Researchers Achieve Spectrally Separable Photon Pairs On-Chip for Quantum Technologies
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Scientists are tackling a key challenge in building scalable quantum technologies: the efficient creation of high-quality entangled photon pairs. Xiaojie Wang, Lin Zhou, and Yue Li, from the National University of Singapore and the Centre for Quantum Technologies, alongside Sakthi Sanjeev Mohanraj, Xiaodong Shi et al, have demonstrated a novel method for generating these pairs directly on a chip, overcoming limitations of previous techniques. Their research, published today, utilises thin-film lithium niobate nanophotonic circuits to produce spectrally separable, co-polarised photons , crucially, without the losses associated with spectral filtering or the complexity of managing differing polarisations. This breakthrough, achieving heralded single-photon purities exceeding 94%, paves the way for more flexible and integrated quantum light sources essential for advanced quantum computing and secure quantum communication networks. This innovative approach bypasses limitations of existing methods, which typically rely on lossy narrowband filtering or complex polarization management for on-chip integration.
The team harnessed higher-order spatial modes to engineer group-velocity matching, effectively separating the spectra of the generated photon pairs, and combined this with a specially designed Gaussian-apodized poling profile to suppress residual spectral correlations. The study reveals a meticulously crafted system where all interacting fields reside in the same polarization, simplifying device design and reducing potential losses. Spectral separability was achieved by precisely engineering the group-velocity matching using higher-order transverse-electric modes, a technique that allows for greater control over the photons’ characteristics. Crucially, the researchers implemented a Gaussian-apodized poling profile, which further minimises inherent spectral correlations typically found in standard periodic poling configurations. This careful engineering resulted in a significant improvement in spectral purity, exceeding 94% as inferred from joint-spectral intensity measurements and 89% from unheralded g(2) measurements, a key metric for assessing the quality of single photon sources. Experiments show that subsequent on-chip mode conversion, boasting an efficiency exceeding 95%, successfully maps the higher-order mode to the fundamental mode, directing the photons into distinct output channels. This efficient mode conversion is vital for seamlessly integrating the quantum light source with existing photonic circuits. The resulting heralded photons exhibit exceptional spectral purities, paving the way for more reliable and efficient quantum information processing. This approach enables flexible spectral and temporal engineering of on-chip quantum light sources, opening doors for advancements in both quantum computing and quantum networking. This work establishes a new paradigm for integrated quantum photonics, offering a pathway to compact, stable, and manufacturable quantum hardware. By leveraging the unique properties of thin-film lithium niobate and higher-order spatial modes, the researchers have overcome significant challenges in generating high-purity single photons without the drawbacks of traditional methods. The demonstrated spectral purities and high mode conversion efficiency represent a substantial leap forward, promising to accelerate the development of practical quantum technologies and facilitate the creation of complex quantum circuits for advanced applications. The ability to flexibly engineer the spectral and temporal properties of these on-chip sources will be invaluable for future quantum communication and computation systems. High-Purity Single Photons via Mode Engineering enable advanced Scientists engineered a novel approach to generating high-purity single photons on a chip, crucial for advancing scalable quantum technologies. Researchers addressed limitations in existing spontaneous parametric down-conversion (SPDC) sources by harnessing higher-order spatial modes within thin-film lithium niobate nanophotonic circuits, maintaining a single polarization for all interacting fields. Spectral separability was achieved through precise engineering of group-velocity matching using higher-order transverse-electric modes, combined with a Gaussian-apodized poling profile designed to suppress residual spectral correlations typically found in standard periodic poling. This innovative technique circumvents the need for narrowband spectral filtering, which often introduces signal loss, and avoids the complexities of managing different polarizations in integrated circuits. The study pioneered a method for on-chip mode conversion, achieving an efficiency exceeding 95%, to map the higher-order mode back to the fundamental mode and direct the generated photons into separate output channels. Experiments employed a 785nm pump laser to initiate the SPDC process within a carefully designed TFLN waveguide, generating signal and idler photons at 1520nm and 1620nm respectively. Simulations and fabrication focused on optimizing the poling profile, specifically implementing a Gaussian-apodized domain inversion to reshape the nonlinear response and minimize spectral sidelobes, resulting in a significantly improved joint spectral intensity distribution.
The team validated the realized domain inversions using scanning-helium-gas microscopy, confirming the precision of the fabrication process. Researchers meticulously designed the device to balance spectral purity with nonlinear strength, targeting a specific phase-matching function that maximizes photon pair generation. The system delivers a spectral purity exceeding 94%, as inferred from joint-spectral intensity analysis, and 89% from unheralded measurement, demonstrating a substantial improvement over conventional SPDC sources. This approach enables flexible spectral and temporal engineering of on-chip quantum light sources, paving the way for advancements in quantum computing and quantum networking. The technique reveals a pathway to create brighter, more reliable single-photon sources without compromising spectral quality, a critical step towards building practical quantum technologies. High-Purity Photons from Lithium Niobate Circuits enable advanced Scientists achieved on-chip generation of high-purity single photons essential for scalable quantum technologies. Experiments revealed a new strategy for generating spectrally separable photon pairs in thin-film lithium niobate nanophotonic circuits by harnessing higher-order spatial modes, all within the same polarization, a significant advancement in quantum light source design. Spectral separability was attained by engineering group-velocity matching using higher-order transverse-electric modes, combined with a Gaussian-apodized poling profile to suppress residual correlations inherent to standard periodic poling.
The team measured an impressive on-chip mode conversion efficiency exceeding 95%, effectively mapping the higher-order mode to the fundamental mode and directing the photons into distinct output channels.
Results demonstrate that the heralded single photons exhibit spectral purities exceeding 94%, as inferred from joint-spectral intensity analysis, and 89% from unheralded measurement, values critical for maintaining quantum coherence. A Gaussian-apodized poling pattern reshaped the nonlinear response and suppressed residual spectral correlations, enabling a measured spectral purity of 94% without the need for spectral filtering. In contrast to type-II configurations, higher-order modes offer stronger dispersion tunability, allowing for flexible control of photon temporal and spectral modes in integrated quantum sources. This tunability is crucial for tailoring quantum light sources to specific application requirements. The biphoton component of the generated state is described by an equation incorporating the joint spectral amplitude (JSA), and achieving spectrally separable photon pairs requires a factorable JSA. Scientists engineered the group velocities of the interacting modes to rotate the phase-matching function to be orthogonal to the pump envelope function, yielding a nearly factorable JSA with strongly suppressed spectral correlations. By increasing the waveguide width to above 2μm with an etch depth of 360nm, the dispersion curves of the TE0 and TE2 modes were arranged to fulfill the group-velocity matching condition. Furthermore, the team designed a target cumulative nonlinear amplitude profile to modify the nonlinear coefficient, achieving a target phase-matching function with a Gaussian form, essential for high spectral purity. Simulations showed that Gaussian-apodized poling increased the theoretical spectral purity from 84% to 99% compared to periodic poling. Scanning laser second-harmonic-generation (SHG) microscope images confirmed clear, uniform, and well-defined domain inversions within the poled region, validating the accurate implementation of the designed poling profile. This breakthrough delivers a pathway towards flexible spectral and temporal engineering of on-chip quantum light sources for quantum key distribution and quantum networking. High-purity single photons from lithium niobate chips enable Scientists have demonstrated a novel method for generating high-purity single photons on a chip, crucial for advancing scalable quantum technologies. Researchers achieved this by employing spontaneous parametric down-conversion (SPDC) within thin-film lithium niobate nanophotonic circuits, utilising higher-order spatial modes to create spectrally separable photon pairs, all while maintaining a single polarization. This innovative approach circumvents limitations of traditional methods that rely on narrowband filtering or polarization manipulation, simplifying on-chip integration and enhancing efficiency.
The team successfully engineered group-velocity matching using higher-order transverse-electric modes, combined with a specifically designed poling profile, to suppress spectral correlations, resulting in heralded single photons exhibiting purities exceeding 94% as inferred from joint-spectral intensity and 89% from unheralded measurement. This modal-dispersion technique offers a flexible framework for phase-matching engineering, overcoming constraints inherent in conventional methods and enabling precise control over spectral properties. The authors acknowledge that residual phase correlations, stemming from the pump envelope and poling imperfections, may slightly reduce heralded purity, and that imperfections in the poling structure can introduce unwanted spectral correlations. Future work could focus on mitigating these effects to further enhance photon purity and explore co-designing photon generation, routing, and spectral manipulation for scalable quantum architectures. This achievement signifies a substantial step towards realising practical and versatile platforms for large-scale quantum technologies, facilitating seamless integration with diverse photonic and hybrid quantum systems. The ability to flexibly engineer spectral characteristics of on-chip quantum light sources is particularly valuable for both fundamental quantum research and the development of quantum networks, paving the way for more complex and functionally rich quantum devices. 👉 More information 🗞 On-Chip Generation of Co-Polarized and Spectrally Separable Photon Pairs 🧠 ArXiv: https://arxiv.org/abs/2601.13740 Tags: