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International Conference on Optical Communication and Optical Information Processing (OCOIP)

Quantum interference of correlated photons generated by cascaded second-order nonlinearity

Yuhang Luo, Tao Xie, Meimei Wang, Chenzhi Yuan, Rui‐Bo Jin

Published Dec 11, 2025• 0 citations
Publisher

Abstract

Cascaded second-order nonlinear processes, specifically second-harmonic generation (SHG) and the following spontaneous parametric down-conversion (SPDC) in a periodically poled lithium niobate (PPLN) waveguide represent significant methods for generating correlated photon pairs. To date, studies on the quantum interference characteristics of entangled photon pairs generated via cascaded SHG and type-0 SPDC process have not been reported. To address this issue, we systematically derive the theoretical expression for the coincidence counts of energy-time entangled photon pairs generated by this nonlinear process after passing through an optical fiber interferometer. The theoretical analysis reveals that the two-photon interference process within the interferometer primarily comprises Hong-Ou-Mandel (HOM) interference and N00N state interference. Experimental results subsequently validate these theoretical findings. In our experiments, a 1550.12 nm laser was used to pump the cascaded SHG/SPDC process in a PPLN waveguide, which satisfies the type-0 quasi-phase matching condition and generates broadband correlated photons in the C-band with a full width at half maximum (FWHM) of approximately 17.5 nm. After being split, the correlated photon pairs were directed into a fiber interferometer. A short-term interference visibility of 76.06% was measured, co-determined by the phase-insensitive HOM effect and the phase-sensitive N00N state interference. Upon increasing the measurement accumulation time, the interference visibility reduced to 49.32%, demonstrating the elimination of N00N state interference under longterm averaging, which is consistent with theoretical predictions. This scheme achieves all-fiber-coupled generation of correlated photon pairs, employing standard fiber components in the optical communication band. This approach significantly reduces the complexity of optical coupling links, thereby offering a feasible pathway for the miniaturization and integration of entangled photon sources and quantum interferometers.

DOI
10.1117/12.3089585

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Cascaded second-order nonlinear processes, specifically second-harmonic generation (SHG) and the following spontaneous parametric down-conversion (SPDC) in a periodically poled lithium niobate (PPLN) waveguide represent significant methods for generating correlated photon pairs. To date, studies on the quantum interference characteristics of entangled photon pairs generated via cascaded SHG and type-0 SPDC process have not been reported. To address this issue, we systematically derive the theoretical expression for the coincidence counts of energy-time entangled photon pairs generated by this nonlinear process after passing through an optical fiber interferometer. The theoretical analysis reveals that the two-photon interference process within the interferometer primarily comprises Hong-Ou-Mandel (HOM) interference and N00N state interference. Experimental results subsequently validate these theoretical findings. In our experiments, a 1550.12 nm laser was used to pump the cascaded SHG/SPDC process in a PPLN waveguide, which satisfies the type-0 quasi-phase matching condition and generates broadband correlated photons in the C-band with a full width at half maximum (FWHM) of approximately 17.5 nm. After being split, the correlated photon pairs were directed into a fiber interferometer. A short-term interference visibility of 76.06% was measured, co-determined by the phase-insensitive HOM effect and the phase-sensitive N00N state interference. Upon increasing the measurement accumulation time, the interference visibility reduced to 49.32%, demonstrating the elimination of N00N state interference under longterm averaging, which is consistent with theoretical predictions. This scheme achieves all-fiber-coupled generation of correlated photon pairs, employing standard fiber components in the optical communication band. This approach significantly reduces the complexity of optical coupling links, thereby offering a feasible pathway for the miniaturization and integration of entangled photon sources and quantum interferometers.
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.
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.
The indistinguishable photon-pair sources are valuable in many quantum information applications, such as quantum microscopy, quantum synchronization, and quantum metrology. Based on cascaded sum-frequency generation (SFG) and spontaneous parametric downconversion (SPDC) processes, we propose and demonstrate a scheme for the generation of spatially separated broadband indistinguishable photon pairs in the telecom band by using only one piece of a fiber-pigtailed periodically poled lithium niobate waveguide in a modified Sagnac loop. The measured joint spectral intensity of the generated entangled photon pairs is 7.27 THz (57.6 nm) at the full width at half-maximum (FWHM). The Hong–Ou–Mandel (HOM) interference of the generated broadband photons is measured with bandwidths of 5.35 THz (∼42.8 nm) and 100 GHz (∼0.8 nm), respectively. Visibility of 94.0±1.4% is achieved with the bandwidth of 5.35 THz, demonstrating good indistinguishability of the generated two-photon states, which could benefit the development of quantum microscopy and quantum synchronization.
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.
The frequency entangled photon pairs generated by spontaneous parametric down-conversion (SPDC) possess wide applications in quantum optics and relevant fields.To facilitate the practical quantum information technologies,particularly in optical fiber links,a frequency entangled source at telecommunication wavelength with features of compactness,portability,high efficiency and miniaturization is highly desired.In this paper,we report the experimental generation of a miniaturized frequency entangled source in telecommunication band from a 10 mm-long type-Ⅱ periodically poled lithium niobate (PPLN) waveguide pumped by a 780 nm distributed Bragg reflector (DBR) laser diode.The frequency entangled photon pairs generated by SPDC possess wide applications in quantum optics and relevant fields.When the DBR laser diode is driven by a current of 170 mA at a temperature controlled to 20℃,the output power is measured to be 70.4 mW with a central wavelength of 780.585 nm.Under this pump,the orthogonally-polarized photon pairs are generated and output from the PPLN waveguide.After filtering out the remaining pump by three high-performance long-pass filters mounted on an adjustable U-type fiber bench,the photon-pair generation rate,spectral and temporal properties of the generated frequency entangled source are measured.The results show that the generation rate of the photon pairs,after being corrected for the detection efficiencies of the single photon detectors and the optical losses,is achieved to be 1.86×107 s-1 at a pump power of 44.9 mW (coupled into the waveguide).Optimizing the working temperature of the waveguide and fixing it at 46.5℃,the frequency degeneracy of the SPDC generated photon pairs is realized.Based on the coincidence measurement setup together with two infrared spectrometers,the spectra of the signal and idler photons are obtained with their center wavelengths of 1561.43 nm and 1561.45 nm,and their 3-dB bandwidths of 3.62 nm and 3.60 nm respectively.The joint spectrum of the photon pair is observed,showing a joint spectrum bandwidth of 3.18 nm.The degree of frequency entanglement is quantified to be 1.13 according to the bandwidth ratio between the single photon spectrum and the joint spectrum.Furthermore,based on the Hong-Ou-Mandel (HOM) interferometric coincidence measurement setup,a visibility of about 96.1% is observed,which indicates the very good frequency indistinguishibility of the down-converted biphotons.The measured 3-dB width of the HOM dip is 1.47 ps and shows good agreement with the measured single-photon spectral bandwidth.The experimental results lay a solid foundation for developing portable,miniaturized frequency entangled sources at telecommunication band for the further applications in quantum information areas,such as quantum time synchronization.
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.