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Optics Letters

Fully-guided polarization-correlated photon pairs at 1560 nm from a type-II SPDC-based source

Akanksha Angural, Joyee Ghosh

Published Dec 8, 2023• 5 citations
Publisher

Abstract

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.

DOI
10.1364/ol.509328

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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.
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.
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.
In this Letter, we report a polarization-entangled photon-pair source based on type-II spontaneous parametric downconversion at telecom O-band in periodically poled silica fiber (PPSF). The photon-pair source exhibits more than 130 nm ( ∼ 24 T H z ) emission bandwidth centered at 1306.6 nm. The broad emission spectrum results in a short biphoton correlation time, and we experimentally demonstrate a Hong–Ou–Mandel interference dip with a full width of 26.6 fs at half-maximum. Owing to the low birefringence of the PPSF, the biphotons generated from type-II SPDC are polarization-entangled over the entire emission bandwidth, with a measured fidelity to a maximally entangled state greater than 95.4%. The biphoton source provides the broadest bandwidth entangled biphotons at O-band to our knowledge.
The time-energy entangled photon source is a core resource in quantum key distribution, quantum teleportation, and quantum metrology. The output of orthogonally polarized photon pairs can enhance the effective coincidence counts of the entangled source in these applications. Additionally, an all-fiber entangled source offers small size and high mobility advantages. However, most current orthogonally polarized all-fiber time-energy entangled sources are limited to a bandwidth of around 1 nm, which is detrimental to the effective utilization of channel resources in fiber networks and restricts precision in quantum metrology. To address this issue, we employ a fiber-coupled 775 nm narrow linewidth pump laser in a periodically poled lithium niobate (PPLN) waveguide to achieve type-II spontaneous parametric down-conversion (SPDC). This results in an all-fiber orthogonally polarized time-energy entangled photon source with a bandwidth greater than 7 nm. We measured the single counts, coincidence counts, coincidence to accidental coincidence ratio (CAR), single photon spectra, and joint spectral intensity of the entangled source, calculating the Schmidt number S = 7.8. Furthermore, we measured Hong-Ou-Mandel (HOM) interference by inserting two 2 nm bandpass filters, and the interference visibility remained around 95.8% after continuous operation for 3 hours. The experimental results indicate that while maintaining an all-fiber structure, this source possesses a broadband range, low noise, and high stability, providing efficient and reliable biphoton resources for applications in quantum communication, quantum networks, and precision metrology.
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.