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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Entanglement Swapping with independent CW-Sources

M. Halder, A. Beveratos, Corentin Jorel, Hugo Zbinden, Christoph Simon, Valerio Scarani, Nicolas Gisin

Published Jun 1, 2007• 1 citation
Publisher

Abstract

We present an entanglement swapping experiment realized for the first time with independent photon pairs created in continuous wave (CW) mode. Pairs of entangled photons are created by the process of spontaneous parametric down conversion in nonlinear periodically poled lithium niobate (PPLN) waveguide crystals. Two photons, one from each pair are combined on a beamsplitter (BS) and projected via a joint measurement onto one of the four Bell states.

DOI
10.1109/cleoe-iqec.2007.4386931

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We present an entanglement swapping experiment realized for the first time with independent photon pairs created in continuous wave (CW) mode. Pairs of entangled photons are created by the process of spontaneous parametric down conversion in nonlinear periodically poled lithium niobate (PPLN) waveguide crystals. Two photons, one from each pair are combined on a beamsplitter (BS) and projected via a joint measurement onto one of the four Bell states.
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We experimentally demonstrated polarization-based entanglement swapping using spatially separated periodically poled lithium niobate waveguides that emit polarization-entangled photon pairs by spontaneous parametric down-conversion (SPDC). Entanglement swapping using such SPDC sources was theoretically analyzed by accounting for losses along photon propagation paths, accidental fourfold coincidence counts (FCC's), and the dead time in single-photon detection. In a partial Bell-state measurement, entanglement was observed between two photons that were initially uncorrelated and that did not directly interact with each other. The obtained fidelity was $52.5\ifmmode\pm\else\textpm\fi{}0.8%$. Accidental FCC's were measured to clarify the effect of simultaneous generation of three pairs in SPDC on the fidelity. The fidelity increased to $80.5\ifmmode\pm\else\textpm\fi{}2.9%$ when accidental coincidence counts were subtracted.
We have demonstrated entanglement swapping with 1550 nm polarization-entangled photon pairs created by spontaneous parametric down conversion in two spatially separated type-II periodically poled lithium niobate bulk crystals. In a partial Bell-state measurement, entanglement was observed between two photons from separate sources. We employed sinusoidally gated InGaAs/InP avalanche photodiodes operated at a repetition frequency of 1.28 GHz, which increased the four-fold coincidence count rate without degrading the fidelity of entanglement swapping. The obtained fidelity exceeds 90%, which indicates that the entangled states obtained in this experiment are high enough for immediate further use in quantum communication protocols such as quantum teleportation or quantum cryptography.
A periodically poled lithium niobate (PPLN) crystal has been used as an efficient source of non-collinearly generated polarization-entangled photon pairs at 810 and 1550 nm. The PPLN crystal was endowed with a specially designed poling pattern and the entangled photons were generated via the nonlinear optical process of spontaneous parametric down conversion (SPDC). A novel design based on overlapping two concurrent type-I quasi-phase-matching structures in a single PPLN crystals produced correlated pairs of alternatively polarized photons in largely separated spectral regions. The phase of the resulting two-photon state is directly linked to parameters of the nonlinear grating. Continuous tunability of the generated Bell state, from Phi(+) to Phi(-), has been demonstrated by translating a slightly wedged crystal perpendicular to the pump beam.