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arXiv:2102.07146 · npj Quantum Information

High-performance quantum entanglement generation via cascaded second-order nonlinear processes

Zichang Zhang, Chenzhi Yuan, Si Qing Shen, Hao Yu, Ruiming Zhang, Heqing Wang, Hao Li, You Wang, Guangwei Deng, Zhiming Wang, Lixing You, Zhen Wang, Hai‐Zhi Song, Guang‐Can Guo, Qiang Zhou

Published Aug 5, 2021• 112 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1038/s41534-021-00462-7

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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.
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.
In this work, we propose and demonstrate the frequency-bin entangled two-photon state via cascaded second harmonic generation and spontaneous parametric down conversion (SHG-SPDC) processes in a single piece of PPLN waveguide. Our scheme is based on all fiber-pigtailed components at 1.5 μm telecom band. Two frequency-bins at 1531.34 nm and 1548.91 nm with bandwidth of 1.04 nm are prepared to be entangled. The frequency entanglement property of our generated two-photon states is measured by spatial two-photon quantum beating. A two-photon beating curve with a visibility of 87.92±0.47% is obtained, showing a good property of frequency-bin entanglement.
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.
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.
Entangled photon pairs are one of the most important resources for the development of quantum communication technologies. In order to produce pulsed photon pairs in the telecommunication band from 1.5 mum pump light, most of the previous experiments used two successive periodically poled lithium niobate (PPLN) waveguides. We report what we believe to be a new method using cascaded second-order nonlinearity in a single PPLN waveguide to produce high-purity time-bin entangled photon pairs. We confirmed the generation of entanglement through a two-photon interference experiment showing a coincidence fringe with a visibility of up to 97%.
In this paper we report experimental investigation in transmission performance over standard single-mode optical fibers (SMFs) of polarization-entangled photon-pairs in a 1.5-μm band generated by cascaded second-harmonic generation and spontaneous parametric down conversion (c-SHG/SPDC) from a periodically poled LiNbO(3) (PPLN) ridge-waveguide device. Clear two-photon interference fringes were observed even after the transmission over 140 km of the SMF spools, remaining small degradation in the visibilities of less than 3%. The performance was also investigated by using optical attenuators, instead of the SMF spools, to study the maximum reach of the distribution of the entanglement in terms of loss penalty. The results show that the quantum entanglement could be distributed even with 50 dB of the transmission loss with violation of Bell inequality by using the c-SHG/SPDC-based photon-pair source.