ar X iv : q ua nt - p h / 05 10 21 8 v 1 28 O ct 2 00 5 Theory and experiment of entanglement in a quasi - phase - matched two - crystal source

We report new results regarding a source of polarization entangled photon-pairs created by the process of spontaneous parametric downconversion in two orthogonally oriented, periodically poled, bulk KTiOPO4 crystals (PPKTP). The source emits light colinearly at the non-degenerate wavelengths of 810 nm and 1550 nm, and is optimized for 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 KTiOPO4, and zero for LiNbO3. 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 standard deviations) and by complete quantum state tomography that the fibers carry high-quality entangled pairs at a maximum rate of 55× 10 sTHzmW.