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Journal of Physics B Atomic Molecular and Optical Physics

High-fidelity entanglement swapping at telecommunication wavelengths

Qing-Lin Wu, Naoto Namekata, Shuichiro Inoue

Published Nov 20, 2013• 8 citations
Publisher

Abstract

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.

DOI
10.1088/0953-4075/46/23/235503

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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 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 report the first entanglement swapping experiment using entangled photon-pair sources based on spontaneous four-wave mixing (SFWM). The 1.5-μm band entangled photon pairs generated by SFWM in two independent 500-m dispersion shifted fibers exhibited quantum interference, thanks to the negligible walk-off between the pump and photon pairs. The use of 500-MHz gated-mode InGaAs/InP avalanche photodiodes based on the sine-wave gating technique increased the fourfold coincidence rate significantly. As a result, the formation of an entanglement between photons from independent sources was successfully observed.
Quantum information protocols tend to involve more and more entangled photon pairs, for enabling advanced quantum tasks. To compensate for the low success efficiency of these protocols, one can increase the repetition rate of the emitted pairs, up to current telecom standard (40 GHz). In this context, we propose a realization of ultra-fast photon pair sources, intended for either heralded quantum state engineering or entanglement swapping at telecom wave- length. In this paper, we present preliminary results consisting of a two-photon interference experiment using four photons where the achieved raw visibility of the Hong-Ou-Mandel (HOM) dip exceeds 98.6 %. The interfering photons are generated through spontaneous parametric down conversion process (SPDC) in two independent, separate, periodically poled lithium niobate (PPLN) waveguides.
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.
Quantum networks based on wavelength-multiplexed entanglement enable parallel distribution of quantum correlations, increasing channel capacity for secure communication and distributed quantum information processing. However, broadband integrated sources capable of generating multipartite entanglement beyond photon pairs remain scarce. Here we report on-chip generation of telecom-band four-photon entanglement in a periodically poled thin-film lithium niobate on insulator (LNOI) waveguide. Type-0 spontaneous parametric down-conversion provides a phase-matching bandwidth exceeding 200 nm, enabling spectrally separable generation of multi-photon entanglement across the telecom band. The generated photons are encoded in time bins for robust fiber compatibility, and a coherent interface enabling reversible conversion between time-bin and polarization degrees of freedom allows complete quantum state tomography. We measure two-photon entanglement with a brightness of 6.7 MHz/mW/nm and a fidelity of 0.874±0.0020.874 \pm 0.0020.874±0.002. At a pump power of 0.08 mW, the four-photon state exhibits a fourfold coincidence rate of 1 Hz and a fidelity of 0.74±0.010.74 \pm 0.010.74±0.01, representing a threefold improvement over previous integrated platforms. Our results establish LNOI as a scalable platform for broadband multi-photon entanglement and provide a practical route toward dense wavelength-multiplexed quantum networks.