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

Abstract

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 KTiOPO₄ 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 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⁻¹/² standard deviations) and by complete quantum state tomography that the fibers carry high-quality entangled pairs at a maximum rate of 55 × 10³ s⁻¹ THz⁻¹ mW⁻¹.