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Advanced Quantum Technologies

All Fiber‐Coupled Noiseless Photon Pair Generation Based on Two Cascaded Periodically Poled LiNbO3 Waveguides

Mei‐Mei Wang, Chenzhi Yuan, Rui‐Bo Jin

Published Jan 6, 2026• 0 citations
Publisher

Abstract

ABSTRACT All‐fiber telecom‐band photon‐pair sources underpin long‐haul quantum networks. While periodically poled lithium niobate (PPLN) waveguides with fiber pigtails enable spontaneous parametric down‐conversion (SPDC) process, direct pumping fixes the wavelength to scarce, costly tunable lasers. Cascaded second harmonic generation (SHG) and SPDC in a single PPLN waveguide allow the use of enormous standard telecom lasers but suffer from noise photons due to spontaneous Raman scattering (SpRS) and spectral discontinuity when the pump is removed. In this paper, we propose and demonstrate an all‐fiber‐coupled method using two PPLN waveguides, one for SHG and the other for SPDC, to generate tunable, low‐noise photon pairs. Pump and SpRS photons are blocked by several mechanisms, including mode mismatch between waveguides and polarization‐maintaining (PM) fibers, high loss of leaky modes in PM fibers, and the high‐extinction ratio filtering effect of isolators. Photon counting shows that the noise photons have been suppressed to the dark count level. A continuous biphoton bandwidth surpassing 35 nm is observed, and 339 MHz photon‐pair generation rate (PPGR) is obtained when the coincidence‐to‐accidental ratio (CAR) is 370. Two‐photon Mach–Zehnder interference (TP‐MZI) reveals that the paired photons are highly indistinguishable. This source could be helpful for scalable quantum networks.

DOI
10.1002/qute.202500652

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ABSTRACT All‐fiber telecom‐band photon‐pair sources underpin long‐haul quantum networks. While periodically poled lithium niobate (PPLN) waveguides with fiber pigtails enable spontaneous parametric down‐conversion (SPDC) process, direct pumping fixes the wavelength to scarce, costly tunable lasers. Cascaded second harmonic generation (SHG) and SPDC in a single PPLN waveguide allow the use of enormous standard telecom lasers but suffer from noise photons due to spontaneous Raman scattering (SpRS) and spectral discontinuity when the pump is removed. In this paper, we propose and demonstrate an all‐fiber‐coupled method using two PPLN waveguides, one for SHG and the other for SPDC, to generate tunable, low‐noise photon pairs. Pump and SpRS photons are blocked by several mechanisms, including mode mismatch between waveguides and polarization‐maintaining (PM) fibers, high loss of leaky modes in PM fibers, and the high‐extinction ratio filtering effect of isolators. Photon counting shows that the noise photons have been suppressed to the dark count level. A continuous biphoton bandwidth surpassing 35 nm is observed, and 339 MHz photon‐pair generation rate (PPGR) is obtained when the coincidence‐to‐accidental ratio (CAR) is 370. Two‐photon Mach–Zehnder interference (TP‐MZI) reveals that the paired photons are highly indistinguishable. This source could be helpful for scalable quantum networks.
We report detailed experimental studies in a single wavelength-band system of correlated photon-pair generation in a 1.5 μm telecommunication wavelength-band using cascaded χ(2):χ(2) processes, second-harmonic generation, and the following spontaneous parametric down conversion (c-SHG/SPDC), in a periodically poled LiNbO3 (PPLN) ridge-waveguide device. By using a PPLN module with 600%/W of the SHG efficiency, we achieved a coincidence-to-accidental ratio (CAR) of 2380 at 1.8×10−4 of the mean number of the signal photon per pulse. Detailed investigation on the origin of uncorrelated noise photons was also discussed in this paper. We revealed that the noise photons mainly originated from spontaneous Raman scattering induced in pigtail optical fibers and also that the PPLN device itself had poor contribution to the noise photons. This feature of the c-SHG/SPDC process is promising for the realization of a noise-photon-free, high-purity quantum entangled photon-pair source.
We generate quantum-correlated photon pairs using cascaded χ ( 2 ) : χ ( 2 ) traveling-wave interactions for second-harmonic generation (SHG) and spontaneous parametric down-conversion (SPDC) in a single periodically-poled thin-film lithium-niobate (TFLN) waveguide. When pulse-pumped at 50 MHz, a 4-mm-long poled region with nearly 300%/Wcm2 SHG peak efficiency yields a generated photon-pair probability of 7±0.2 × 10−4 with corresponding coincidence-to-accidental ratio (CAR) of 13.6±0.7. The CAR is found to be limited by Stokes/anti-Stokes Raman-scattering noise generated primarily in the waveguide. A Raman peak of photon counts at 250 cm−1 Stokes shift from the fundamental-pump wavenumber suggests most of the noise that limits the CAR originates within the lithium niobate material of the waveguide.
We report the generation of high-purity correlated photon-pairs and polarization entanglement in a 1.5 μm telecommunication wavelength-band using cascaded χ((2)):χ((2)) processes, second-harmonic generation (SHG) and the following spontaneous parametric down conversion (SPDC), in a periodically poled LiNbO(3) (PPLN) ridge-waveguide device. By using a PPLN module with 600%/W of the SHG efficiency, we have achieved a coincidence-to-accidental ratio (CAR) higher than 4000 at 7.45×10(-5) of the mean number of the photon-pair per pulse. We also demonstrated that the maximum reach of the CAR was truly dark-count-limited by the single-photon detectors used here. This indicates that the fake (noise) photons were negligibly small in this system, even though the photon-pairs, the Raman noise photons, and the pump photons were in the same wavelength band. Polarization entangled photon pairs were also generated by constructing a Sagnac-loop-type interferometer which included the PPLN module and an optical phase-difference compensator to observe maximum entanglement. We achieved two-photon interference visibilities of 99.6% in the H/V basis and 98.7% in the diagonal basis. The peak coincidence count rate was approximately 50 counts per second at 10(-3) of the mean number of the photon-pair per pulse.
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.
Abstract In this paper, we demonstrate the generation of high-performance entangled photon-pairs in different degrees of freedom from a single piece of fiber pigtailed periodically poled LiNbO 3 (PPLN) waveguide. We utilize cascaded second-order nonlinear optical processes, i.e., second-harmonic generation (SHG) and spontaneous parametric downconversion (SPDC), to generate photon-pairs. Previously, the performance of the photon-pairs is contaminated by Raman noise photons. Here by fiber-integrating the PPLN waveguide with noise-rejecting filters, we obtain a coincidence-to-accidental ratio (CAR) higher than 52,600 with photon-pair generation and detection rate of 52.36 kHz and 3.51 kHz, respectively. Energy-time, frequency-bin, and time-bin entanglement is prepared by coherently superposing correlated two-photon states in these degrees of freedom, respectively. The energy-time entangled two-photon states achieve the maximum value of CHSH-Bell inequality of S = 2.71 ± 0.02 with two-photon interference visibility of 95.74 ± 0.86%. The frequency-bin entangled two-photon states achieve fidelity of 97.56 ± 1.79% with a spatial quantum beating visibility of 96.85 ± 2.46%. The time-bin entangled two-photon states achieve the maximum value of CHSH-Bell inequality of S = 2.60 ± 0.04 and quantum tomographic fidelity of 89.07 ± 4.35%. Our results provide a potential candidate for the quantum light source in quantum photonics.
Broadband nonlinear optical devices play a critical role in both classical and quantum optics. Here, we design and fabricate a 6.82-mm-long step-chirped periodically poled lithium niobate (CPPLN) waveguide on lithium niobate on insulator, which enables quasi-phase matching over a broad bandwidth for second-harmonic generation (SHG) and spontaneous parametric down-conversion (SPDC). The SHG achieves an average efficiency of 91%/W/cm 2 over the first-harmonic wavelength range of 1510 nm–1620 nm, paving the way for realizing SPDC across a wide range of pump wavelengths. For SPDC, by tuning the pump wavelength to 775 nm, 780 nm, and 785 nm, we achieve broadband photon-pair generation with a maximum full bandwidth and brightness up to 99 THz (846 nm) and 21 MHz/mW/nm, respectively. Our findings provide an efficient and experiment-friendly approach for generating broadband photon pairs, which holds significant promise for advancing applications in quantum metrology.