}0.2%, including background counts. We confirm by a violation of the CHSH-inequality (S=2.679{+-}0.004 at 55 s{sup -1/2} standard deviations) and by complete quantum state tomography that the fibers carry high-quality entangled pairs at a maximum rate of 55x10{sup 3} s{sup -1} THz{sup -1} mW{sup -1}.