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.
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.
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.
The quantum entangled photon-pair source, as an essential component of optical quantum systems, holds great potential for applications such as quantum teleportation, quantum computing, and quantum imaging. The current workhorse technique for preparing photon pairs involves performing spontaneous parametric down conversion (SPDC) in bulk nonlinear crystals. However, the current power consumption and cost of preparing entangled photon-pair sources are relatively high, posing challenges to their integration and scalability. In this paper, we propose a low-power system model for the quantum entangled photon-pair source based on SPDC theory and phase matching technology. This model allows us to analyze the performance of each module and the influence of component characteristics on the overall system. In our experimental setup, we utilize a 5 mW laser diode and a typical type-II barium metaborate (BBO) crystal to prepare an entangled photon-pair source. The experimental results are in excellent agreement with the model, indicating a significant step towards achieving the goal of low-power and low-cost entangled photon-pair sources. This achievement not only contributes to the practical application of quantum entanglement lighting, but also paves the way for the widespread adoption of optical quantum systems in the future.
Time-correlated photons can be used to demonstrate emerging technical applications such as ghost imaging, and they can demonstrate quantum-based phenomena in order to interest undergraduate students in engineering applications involving optics and quantum mechanics. In this work, we use spontaneous parametric down-conversion to generate time-correlated photons. To achieve this, we simulated laboratory parameters with MATLAB and designed optical and electrical setups. Specifically, we used a 405 nm diode laser, various common optical components, and a nonlinear crystal designed to generate time-correlated photons, and we used single-photon detectors, time-to-amplitude converters, custom flash-conversion analog-to-digital converters, a Teensy 4.1 microcontroller, and the Arduino IDE to detect the time-correlated photons. We present two significant results. First, we measured >100,000 counts per second 5 ± 1° on either side of our input laser which was within our theoretical expectations of 4–9° and much greater than our < 2000 dark counts per second. Our second major result was finding 15.8 ± 0.7 time-correlated counts per second with an SNR of 2.7. We also outline future extensions of our work including ghost imaging and generating polarization-entangled photons.
Entangled photon pairs play a crucial role in emerging quantum technologies, acting as tamper-proof padlocks in secure quantum communication, ultra-precise probes in quantum metrology and high-fidelity information carriers in photonic quantum computing. Amongst the many technological possibilities for generating photon pairs, sources based on spontaneous parametric down-conversion (SPDC) in second-order nonlinear crystals are still the workhorse tool in quantum optics laboratories, and more recently even in long-distance quantum communication with satellites. SPDC is a widely used method to create entangled photon pairs. The number of photon pairs that can be detected in a particular choice of collection modes, typically being optical single-mode fibers, depends critically on the spatial characteristics of the multi-mode SPDC emission as well as the particular beam waist of the collection modes. While several studies have already addressed the issue of optimal fiber coupling of SPDC photons in theory and experiment, the results of these studies arrive at different conclusions. Here, we present the results of a comprehensive experimental study on the optimal collection of photon pairs into single-mode optical fibers. Our approach is based on quasi-phase-matched type II SPDC from a periodically poled KTiOPO4 (ppKTP) crystal. We discuss the influence of pump and collection focal parameters on the spectral brightness and heralding efficiency, as well as practical issues of alignment tolerances into an optical single mode fiber. Further, Using two-photon interference (Hong-Ou-Mandel interference), we assess the spectral bandwidth of the photon pairs for variable crystal lengths. The results are in good agreement with our theoretical model, thus providing the recipe of building ultra bright and stable entangled photon sources.
Miniaturised entangled photon sources are highly demanded for the development of integrated quantum photonics. Since the invention of subwavelength optical metasurfaces and their successes at replacing bulky optical components, the possibility of implementing entangled photon sources on such devices is actively investigated. Here, as a first step towards the development of quantum optical metasurfaces (QOM), we demonstrate photon pair generation via spontaneous parametric down-conversion (SPDC) from subwavelength films. We achieved photon pair generation with a high coincidence-to-accidental ratio in lithium niobate and gallium phosphide nanofilms. In addition, we have measured the SPDC frequency spectrum via fibre spectroscopy, obtaining photon pairs with a spectral bandwidth of 500\;nm, limited only by the overall detection efficiency. Moreover, we have observed the vacuum field enhancement due to a Fabry-Perot resonance inside the nonlinear films. Our experiments lay the groundwork for the future development of flat SPDC sources, including QOM.
Spontaneous parametric down-conversion (SPDC) is a fundamental quantum optical technique for generating entangled photon pairs, which serves as the foundation for various quantum applications, including quantum cryptography, quantum simulation, quantum metrology, and the exploration of fundamental physical laws in quantum mechanics. Nevertheless, SPDC is plagued by low efficiency due to the extremely weak nature of this spontaneous quantum nonlinear process. In this work, we design a simple dielectric metasurface structure composed of lithium niobate (LN) defect disks that can support dual quasi-bound states in the continuum (quasi-BICs) in the near-infrared range. By aligning the high-Q quasi-BIC resonances with the generated signal and idler frequencies, our numerical simulation results demonstrate that the local electric field in the nonlinear medium reaches a substantial enhancement of up to two orders of magnitude, resulting in a significant improvement in the efficiency of the classical nonlinear optical sum frequency generation (SFG) and its inverse SPDC quantum process. Our findings pave the way for the development of high-quality compact nanosources of entangled photon pairs with potential applications spanning across cutting-edge fields such as quantum communications, computing, and imaging that can potentially revolutionize the field of quantum technology.