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Optics Letters

Broadband telecom photon pairs from a fiber-integrated PPLN ridge waveguide

Vikash Kumar Yadav, Vivek Venkataraman, Joyee Ghosh

Published Sep 14, 2022• 20 citations
Publisher

Abstract

We demonstrate a spectrally correlated photon-pair source at telecom wavelengths (spanning across the S-, C-, and L-bands), based on type-0 spontaneous parametric downconversion (SPDC) in a fiber-coupled Zn-indiffused MgO doped periodically poled lithium niobate (PPLN) ridge waveguide. Modal analysis of the waveguide performed through numerical finite element method (FEM) simulation indicates that device temperature can be used to dramatically vary and control the emission spectrum. Efficient photon-pair generation is measured over a broad wavelength range from ∼1520 − 1580 nm [full width at half maximum (FWHM) > 45 nm] with a coincidence-to-accidental ratio (CAR) as high as ∼668 and spectral brightness ∼2.5 × 10 7 pairs/s/mW/nm. Such sources can be employed in wavelength division multiplexed (WDM) quantum key distribution (QKD) over existing fiber-optic networks.

DOI
10.1364/ol.472045

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We demonstrate a spectrally correlated photon-pair source at telecom wavelengths (spanning across the S-, C-, and L-bands), based on type-0 spontaneous parametric downconversion (SPDC) in a fiber-coupled Zn-indiffused MgO doped periodically poled lithium niobate (PPLN) ridge waveguide. Modal analysis of the waveguide performed through numerical finite element method (FEM) simulation indicates that device temperature can be used to dramatically vary and control the emission spectrum. Efficient photon-pair generation is measured over a broad wavelength range from ∼1520 − 1580 nm [full width at half maximum (FWHM) > 45 nm] with a coincidence-to-accidental ratio (CAR) as high as ∼668 and spectral brightness ∼2.5 × 10 7 pairs/s/mW/nm. Such sources can be employed in wavelength division multiplexed (WDM) quantum key distribution (QKD) over existing fiber-optic networks.
We report a fibre-integrated heralded single-photon source based on a commercial Zn-indiffused MgO:PPLN ridge waveguide operated in Type-0 degenerate spontaneous parametric down-conversion (SPDC). The device emits a broadband spectrum spanning approximately 70 nm across the telecom C-and L-bands, with an absolute brightness of 9.1×1099.1\times 10^{9}9.1×109 pairs/(s mW). After accounting for optical losses, we determine an internal heralding efficiency of 58(5)%. The source maintains high purity, exhibiting a coincidences-to-accidentals ratio exceeding 9×1049\times 10^{4}9×104 and a heralded second-order correlation of gh(2)(0)=(5.53±0.46)×10−4g_{\text{h}}^{(2)}(0) = (5.53 \pm 0.46)\times 10^{-4}gh(2)​(0)=(5.53±0.46)×10−4. Low multi-photon noise and broadband emission demonstrate the viability of such platforms for wavelength-multiplexed quantum networking.
The study, reliable generation, and application of spectral and polarization-correlated biphotons is a widely researched area in the field of quantum technology. In this Letter, we report a bright narrowband source of spectral and polarization-correlated orthogonal photon-pairs around the telecom wavelength of 1560 nm from a fiber-pigtailed, type-II quasi-phase-matched, MgO-doped periodically poled lithium niobate (MgO:ppLN) ridge waveguide. We achieved a high spectral brightness of ∼5×10 6 photon pairs/s/mW/nm with a coincidence-to-accidental ratio (CAR) value of ∼427 and an emission bandwidth of ∼2.4 nm. Owing to its compact and narrowband nature, this source can reliably be used in commercial quantum communication systems for long-distance transmissions. The generation of such high rates of correlated orthogonal twin photons from a fully guided type-II SPDC-based source has not been thoroughly investigated to the best of our knowledge.
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.
We demonstrate a fully fibre-integrated source of broadband polarisation-entangled photon pairs at telecom wavelengths based on a commercially available Zn-indiffused MgO:PPLN ridge waveguide incorporated within a Sagnac interferometer. Operating across the full degenerate SPDC spectrum using pump-suppression filtering only, the source exhibits a measured bandwidth of approximately 67 nm. Strong non-classical correlations are observed, with an average two-photon interference visibility of 96.9 +/- 0.2 %, a fidelity to the Phi + Bell state of 78.1 %, and a CHSH Bell parameter of S = 2.501 +/- 0.005. We further explore the effect of increasing source brightness on Bell-state performance, revealing the limitations of accidental-corrected metrics in high-flux operating regimes.
Spontaneous parametric down-conversion (SPDC) presents a convenient method of producing highly correlated heralded entangled photon pairs for quantum applications and networking. Of particular interest is Type-II SPDC, where signal and idler photons possess orthogonal polarisations, permitting straightforward entanglement in the useful polarisation basis. To the best of our knowledge, this has hitherto not been demonstrated in Zn-indiffused MgO:PPLN ridge waveguides, which offer excellent mode matching to standard telecoms fibers [1]. A particular difficulty in achieving SPDC in waveguides is to establish optimal phase-matching conditions. A common approach [2] is to perform standard SHG from the target SPDC wavelength first, leveraging the greater conversion efficiencies available in this regime. For example, in this work we target generation of Type-II SPDC photon pairs at 1310nm telecoms wavelengths suited to long-distance quantum networking protocols such as quantum key distribution [3], prompting initial testing of phase matching conditions via Type-II SHG.