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Experimental Proposal: Testing Frame Width of Coupling in Entangled Photon Correlations

Wells, A. R.

Published Nov 4, 2025• 0 citations
Publisher

Abstract

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).

DOI
10.5281/zenodo.17518601

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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).
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 (gsi2(0)g_{si}^{2}(0)gsi2​(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 togsi2(0)=∼104g_{si}^{2}(0)=\sim 10^{4}gsi2​(0)=∼104at 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∼99±1 %\sim 99\pm 1\ \%∼99±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.
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.
Abstract Qudit entanglement is an indispensable resource for quantum information processing since increasing dimensionality provides a pathway to higher capacity and increased noise resilience in quantum communications, and cluster-state quantum computations. In continuous-variable time–frequency entanglement, encoding multiple qubits per photon is only limited by the frequency correlation bandwidth and detection timing jitter. Here, we focus on the discrete-variable time–frequency entanglement in a biphoton frequency comb (BFC), generating by filtering the signal and idler outputs with a fiber Fabry–Pérot cavity with 45.32 GHz free-spectral range (FSR) and 1.56 GHz full-width-at-half-maximum (FWHM) from a continuous-wave (cw)-pumped type-II spontaneous parametric downconverter (SPDC). We generate a BFC whose time-binned/frequency-binned Hilbert space dimensionality is at least 324, based on the assumption of a pure state. Such BFC’s dimensionality doubles up to 648, after combining with its post-selected polarization entanglement, indicating a potential 6.28 bits/photon classical-information capacity. The BFC exhibits recurring Hong–Ou–Mandel (HOM) dips over 61 time bins with a maximum visibility of 98.4% without correction for accidental coincidences. In a post-selected measurement, it violates the Clauser–Horne–Shimony–Holt (CHSH) inequality for polarization entanglement by up to 18.5 standard deviations with an S-parameter of up to 2.771. It has Franson interference recurrences in 16 time bins with a maximum visibility of 96.1% without correction for accidental coincidences. From the zeroth- to the third-order Franson interference, we infer an entanglement of formation (Eof) up to 1.89 ± 0.03 ebits—where 2 ebits is the maximal entanglement for a 4 × 4 dimensional biphoton—as a lower bound on the 61 time-bin BFC’s high-dimensional entanglement. To further characterize time-binned/frequency-binned BFCs we obtain Schmidt mode decompositions of BFCs generated using cavities with 45.32, 15.15, and 5.03 GHz FSRs. These decompositions confirm the time–frequency scaling from Fourier-transform duality. Moreover, we present the theory of conjugate Franson interferometry—because it is characterized by the state’s joint-temporal intensity (JTI)—which can further help to distinguish between pure-state BFC and mixed state entangled frequency pairs, although the experimental implementation is challenging and not yet available. In summary, our BFC serves as a platform for high-dimensional quantum information processing and high-dimensional quantum key distribution (QKD).
Entanglement swapping enables the distribution of quantum states over long distances. In this protocol, a Bell state measurement projects photons, which were created in separate entangled photon sources, into an entangled state when their corresponding pairs interfere at a beamsplitter. This process is optimally realized when the photons are indistinguishable in all degrees of freedom, resulting in Hong-Ou-Mandel (HOM) interference. Since its initial proposal by [1], several experimental realizations have used pulsed, synchronized pumps, as in [2] to achieve temporal indistiguishability. In the case of continuous wave (CW) pumping, the sources are not synchronized, requiring post-selection of the coincidence events after detection. This requires detectors with a temporal resolution greater than the coherence time of the photons. Previous experiments have successfully observed HOM interference using periodically poled Lithium Niobate (ppLN) waveguides and superconducting nanowire single-photon detectors (SNSPDs) [3], also with hybrid sources to improve coincidence rates and HOM visibility [4], [5].
We present experimental observations of a nonclassic and nonlocal interference effect with frequency-entangled photon pairs. We observed two-photon interference in a Hong–Ou–Mandel interferometer by monitoring the coincidence-counting rate as we varied the path-length difference or the relative time delay between two photons. The quantum-mechanical probabilities of joint detection by two detectors that respond to different frequency components exhibit differences that depend not only on the arrangement of the detector pairs but on the spectral filtering. The experimental results provide more understanding of two-photon entanglement and of the interference effects that arise from superposition of indistinguishable probability amplitudes.
Abstract Over the last several decades, quantum entanglement has been intensively studied for potential applications in quantum information science. The Hong-Ou-Mandel (HOM) dip is the most important test tool for direct proof of entanglement between paired photons, whose coincidence detection results in anticorrelation due to photon bunching on a beam splitter. Although anticorrelation is due to destructive quantum interference between paired photons, a wavelength-sensitive interference fringe has never been observed in any HOM-type experiments. Here, a typical HOM dip is investigated for entangled photon pairs generated by parametric down conversion processes (SPDC) to understand fundamental physics of anticorrelation. In addition, a pure coherence optics-based Hong-Ou-Mandel scheme is proposed and analyzed for general understanding of anticorrelation in an interferometric system. This study sheds light on deterministic quantum correlation control and opens the door to potential applications of on-demand quantum information science.