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

Waveguide source for an on-demand entanglement distribution network

Isabelle Herbauts, B. Blauensteiner, Andreas Poppe, Thomas D. Jennewein, Hannes Hübel

Published May 1, 2011• 0 citations
Publisher

Abstract

Most quantum communication protocols rely on the distribution of entangled photon pairs. If the transmission channel is an optical fiber both photons should have a wavelength around 1550nm. We present a source of polarisation entanglement based on spontaneous parametric down-conversion (SPDC) at telecom wavelengths. Using fiber-pigtailed ppLN crystals we can profit from the clean χ2SPDC process, as opposed to four-wave mixing techniques, and also from a stable and efficient coupling to single mode fibers. Using a cw-pump, a tight energy correlation between the entangled photons is achieved. This fact is used here to distribute the entanglement through multiple channels employing only passive wavelength multiplexing. We demonstrate a possible application scenario with a single central source distributing entanglement to four users simultaneously. The source can also be used for creating tri-partite entanglement using the recently demonstrated cascaded SPDC scheme.

DOI
10.1109/cleoe.2011.5943440

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Most quantum communication protocols rely on the distribution of entangled photon pairs. If the transmission channel is an optical fiber both photons should have a wavelength around 1550nm. We present a source of polarisation entanglement based on spontaneous parametric down-conversion (SPDC) at telecom wavelengths. Using fiber-pigtailed ppLN crystals we can profit from the clean χ2SPDC process, as opposed to four-wave mixing techniques, and also from a stable and efficient coupling to single mode fibers. Using a cw-pump, a tight energy correlation between the entangled photons is achieved. This fact is used here to distribute the entanglement through multiple channels employing only passive wavelength multiplexing. We demonstrate a possible application scenario with a single central source distributing entanglement to four users simultaneously. The source can also be used for creating tri-partite entanglement using the recently demonstrated cascaded SPDC scheme.
We implement an entanglement distribution network based on wavelength-multiplexing and optical switching for quantum communication applications. Using a high-brightness source based on spontaneous parametric down-conversion in periodically-poled lithium niobate waveguides, we generate polarisation entangled photon pairs with a broad spectrum covering the telecom wavelengths around 1550 nm. The photon pairs have entanglement fidelities up to 99%, and are distributed via passive wavelength multiplexing in a static multi-user network. We furthermore demonstrate a possible network application in a scenario with a single centralised source dynamically allocating two-party entanglement to any pair of users by means of optical switches. The whole system, from the pump laser up to the receivers, is fibre and waveguide based, resulting in maximal stability, minimal losses and the advantage of readily integrable telecom components in the 1550 nm range.
We report a broadband energy-time entangled photon-pair source based on a fiber-pigtailed periodically poled lithium niobate (PPLN) waveguide, designed for applications in the quantum secure network.Utilizing the spontaneous parametric down-conversion nonlinear optical process, the source generates entangled photon pairs within a wavelength range of 64 nm in the telecom band at a pump wavelength of 770.3 nm.Photon pairs from eight paired International Telecommunication Union (ITU) channels are selected, and their correlation and entanglement properties are characterized.The measured coincidence counts of photon pairs from eight paired ITU channels are larger than 152.9 kHz when the coincidence-to-accidental ratios are greater than 260.Entanglement properties are measured through two-photon interference in the Franson interferometer, with all visibilities of interference curves exceeding 98.13%.Our demonstration provides a broadband energy-time entangled photon-pair source, contributing to the development of a large-scale quantum secure network.
Studies on telecom-band entangled photon-pair sources for entanglement distribution have so far focused on their narrowband operations. Fiber-based sources are seriously limited by spontaneous Raman scattering while sources based on quasi-phase-matched crystals or waveguides are usually narrowband because of long device lengths and/or operations far from degeneracy. An entanglement distributor would have to multiplex many such narrowband sources before entanglement distribution to fully utilize the available fiber transmission bandwidth. In this work, we demonstrate a broadband source of polarization-entangled photon-pairs suitable for wavelength-multiplexed entanglement distribution over optical fiber. We show that our source is potentially capable of simultaneously supporting up to forty-four independent wavelength channels.
Generation of entangled photon pairs at telecom wavelength has been garnering a growing interest in the context of quantum communications and quantum computing. Several generation techniques have been proposed, including spontaneous Four-Wave Mixing in resonators or Spontaneous Parametric Down Conversion (SPDC) in Periodically Polled Lithium Niobate (PPLN) waveguides. Large photon pair efficiencies have been demonstrated in particular in PPLN on insulator [3]. However, polarization entangled photon pair generation (Type II phase matching configuration) suffers from a less favourable non-linear coefficient and optical mode birefringence in PPLN.
The scalability of quantum communication networks requires compact, fiber-integrated, easy-to-deploy, and efficient wavelength-division-multiplexed (WDM) sources for multi-user secure key distribution. Here, we demonstrate such a multi-channel source of polarization-entangled photon pairs in the low-loss telecom C-band based on type-0 spontaneous parametric downconversion in a fiber-coupled Zn-indiffused MgO-doped periodically poled lithium-niobate (MgO:PPLN) ridge waveguide in the Sagnac configuration. The source can be easily tuned to generate the |Φ+⟩ or |Φ−⟩ Bell state in 14 channel pairs of the International Telecommunication Union dense WDM (100-GHz spacing) grid around 1550-nm with a raw fidelity of ≳89%, with a maximum value of ≳94%. The raw concurrence is ≳0.8 for both Bell states in all channel pairs, and the observed S-parameter (>2.56 ± 0.04 in all 14-channel pairs) shows a strong violation of CHSH-Bell’s inequality. The source’s suitability for long-distance entanglement transmission is also demonstrated by the successful transfer of entangled photons up to 100 km while maintaining fidelity >85% and quantum bit error rate <9%. The effect of polarization mode dispersion on entanglement distribution among remote users is also studied in detail. All these performance metrics are measured using conventional room-temperature semiconductor-based single-photon avalanche detectors, and these are the best reported with these detectors. Our highly flexible source can support up to ∼40 user pairs to communicate simultaneously, and it can be easily deployed into the current metro-area fiber-optic telecom infrastructure to form a complete WDM-based quantum communication network.
A fully fibered source of entangled photon pairs based on a nonlinear Sagnac interferometer is reported. Operating at telecom wavelengths, the source relies exclusively on standard fiber-optic components and periodically poled lithium niobate (PPLN) waveguides, resulting in a compact, robust, and field-deployable architecture. The generation stage supports both polarization and energy-time entanglement without modification, enabling versatile operation depending on the targeted application. Broadband spontaneous parametric down-conversion allows dense wavelength-division multiplexing over the telecom C and L bands. High normalized brightness (10.3 kpairs/s/nm/mW2^22) is achieved on a standard 100 GHz ITU channel pair, together with high entanglement quality. Polarization and energy-time encodings are characterized through state tomography and two-photon interference measurements, yielding fidelities, purities, and visibilities exceeding 96 % over multiple wavelength channels. The stability and reproducibility of the source are further evaluated through long-duration operation in a network environment. These results demonstrate that the proposed Sagnac source constitutes a practical and scalable building block for future plug-and-play quantum communication and quantum networking platforms.