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arXiv:2601.05671 · Japanese Journal of Applied Physics

Improving quantum interference visibility between independent sources by enhancing the purity of correlated photon pairs

Hsin-Pin Lo, Kai Asaoka, Hiroki Takesue

quant-phSubmitted Jan 9, 2026• Announced Jan 12, 2026• 1 citation
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Abstract

High-visibility quantum interference between independent photons is essential for demonstrating multi-photon quantum information processing, and it is closely linked to the spectral purity of correlated photon pairs. In this study, we investigate two approaches to enhance the purity of photon pairs generated from a type-0 PPLN waveguide by systematically varying both the pump bandwidth and the interference-filter bandwidth, and we directly compare their performance under identical experimental conditions. The spectral purity is evaluated from measured joint spectral intensities using Schmidt decomposition. Both methods significantly improve the Hong-Ou-Mandel interference visibility to approximately 80%. However, the former approach also yields a higher three-fold coincidence rate, which is advantageous for our ongoing efforts to increase the state fidelity and generation rate of multi-photon time-bin Greenberger-Horne-Zeilinger (GHZ) states.

DOI
10.35848/1347-4065/ae35f7
Comments
Accepted for publication in Japanese Journal of Applied Physics (JJAP). 10 pages, 3 figures (2026.1.8)

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High-visibility quantum interference between independent photons is essential for demonstrating multi-photon quantum information processing, and it is closely linked to the spectral purity of correlated photon pairs. In this study, we investigate two approaches to enhance the purity of photon pairs generated from a type-0 PPLN waveguide by systematically varying both the pump bandwidth and the interference-filter bandwidth, and we directly compare their performance under identical experimental conditions. The spectral purity is evaluated from measured joint spectral intensities using Schmidt decomposition. Both methods significantly improve the Hong-Ou-Mandel interference visibility to approximately 80%. However, the former approach also yields a higher three-fold coincidence rate, which is advantageous for our ongoing efforts to increase the state fidelity and generation rate of multi-photon time-bin Greenberger-Horne-Zeilinger (GHZ) states.
We implement an ultrafast pulsed type-II parametric down conversion source in a periodically poled KTP waveguide at telecommunication wavelengths with almost identical properties between signal and idler. As such, our source resembles closely a pure, genuine single mode photon pair source with indistinguishable modes. We measure the joint spectral intensity distribution and second order correlation functions of the marginal beams and find with both methods very low effective mode numbers corresponding to a Schmidt number below 1.16. We further demonstrate the indistinguishability as well as the purity of signal and idler photons by Hong-Ou-Mandel interferences between signal and idler and between signal/idler and a coherent field, respectively. Without using narrowband spectral filtering, we achieve a visibility for the interference between signal and idler of 94.8% and determine a purity of more than 80% for the heralded single photon states. Moreover, we measure raw heralding efficiencies of 20.5% and 15.5% for the signal and idler beams corresponding to detector-loss corrected values of 80% and 70%. (C) 2013 Optical Society of America
We implement an ultrafast pulsed type-II parametric down conversion source in a periodically poled KTP waveguide at telecommunication wavelengths with almost identical properties between signal and idler. As such, our source resembles closely a pure, genuine single mode photon pair source with indistinguishable modes. We measure the joint spectral intensity distribution and second order correlation functions of the marginal beams and find with both methods very low effective mode numbers corresponding to a Schmidt number below 1.16. We further demonstrate the indistinguishability as well as the purity of signal and idler photons by Hong-Ou-Mandel interferences between signal and idler and between signal/idler and a coherent field, respectively. Without using narrowband spectral filtering, we achieve a visibility for the interference between signal and idler of 94.8% and determine a purity of more than 80% for the heralded single photon states. Moreover, we measure raw heralding efficiencies of 20.5% and 15.5% for the signal and idler beams corresponding to detector-loss corrected values of 80% and 70%.
Heralded single photons (HSPs) generated by spontaneous parametric down-conversion (SPDC) are useful resource to achieve various photonic quantum information processing. Given a large-scale experiment which needs multiple HSPs, increasing the generation rate with suppressing higher-order pair creation is desirable. One of the promising ways is to use a pump laser with a GHz-order repetition rate. In such a high repetition rate regime, however, single-photon detectors can only partially identify the pulses. Hence, we develop a simple model to consider that effect on the spectral purity, and experimentally demonstrate a high-visibility Hong-Ou-Mandel interference between two independent HSPs generated by SPDC with 3.2 GHz-repetition-rate mode-locked pump pulses. The observed visibility of 0.88(3) is in good agreement with our theoretical model.
Guided-wave platforms such as fiber and silicon-on-insulator waveguide show great advances over traditional free space implementations in quantum information technology for significant advantages of low transmission loss, low cost, integrability and compatible with mature fiber communication systems. Interference between independent photon sources is the key to realize complex quantum systems for more sophisticated applications such as multi-photon entanglement generation and quantum teleportation. In this work, we report Hong-Ou-Mandel interference between two independent all fiber photon pair sources over two 100GHz dense wave division multiplexing channels, the visibility reaches 53.2(8.4)% (82.9(5.3)%) without (with) back ground counts subtracted. In addition, we give a general theoretical description of the purity of the photon pair generation in dispersion shifted fiber and obtain the optimized condition for high purity photon pair generation. We also obtain a maximum coincidence to back ground ratio of 131 by cooling the fiber in liquid nitrogen. Our study shows great promising of integrated optical elements for future scalable quantum information promising.
We experimentally engineer a high-spectral-purity single-photon source using a single-interferometer-coupled silicon microring. By the reconfiguration of the interferometer, different coupling conditions can be obtained, corresponding to different quality factors for the pump and signal/idler. The ratio between the quality factor of the pump and signal/idler ranges from 0.29 to 2.57. By constructing the signal–idler joint spectral intensity, we intuitively demonstrate the spectral correlation of the signal and idler. As the ratio between the quality factor of the pump and signal/idler increases, the spectral correlation of the signal and idler decreases, i.e., the spectral purity of the signal/idler photons increases. Furthermore, time-integrated second-order correlation of the signal photons is measured, giving a value up to 94.95 ± 3.46%. Such high-spectral-purity photons will improve the visibility of quantum interference and facilitate the development of on-chip quantum information processing.