Spontaneous Parametric Down-Conversion (SPDC) serves as a fundamental physical resource for generating entangled photon pairs. However, modeling its multi-dimensional phase-space dynamics under realistic laboratory conditions remains computationally demanding. Consequently, there is a growing demand for physically faithful digital datasets that can support quantum-optics research prior to physical laboratory implementation. In this work, we present an end-to-end, physically constrained numerical framework designed to generate reusable synthetic datasets for degenerate Type-I SPDC within β-BBO crystals (405 nm → 810 nm). To ensure the numerical outputs can function as a reliable resource for downstream analysis, the framework is subjected to a multi-level validation architecture. The structural integrity of the generated data is first confirmed through independent conservation-law checks, achieving energy conservation with a relative error of 0.000247% and preserving strict transverse momentum anti-correlations (ρ ≈ −0.99999). Furthermore, the dataset’s capacity to retain hidden quantum correlations is verified through high-dimensional modal analysis. Laguerre-Gaussian projections confirm the preservation of nonclassical modal correlations through an effective Schmidt number of K = 2.668 within the computational basis. Finally, The Bell-CHSH analysis validates the implementation of the post-processing pipeline (Sideal = 2.82843) rather than the physical fidelity of the generated SPDC source. When practical laboratory constraints—such as spatial aperture masking and dark counts—are applied, the dataset robustly maintains a mathematically verified violation of Sexp = 2.72992. By bridging fundamental quantum optics with modeled empirical imperfections, this open-science platform yields structurally validated synthetic data, providing a reusable foundation for future studies in entanglement, interference, and quantum information processing.
Spontaneous Parametric Down-Conversion (SPDC) serves as a fundamental physical resource for generating entangled photon pairs. However, modeling its multi-dimensional phase-space dynamics under realistic laboratory conditions remains computationally demanding. Consequently, there is a growing demand for physically faithful digital datasets that can support quantum-optics research prior to physical laboratory implementation. In this work, we present an end-to-end, physically constrained numerical framework designed to generate reusable synthetic datasets for degenerate Type-I SPDC within β-BBO crystals (405 nm → 810 nm). To ensure the numerical outputs can function as a reliable resource for downstream analysis, the framework is subjected to a multi-level validation architecture. The structural integrity of the generated data is first confirmed through independent conservation-law checks, achieving energy conservation with a relative error of 0.000247% and preserving strict transverse momentum anti-correlations (ρ ≈ −0.99999). Furthermore, the dataset’s capacity to retain hidden quantum correlations is verified through high-dimensional modal analysis. Laguerre-Gaussian projections confirm the preservation of nonclassical modal correlations through an effective Schmidt number of K = 2.668 within the computational basis. Finally, The Bell-CHSH analysis validates the implementation of the post-processing pipeline (Sideal = 2.82843) rather than the physical fidelity of the generated SPDC source. When practical laboratory constraints—such as spatial aperture masking and dark counts—are applied, the dataset robustly maintains a mathematically verified violation of Sexp = 2.72992. By bridging fundamental quantum optics with modeled empirical imperfections, this open-science platform yields structurally validated synthetic data, providing a reusable foundation for future studies in entanglement, interference, and quantum information processing.
Spontaneous Parametric Down-Conversion (SPDC) is a cornerstone phenomenon in nonlinear optics, where an incident high-energy photon is transformed into a pair of lower-energy photons. This quantum process serves as a fundamental tool for generating entangled photon pairs, heralded single photons, and squeezed light, making it indispensable in the realms of quantum communication, quantum computing, and precision metrology. In this paper, we present a comprehensive study on the theoretical foundations and practical realization of an SPDC experimental setup, developed for the first time at the Photonics Research Laboratory at Amirkabir University of Technology. Our setup features a self-grown potassium dihydrogen phosphate (KDP) nonlinear crystal, illuminated by a 450 nm blue laser, to achieve parametric down-conversion into infrared photon pairs at 900 nm. By integrating a custom-designed transimpedance amplifier with a BPX65 photodiode, we have successfully amplified and detected the weak SPDC signal, demonstrating the feasibility of achieving high sensitivity with low-cost, widely available components. The experimental results align closely with theoretical predictions, showcasing the effectiveness of the system in detecting photon pairs with minimal resources. This study underscores the accessibility and scalability of SPDC technology, providing a blueprint for constructing versatile quantum optics experiments in resource-limited settings. By bridging the gap between theoretical insight and experimental realization, we aim to inspire further exploration of SPDC's potential in both foundational research and practical applications, laying the groundwork for future innovations in quantum technologies.
This portfolio of work contributes to the remit of the National Physical Laboratory (NPL) to develop the underpinning expertise and tools for validating nascent and future optical quantum technologies based on the discrete and quantum properties of photons. This requirement overlaps with the requirement to provide validation for devices operating in the photon-counting regime. A common theme running through the portfolio is photon pairs generated through spontaneous parametric downconversion (SPDC). \n \nA Hong-Ou-Mandel (HOM) interferometer sourced with visible wavelength photon pairs from an SPDC process in beta-barium borate (BBO) was designed, built and characterised. The visibility of the HOM interference is dependent on the indistinguishability of the interfering photons, but is also influenced by imperfections of the interferometer; therefore an investigation was carried out to quantify the effects of the interferometer imperfections on the measured visibility so that the true photon indistinguishability could be measured with a quantified uncertainty. \n \nA bright source of correlated pair photons in the telecoms band based upon a pump enhanced SPDC process in periodically-poled potassium titanyl phosphate (PPKTP) was designed, built and characterised. From the characterisation measurements the source brightness was estimated to be 6.2×10⁴ pairs/ s/ mw pump. The photon pairs were further characterised through their incorporation as a source in a HOM interference experiment. \n \nThe developed correlated photon pair source was at the heart of a novel scheme for the generation of polarisation entangled photon pairs, for which the design, build and characterisation work is presented. The source was demonstrated to produce two of the four maximally entangled Bell states with quantum interference visibilities of around 0.95. The generated states were also shown to break a form of Bell’s inequality by around six standard deviations. The polarisation entangled photon pair source was originally built at the University of St Andrews and was later transferred to the NPL where it will extend NPL’s capabilities to this key spectral region. \n \nFinally a study was carried out to investigate the possibility of a wavelength tuneable device for the absolute measurement of single photon detector quantum efficiencies based upon an established SPDC technique.
Photon sources based on spontaneous parametric down-conversion (SPDC) are central to experimental quantum optics and quantum technologies. Their performance is commonly quantified by three metrics: pair-collection probability, heralding efficiency, and spectral purity. In bulk-crystal SPDC, these metrics are known to be mutually constrained, yet the physical origin of the resulting trade-offs is often obscured. We show that these trade-offs originate from the frequency-dependent population of discrete spatial modes in the SPDC emission. By performing a Laguerre-Gauss mode decomposition at each frequency component, we show how spectral-spatial non-separability impacts collection probability, heralding efficiency, and purity. We apply this framework to two widely used quasi-phase-matching configurations: collinear degenerate type-0 and type-II SPDC in periodically poled bulk crystals, and quantify how different phase-matching functions shape the spectral-spatial mode structure. In particular, for type-II SPDC we compare standard periodically poled and aperiodically poled Gaussian phase matching. We experimentally validate some of our theoretical results using spatial- and spectral-projection measurements. This spectral-spatial mode analysis provides a quantitative and predictive framework for understanding and engineering bulk-crystal photon sources, enabling systematic multi-parameter optimization beyond qualitative design guidelines.
Spontaneous Parametric Down-Conversion (SPDC) is a cornerstone phenomenon in nonlinear optics, where an incident high-energy photon is transformed into a pair of lower-energy photons. This quantum process serves as a fundamental tool for generating entangled photon pairs, heralded single photons, and squeezed light, making it indispensable in the realms of quantum communication, quantum computing, and precision metrology. In this paper, we present a comprehensive study on the theoretical foundations and practical realization of an SPDC experimental setup, developed for the first time at the Photonics Research Laboratory at Amirkabir University of Technology. Our setup features a self-grown potassium dihydrogen phosphate (KDP) nonlinear crystal, illuminated by a 450 nm blue laser, to achieve parametric down-conversion into infrared photon pairs at 900 nm. By integrating a custom-designed transimpedance amplifier with a BPX65 photodiode, we have successfully amplified and detected the weak SPDC signal, demonstrating the feasibility of achieving high sensitivity with low-cost, widely available components. The experimental results align closely with theoretical predictions, showcasing the effectiveness of the system in detecting photon pairs with minimal resources. This study underscores the accessibility and scalability of SPDC technology, providing a blueprint for constructing versatile quantum optics experiments in resource-limited settings. By bridging the gap between theoretical insight and experimental realization, we aim to inspire further exploration of SPDC's potential in both foundational research and practical applications, laying the groundwork for future innovations in quantum technologies.
Stimulated-emission tomography (SET) is an excellent tool for characterizing the process of spontaneous parametric down conversion (SPDC), which is commonly used to create pairs of entangled photons for use in quantum information protocols. The use of stimulated emission increases the average number of detected photons by several orders of magnitude compared to the spontaneous process. In a SET measurement, the parametric down conversion is seeded by an intense signal field prepared with specified mode properties rather than by broadband multimodal vacuum fluctuations, as is the case for the spontaneous process. The SET process generates an intense idler field in a mode that is the complex conjugate to the signal mode. In this work we use SET to estimate the joint spatial mode distribution (JSMD) in the Laguerre-Gaussian (LG) basis of the two photons of an entangled photon pair. The pair is produced by parametric down conversion in a beta barium borate (BBO) crystal with type-II phase matching pumped at a wavelength of 405 nm along with a 780 nm seed signal beam prepared in a variety of LG modes to generate an 842 nm idler beam of which the spatial mode distribution is measured. We observe strong idler production and good agreement with the theoretical prediction of its spatial mode distribution. Our experimental procedure should enable the efficient determination of the photon-pair wavefunctions produced by low-brightness SPDC sources and the characterization of high-dimensional entangled-photon pairs. Published by the American Physical Society 2024
Spontaneous parametric down-conversion (SPDC) is one of the most important sources of entangled photons in quantum optics and underpins a wide range of applications in quantum communication, quantum imaging, quantum sensing, and photonic quantum information processing. In this nonlinear optical process, a pump photon is converted into a pair of lower-energy photons that exhibit strong correlations in multiple degrees of freedom. While the generation of photon pairs is governed by energy and momentum conservation, the resulting biphoton state possesses a rich spatial structure whose properties are determined by the interplay among the pump field, the nonlinear medium, and subsequent propagation. This thesis investigates the spatial properties of entangled photon pairs generated through SPDC, focusing on the evolution of their correlations in the near field and on the behavior of orbital-angular-momentum (OAM) entangled states under tight focusing conditions. A common theme throughout this work is the examination of physical effects that emerge when the full biphoton wavefunction is retained rather than reduced through the simplifying approximations often employed in theoretical treatments. The principal contribution of this thesis concerns the near-field propagation of SPDC photon pairs. The spatial correlations of SPDC-generated photons are commonly analyzed within the thin-crystal approximation, where the biphoton wavefunction is assumed to factorize into independent functions of the sum and difference coordinates. We demonstrate that birefringence-induced transverse walk-off breaks this factorization and introduces an intrinsic coupling between these coordinates. More importantly, this coupling survives free-space propagation and remains observable even for crystals that nominally satisfy the thin-crystal condition. As a result, the joint spatial correlations develop a characteristic tapering near the crystal image plane that cannot be captured by conventional factorized models. Numerical simulations and experimental measurements are found to be in excellent agreement, revealing a previously overlooked feature of SPDC and providing a more complete description of photon-pair generation and propagation in birefringent nonlinear media. The second part of the thesis explores the propagation of OAM-entangled photon pairs beyond the paraxial regime. While OAM correlations are typically studied under weak-focusing conditions, many emerging quantum technologies require structured quantum states to be manipulated in highly confined optical systems. We show that tight focusing modifies the OAM correlation spectrum, leading to a broadening of the accessible mode distribution. This broadening exhibits distinct transverse and longitudinal characteristics and arises from the emergence of nonparaxial field components absent in conventional scalar descriptions. These results provide new insight into the behavior of structured quantum light in strongly focused optical fields and establish a framework for understanding OAM entanglement beyond the paraxial approximation. Together, the results presented in this thesis reveal previously unexplored aspects of the structure and propagation of entangled photon pairs. By moving beyond widely used approximations, this work uncovers physical effects that influence the generation, evolution, and measurement of spatially entangled states, with implications for quantum imaging, high-dimensional quantum communication, and the broader development of structured-light-based quantum technologies.