Quantum SPDC Source at 900-950nm Range Enhances Single Photon Generation Probability Via Time-Multiplexing

Single photons represent a crucial resource for advancing quantum technologies, and researchers continually seek ways to improve their generation, particularly through heralded single photon sources. V. O. Gotovtsev, I. V. Dyakonov, and O. V. Borzenkova, alongside colleagues including K. A. Taratorin and T. B. Dugarnimaev, have significantly enhanced these sources by optimising a time-multiplexed approach to spontaneous parametric down-conversion. This work addresses the inherent challenge of low single photon generation probability, demonstrating accurate calculations and modelling of key source characteristics such as purity and heralding efficiency. By analysing and approximating the probability of single photon generation after applying time multiplexing, the team provides a pathway towards brighter and more reliable single photon sources for future quantum applications.

High-Purity Entangled Photons via SPDC Generation

This research focuses on maximizing the purity of entangled photon pairs created through spontaneous parametric down-conversion, a process where a laser beam splits into two lower-energy photons with correlated properties. Purity, in this context, refers to how well-defined the quantum state of the photons is, crucial for applications like quantum communication, computing, and sensing. Scientists achieve this by carefully structuring the nonlinear crystal used to generate the photons, a technique known as domain engineering. Domain engineering deliberately shapes the crystal’s internal structure to control the down-conversion process efficiently, aiming for a predictable and single-mode output leading to higher purity.

The shape of the phase-matching function is critical, with a Gaussian shape being particularly desirable. Unwanted spectral correlations between the generated photons degrade purity, and domain engineering effectively removes these correlations. Using a narrow-bandwidth laser and carefully selecting the crystal length further enhances purity. The team developed a detailed mathematical model to describe the process, incorporating the crystal structure and laser characteristics, allowing for precise optimization of the domain structure through numerical simulations. By comparing their custom domain structure to standard designs, researchers demonstrated a significant improvement in purity, achieving up to a 10% increase. This work highlights the importance of precise control over the nonlinear crystal’s structure for generating high-quality entangled photons, paving the way for advanced quantum technologies.

Time Multiplexing for Single Photon Generation

Scientists developed a method to increase the probability of detecting a single photon, a key challenge in many quantum technologies. The technique, called time-multiplexing, utilizes spontaneous parametric down-conversion to generate pairs of photons. A series of laser pulses pumps a nonlinear crystal, and the team stores the signal photon in an optical memory triggered by detecting its entangled partner, the idler photon. This effectively increases the chances of detecting a single photon within a specific time window. The core of the method lies in the precise control of photon storage and release.

Following idler photon detection, the signal photon is directed into an optical memory cell, remaining there until the end of the pulse series. Researchers carefully calculated the probability of obtaining a single photon after multiplexing, considering factors like photon pair generation and the efficiency of the detection system. The experimental setup incorporated a femtosecond laser and a sophisticated field-programmable gate array to control the process. The team implemented a technique to extend the opening time of the optical memory cell for the final photon in the series, ensuring accurate storage and release. This innovative approach allows for precise control over the photon storage and release process, maximizing the efficiency of single-photon generation. This work demonstrates a promising pathway towards brighter and more reliable single-photon sources for quantum technologies.

Optimizing Single-Photon Purity and Heralding Probability

This research focuses on enhancing single-photon generation using time-multiplexing, applied to a source based on spontaneous parametric down-conversion. Researchers concentrated on optimizing the probability of generating a photon within a very narrow temporal window, by carefully analyzing the characteristics of the photon pairs produced. To quantify this effect, the team introduced the concept of “heralding probability,” defined in terms of the joint spectral amplitude of the photon pairs, and explored how this, alongside photon purity, could be maximized. The study centers on a thorough analysis of the joint spectral amplitude, which fully characterizes the two-photon state and describes the spectral correlations between signal and idler photons.

This provides a framework for evaluating both heralding probability and the purity of the heralded single-photon state. Researchers developed a mathematical model describing the biphoton state generated by the down-conversion crystal, incorporating factors like pump beam characteristics and crystal properties. Experiments utilized a periodically poled potassium titanyl phosphate crystal pumped by a laser. Through careful manipulation of the beam parameters and crystal properties, the team determined the optimal conditions for achieving single-mode generation. The model predicts and experiments confirm that precise control over phase matching and beam confinement is crucial for maximizing the heralding probability and ensuring high photon purity. This work demonstrates a pathway towards brighter and more reliable single-photon sources, essential for advancements in quantum technologies.

Time-Multiplexed Single Photons From Down-Conversion

This research demonstrates a time-multiplexed heralded single-photon source, built upon the process of spontaneous parametric down-conversion. The team accurately modeled and calculated key characteristics of the source, including its purity and heralding efficiency, and derived an approximation for the probability of single-photon generation following time multiplexing. This work addresses a significant challenge in quantum photonics, namely the low generation probability often associated with single-photon sources. The experimental setup incorporated a femtosecond laser and a periodically poled potassium titanyl phosphate crystal to generate photon pairs, with a field-programmable gate array used to identify and store signal photons in an optical memory cell.

Through careful analysis and optimization of the system, the researchers achieved a measured frequency of single photons and determined values for heralding efficiency, accounting for fiber losses and detector performance. The team acknowledges limitations imposed by the actuation speed of the optical memory cell used for photon storage, requiring adjustments to the calculation of multiplexed probability. Future work could focus on improving the speed and capacity of the optical memory cell to further enhance the efficiency of photon storage and retrieval. Additionally, exploring alternative methods for heralding and detection could lead to even greater improvements in the overall performance of single-photon sources for quantum technologies. These advancements promise to contribute to the development of more robust and practical quantum communication and computation systems.