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Time-Correlated Photons from Spontaneous Parametric Down-Conversion (SPDC)

Aaron Dubois, Islam, Tanvir, , ‘26, Thapa Magar, Prashan, , ‘26

Published Apr 1, 2026• 0 citations
OpenAlex

Abstract

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.

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Experimental Realization of Spontaneous Parametric Down Conversion

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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.

•Dec 14, 2024•2 citations

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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.

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Quantum interference using ultrafast spontaneous parametric down-conversion

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Summary form only given. Pairs of entangled photons display a complex quantum state that cannot be represented through a product of single-particle states. The spontaneous parametric down-conversion (SPDC) process, that takes place when a laser pulse pumps a second-order nonlinear crystal has become recently a convenient source of such nonclassical states of light in quantum optics experiments. Nonstationary effects are mainly observed when the pump pulse temporal duration is significantly shorter than the characteristic interaction time in the nonlinear crystal. We introduce a nonstationary theory of SPDC to describe the results of fourth-order quantum-interference experiments when no spectral filtering is applied to the down-converted fields. Experiments involving nonlinear SPDC crystals with three different lengths were performed. The ultraviolet laser pulses were obtained from a Ti:sapphire femtosecond laser with a second-harmonic generator. These pulses were used to pump nonlinear crystals to initiate the process of nonstationary spdc and generate orthogonally-polarized photon pairs. These pairs then went into a polarization interferometer, and the coincidence counts of the detectors at the two output ports of the interferometer were recorded.

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There is an important class of experiments that characterize the properties of quantum mechanical systems using correlated photons, and a typical source for correlated photons is the process of Spontaneous Parametric Down Conversion (SPDC). SPDC is a quantum mechanical process in which a single “pump” photon is converted into two photons that have half the frequency and double the wavelength of the original photon. This process takes place inside a crystal with particular optical properties that allow for SPDC, and we used a uniaxial Beta Barium Borate crystal. A single photon counting module (SPCM) was used as a detector for the photons. Our goal was to characterize a 474 nanometer (nm) Diode Pumped Solid State (DPSS) laser as a possible source for SPDC correlated photons. Measurements of the DPSS laser radiation showed that it emitted photons with a wavelength of both 474 nm and 948 nm, which was expected. The SPDC photons produced by our apparatus would also have a wavelength of 948 nm, so we tested whether the down converted photons could be detected without the 948 nm signal from the DPSS laser overwhelming the SPCM. Software for determining the parameters of SPDC in a given system is available from the National Institute of Standards and Technology, and we used it to determine where we could expect to see SPDC photons. Initial measurements suggest that the system can produce SPDC photons, and we will continue to characterize the apparatus.

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The source of time-correlated photon-pairs at 1064 nm is described. The source consists of the spontaneous parametric down-conversion (SPDC) generator, pumped by cw laser operating at 532 nm, and the measuring and control appliances. One of the main parts of the electronic systems is the “time-to-digital converter” which is designed and built by our group. The system allows to create and detect correlation of photon pairs with resolution better than 1 ns. We adduce the results of a quantum key distribution through open air. The key length was about 5000 bits and the accuracy ~0.1%.

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Second-order correlations and purity of unheralded single photons from spontaneous parametric down-conversion

A. Padhye, K. Muhammed Shafi, C. M. Chandrashekar

Various quantum technology applications require high-purity single photons with high generation rate. Although different methods are employed to generate such photons, heralded single photons from spontaneous parametric down-conversion (SPDC) is the most commonly used approach. Photon generation rate from the heralded single-photon sources are limited by the efficiency of the detectors to record coincidence detection of the photon pairs which are lower than the single-photon counts recorded separately on each detector. In this paper we present a revised expression to calculate second-order temporal correlation function, $g^{(2)}$ for any fixed time window (bin) and report the experimental characterization of purity of unheralded and heralded single photons from the SPDC process. With an appropriate choice of time bin for a given pump power, without heralding we show that higher rate of single photons with $g^{(2)}(0) = 0$ can be generated with very high probability.

arXiv:2207.06984•Jul 14, 2022•0 citations
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.
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.
Summary form only given. Pairs of entangled photons display a complex quantum state that cannot be represented through a product of single-particle states. The spontaneous parametric down-conversion (SPDC) process, that takes place when a laser pulse pumps a second-order nonlinear crystal has become recently a convenient source of such nonclassical states of light in quantum optics experiments. Nonstationary effects are mainly observed when the pump pulse temporal duration is significantly shorter than the characteristic interaction time in the nonlinear crystal. We introduce a nonstationary theory of SPDC to describe the results of fourth-order quantum-interference experiments when no spectral filtering is applied to the down-converted fields. Experiments involving nonlinear SPDC crystals with three different lengths were performed. The ultraviolet laser pulses were obtained from a Ti:sapphire femtosecond laser with a second-harmonic generator. These pulses were used to pump nonlinear crystals to initiate the process of nonstationary spdc and generate orthogonally-polarized photon pairs. These pairs then went into a polarization interferometer, and the coincidence counts of the detectors at the two output ports of the interferometer were recorded.
There is an important class of experiments that characterize the properties of quantum mechanical systems using correlated photons, and a typical source for correlated photons is the process of Spontaneous Parametric Down Conversion (SPDC). SPDC is a quantum mechanical process in which a single “pump” photon is converted into two photons that have half the frequency and double the wavelength of the original photon. This process takes place inside a crystal with particular optical properties that allow for SPDC, and we used a uniaxial Beta Barium Borate crystal. A single photon counting module (SPCM) was used as a detector for the photons. Our goal was to characterize a 474 nanometer (nm) Diode Pumped Solid State (DPSS) laser as a possible source for SPDC correlated photons. Measurements of the DPSS laser radiation showed that it emitted photons with a wavelength of both 474 nm and 948 nm, which was expected. The SPDC photons produced by our apparatus would also have a wavelength of 948 nm, so we tested whether the down converted photons could be detected without the 948 nm signal from the DPSS laser overwhelming the SPCM. Software for determining the parameters of SPDC in a given system is available from the National Institute of Standards and Technology, and we used it to determine where we could expect to see SPDC photons. Initial measurements suggest that the system can produce SPDC photons, and we will continue to characterize the apparatus.
The source of time-correlated photon-pairs at 1064 nm is described. The source consists of the spontaneous parametric down-conversion (SPDC) generator, pumped by cw laser operating at 532 nm, and the measuring and control appliances. One of the main parts of the electronic systems is the “time-to-digital converter” which is designed and built by our group. The system allows to create and detect correlation of photon pairs with resolution better than 1 ns. We adduce the results of a quantum key distribution through open air. The key length was about 5000 bits and the accuracy ~0.1%.
Various quantum technology applications require high-purity single photons with high generation rate. Although different methods are employed to generate such photons, heralded single photons from spontaneous parametric down-conversion (SPDC) is the most commonly used approach. Photon generation rate from the heralded single-photon sources are limited by the efficiency of the detectors to record coincidence detection of the photon pairs which are lower than the single-photon counts recorded separately on each detector. In this paper we present a revised expression to calculate second-order temporal correlation function, g(2)g^{(2)}g(2) for any fixed time window (bin) and report the experimental characterization of purity of unheralded and heralded single photons from the SPDC process. With an appropriate choice of time bin for a given pump power, without heralding we show that higher rate of single photons with g(2)(0)=0g^{(2)}(0) = 0g(2)(0)=0 can be generated with very high probability.