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Physical Review A

Polarization-based entanglement swapping at the telecommunication wavelength using spontaneous parametric down-conversion photon-pair sources

Yinghong Xue, Akio Yoshizawa, Hidemi Tsuchida

Published Mar 30, 2012• 9 citations
Publisher

Abstract

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.

DOI
10.1103/physreva.85.032337

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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±\else\textpm\fi0.852.5\ifmmode\pm\else\textpm\fi{}0.8%52.5\ifmmode±\else\textpm\fi0.8. 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±\else\textpm\fi2.980.5\ifmmode\pm\else\textpm\fi{}2.9%80.5\ifmmode±\else\textpm\fi2.9 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.
We experimentally demonstrate a high-fidelity entanglement swapping and a generation of the Greenberger-Horne-Zeilinger (GHZ) state using polarization-entangled photon pairs at telecommunication wavelength produced by spontaneous parametric down conversion with continuous-wave pump light. While spatially separated sources asynchronously emit photon pairs, the time-resolved photon detection guarantees the temporal indistinguishability of photons without active timing synchronizations of pump lasers and/or adjustment of optical paths. In the experiment, photons are sufficiently narrowed by fiber-based Bragg gratings with the central wavelengths of 1541 nm & 1580 nm, and detected by superconducting nanowire single-photon detectors with low timing jitters. The observed fidelities of the final states for entanglement swapping and the generated three-qubit state were 0.84 ± 0.04 and 0.70 ± 0.05, respectively.
We develop and demonstrate a source of polarization-entangled photon pairs using spontaneous parametric down-conversion (SPDC) in domain-engineered, periodically poled lithium niobate (PPLN) at telecom wavelengths. Pumped at 775 nm, this domain-engineered type-II SPDC source produces non-degenerate signal and idler pairs at 1530 nm and 1569 nm. Because of birefringence, the photon pair with horizontally polarized signal and vertically polarized idler has a different phasematching condition than the pair with vertically polarized signal and horizontally polarized idler. Using phase-modulation of the domain structure, we produced a crystal that can simultaneously generate both states in a distributed fashion throughout a single crystal. Performing SPDC using this aperiodically poled crystal, we observed polarization entanglement visibility above 93%. We compare the phase-modulated crystal to other aperiodic structures, including dual-periodically-poled and interlaced biperiodic structures.