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Journal of Lightwave Technology

Enhancing Quantum Temporal Cross-Correlation Measurements by Manipulating Detection Parameters

Akanksha Angural, Joyee Ghosh

Published May 26, 2025• 0 citations
Publisher

Abstract

Advancements in the field of compact quantum photonics have enabled researchers to reliably generate and detect spectral and temporal correlated biphotons, opening avenues for diverse applications in quantum technology. Although many routine quantum interferometry and entanglement generation experiments are performed worldwide, only a few studies discuss how different experimental detection and correlation measurement parameters can significantly influence the raw outcomes of these experiments. In this work, we analytically and experimentally elucidate the impact of quantum efficiency and dead time of single-photon detectors, and bin width of the time correlator on the coincidence-to-accidental ratio (CAR) and hence the quantum temporal cross-correlations ($g_{si}^{2}(0)$) of photon pairs generated at 1560 nm via a type-II MgO:ppLN ridge waveguide-based source. We experimentally achieved a high cross-correlation of up to$g_{si}^{2}(0)=\sim 10^{4}$at different detection settings, thus revealing the high-quality of photon-pair correlations, surpassing several state-of-the-art biphoton sources. Additionally, we demonstrate how this analysis anticipates the effect of afterpulsing, temporal distinguishability between signal and idler photons, and temporal resolution of the correlation analysis. We further use this analysis for application in Hong-Ou-Mandel (HOM) interferometry and achieve raw visibility$\sim 99\pm 1\ \%$. To the best of our knowledge, such high raw HOM visibility has not been previously reported in a fiber-based type-II SPDC configuration. This study is important for understanding the quantum source and finding the optimal detection settings to operate it on for its real-time practical implementation in various quantum optical experiments.

DOI
10.1109/jlt.2025.3573861

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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.
Hong-Ou-Mandel interference between independent photon sources (HOMI-IPS) plays an important role in quantum information processing. A variety of HOMI-IPS experiments have been demonstrated at near-infrared wavelengths or telecom wavelengths [1]. In all the previous HOMI-IPS experiments, the HOM dips are obtained by recording the four-fold coincidence counts as a function of the temporal delay. However, the spectral correlation during the interference is omitted, partially due to technical difficulties, e.g., the acquisition time is too long for measuring a joint spectral distribution. Spectral information, as the conjugate of the temporal information in the Fourier transformation, is definitely of great importance and should be investigated. Very recently, a new technique of spectral correlation measurement in a HOM interference has been developed by Gerrits, et al [2]. This technique has the merits of high speed and high resolution, therefore, has been used as a powerful tool to analyse the spectral correlation in a HOM interference between signal and idler photons from one spontaneous parametric down conversion (SPDC) source. In this work, we measure the joint spectral distribution in HOMI-IPS using the new technique developed in [2]. The experimental setup is shown in Fig. 1 (Left). The two independent sources are based on SPDC from two 30-mm-long periodically poled KTiOPO4 (PPKTP) crystals pumped by picosecond laser pulses (76 MHz, 792 nm, temporal duration ~ 2 ps). In Fig. 1 (Left), the dispersion compensation fiber model (DCFMs), the superconducting nanowire single-photon detectors (SNSPDs) [3,4,5] and the time interval analyzer (TIA) constitute a fiber spectral analyzer [6]. Fig. 1(Right: (a)) is the measured four-fold HOM interference between signal1 and signal2, with idler1 and idler2 as heralders. Without subtraction of any background counts, the raw visibility of the HOM dip is 69.9±3.2%, which is consistent with our previous results in [1,7,8]. Fig. 1(Right: (b)-(g)) are the measured joint spectral intensities at different delay positions. The splitting phenomenon in the joint spectral distribution is clearly observed. The experimental results agree well with our theoretical simulations. This experiment not only can deepen our understanding of HOMI-IPS from the viewpoint of spectral domain, but also can be used to improve the visibility of HOMI-IPS by temporal filtering.
Quantum entanglement correlations, while predicted probabilistically by quantum mechanics, lack a structural mechanism explaining their persistence across space-like separations. We propose an experimental test of bounded temporal coupling: a finite Frame Width of Coupling (FWC) limiting correlations, rather than assuming unbounded nonlocality. This could arise in alternative models positing quantized adjacency in an extended temporal domain (e.g., reciprocal space-time frameworks), projecting as a measurable structure in the two-photon interference profile. We employ a type-II SPDC source (pumped at 405 nm via frequency-doubling of an ∼14 fs Ti:Sapphire laser, yielding ∼20 fs pulses at 405 nm) to generate polarization-entangled 810 nm photon pairs and read out with a Hong–Ou–Mandel (HOM) interferometer. In a HOM measurement, the coincidence-versus-delay shape is governed by the joint temporal amplitude (JTA), set by the source spectrum and filtering; detector timing jitter does not set the HOM width, though the coincidence window sets the accidental background. A variable optical delay line (sub-10 fs resolution) scans the dip. Scanning ±2 ps in 10 fs steps and accumulating ∼ 5 × 10³ coincidences per step, we compare a pure Gaussian dip (standard model class) against a rect-convolved-Gaussian dip (FWC class) that can produce a flat bottom with steeper edges. Model comparison (Poisson-likelihood AIC/BIC), an edge-steepness metric, and control sweeps (filter bandwidth, pump power, pump chirp) discriminate the hypotheses. Observation of FWC would introduce a new entanglement observable; null results set lower bounds. The test uses standard components (incremental cost ∼$55 k with an existing rig).
Abstract Frequency entangled biphoton sources have played an important role in quantum information. There are two key characteristics which determine the extent of their applications: the spectral indistinguishability and the degree of frequency entanglement. Previously, the spectral indistinguishability of the biphoton state is measured by the Hong–Ou–Mandel (HOM) interference visibility, whereas the degree of frequency entanglement is normally characterized with independent spectral measurements. However, these two quantum features have not been successfully measured with a single experimental setup. In this paper, we deduce the temporal distribution of a short-pulse pumped type-II spontaneous parametric down converted (SPDC) biphoton source and its relevant temporal entanglement parameter R t for operationally quantifying the degree of frequency entanglement. Furthermore, we theoretically and experimentally demonstrate, due to the inherent group delay, the difference between the signal and idler photons at the exit of the type-II nonlinear crystal, resulting in a HOM-shape dip at both sides of the center fringed envelope of the MZ interferometric coincidence diagram. By measuring the HOM-shape dip depth, the separation between the two sideband dips as well as the MZ fringed envelope width, both the spectral indistinguishability and the temporal entanglement parameter can thus be simultaneously quantified. This implementation provides us a unified and convenient method to characterize the frequency entanglement of the short-pulse pumped biphotons.
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.