High-dimensional Temporal Entanglement Achieves Robust Quantum Key Distribution with Novel Protocol

Harnessing the power of high-dimensional entanglement represents a significant step towards secure quantum communication, particularly in challenging environments with signal loss and noise. Dorian Schiffer, Robert Kindler, and Alexandra Bergmayr-Mann, alongside Florian Kanitschar, Amin Babazadeh, Paul Erker, and colleagues from the Atominstitut (Technische Universität Wien) and the Institute for Quantum Optics and Quantum Information (Austrian Academy of Sciences), have developed a new source of highly bright, time-bin entangled photons designed to overcome these limitations. The researchers demonstrate a robust and stable system, certifying the generated entanglement using a novel measurement technique based on nested Franson interferometry. This work is particularly important as it establishes a flexible evaluation method allowing optimisation of performance, and reveals achievable key rates for high-dimensional quantum key distribution exceeding those possible with conventional systems. Ultimately, this research paves the way for more practical and secure quantum communication networks.

This research addresses the critical challenge of secure communication in environments plagued by signal loss and noise, leveraging the inherent robustness of high-dimensional entanglement. The team achieved this by building a source of time-bin entangled photons specifically optimized for brightness, simplicity, and long-term operational stability, crucial factors for practical QKD systems. This innovative source utilizes spontaneous parametric down-conversion within a periodically poled Potassium Titanyl Phosphate crystal, pumped by a wavelength-locked laser exhibiting exceptional stability of 3 fm per hour over 24 hours, generating photons centered at wavelengths of 774.6nm and 845.4nm with linewidths of 2.2nm and 2.3nm respectively.

The study unveils a novel method for certifying high-dimensional entanglement, employing a nested Franson interferometry technique. This approach allows for precise characterization of the generated entangled states, specifically quantifying the Schmidt number and entanglement rate. Researchers then implemented a new, noise-resilient QKD protocol, demonstrating key rate performance exceeding that of traditional qubit-based systems. The experimental setup, detailed in the work, incorporates stabilized interferometers and utilizes a dichroic mirror for efficient photon separation, achieving coincidence rates of 2.2x 10 6 counts per second per milliwatt of pump power and a heralding efficiency of approximately 30% at 0.1mW.

A key innovation lies in the flexible evaluation method developed by the scientists, centered around discretizations of the time stream. This allows the same experimental dataset to be processed with varying parameters, such as state dimensionality and time bin length, enabling optimization of performance under diverse environmental conditions. Results indicate the existence of accessible parameter regions where high key rates are achievable for dimensionalities greater than two, signifying a substantial improvement over conventional QKD approaches. This adaptability is particularly valuable for real-world applications, including the prospect of full daytime QKD over free-space channels.

The research establishes a pathway towards practical, high-performance QKD systems, particularly for challenging scenarios like ground-to-satellite quantum communication links. By focusing solely on time-bin entanglement, the team circumvented the limitations inherent in polarization-based approaches, constructing a low-complexity source with enhanced stability and fewer technical constraints. The ability to optimize performance through post-processing of time-of-arrival statistics further enhances the protocol’s versatility and potential for widespread deployment, paving the way for secure quantum communication networks of the future.

High-Dimensional Entangled Photons for Robust QKD

The research detailed a novel approach to quantum key distribution (QKD), focusing on high-dimensional time-bin entangled photons to overcome limitations in high-loss and noisy environments. Scientists engineered a source of these entangled photons, prioritizing brightness, simplicity, and long-term stability, crucial for practical applications like satellite-based quantum communication. This source utilizes a Type-0 periodically poled potassium titanyl phosphate (ppKTP) crystal pumped by a wavelength-locked 404.5nm laser, with careful collimation and filtering to isolate the desired photon pairs. The generated photons are then coupled into single-mode fibers for distribution to detection modules, designated Alice and Bob.

To rigorously verify the generated entanglement, the study pioneered a new certification method employing nested Franson interferometry. Each detection module comprises two imbalanced Mach-Zehnder interferometers (MZIs) constructed with polarizing beam splitters (PBSs) and half-wave plates (HWPs). These components impose diagonal polarization and serve as filters against background noise, enhancing the signal quality. The nested configuration allows precise measurement of the Schmidt number and entanglement rate, providing a detailed characterization of the photonic states. This innovative technique moves beyond traditional qubit-based entanglement, addressing inherent limitations in dimensionality and stability.

A key methodological innovation lies in the flexible evaluation method centered around discretizations of the time stream. This approach enables the same experimental dataset to be processed with varying parameters, including state dimensionality and time bin length, facilitating optimization of performance under diverse environmental conditions. The system records time-of-arrival statistics from all detectors in two distinct settings, allowing for post-processing analysis and exploration of different figures of merit. This adaptability is critical for achieving high key rates, demonstrating the potential for QKD in challenging scenarios, including daytime free-space channels.

The research culminated in the demonstration of a novel, noise-resilient QKD protocol, agnostic to polarization, and built solely on time-bin entanglement. By avoiding the complexities of polarization entanglement, the team achieved a low-complexity source with enhanced stability and fewer technical constraints. Results indicate achievable high key rates for dimensionalities exceeding two, paving the way for practical, long-distance quantum communication links.

High-Dimensional Entanglement Source and Key Rates

Scientists achieved a breakthrough in high-dimensional quantum key distribution (QKD) by constructing a novel source of time-bin entangled photons. The research focused on optimizing brightness, reducing complexity, and ensuring long-term stability, critical factors for practical quantum communication systems. Experiments revealed the successful generation of high-dimensional entanglement, certified using a new witness based on nested Franson interferometry, a technique allowing precise characterization of entangled states. This innovative approach enables the same dataset to be processed with varying parameters, such as state dimensionality and time bin length, facilitating performance optimization under diverse environmental conditions.

The team measured key rates using a newly developed, noise-resilient QKD protocol, demonstrating the potential for secure communication in challenging scenarios. Results demonstrate regions within the accessible parameter space where high key rates per time unit are achievable for dimensionalities exceeding two, a significant advancement beyond traditional qubit-based systems. The entangled-photon source utilizes a 3cm periodically poled Potassium Titanyl Phosphate (ppKTP) crystal pumped with a wavelength-locked 404.5nm laser, achieving high brightness through a larger non-linear coefficient. Careful temperature tuning of the crystal allows for deterministic separation of signal and idler photons via a dichroic mirror, a crucial step in the process.

Measurements confirm the source’s ability to simultaneously optimize for both high brightness and efficient heralding, with significantly fewer technical constraints than existing polarization-based approaches. The experimental setup incorporates nested, imbalanced Mach-Zehnder interferometers (MZIs) for assessing entanglement properties, specifically the Schmidt number and entanglement rate. By monitoring the classical interference of a stabilized reference laser through the MZIs, scientists stabilized the interferometers using a piezo-actuated mirror stage, ensuring consistent performance over time. This work paves the way for full daytime QKD over free-space channels, including ground-to-satellite quantum communication links, by employing only time-bin entanglement and avoiding the limitations inherent in polarization-based systems. The flexible evaluation method, centered on discretizations of the time stream, allows for post-processing optimization, tailoring performance to specific figures of merit and environmental conditions. The breakthrough delivers a low-complexity entangled-photon source with enhanced stability, crucial for real-world deployment and long-distance quantum communication.

High-Dimensional Entanglement Boosts Quantum Key Distribution

This work demonstrates a high-brightness, stable source of high-dimensional time-bin entangled photons specifically designed for use in challenging lossy and noisy environments. Researchers characterised the generated photonic states using a nested Franson interferometer, quantifying entanglement dimensionality, rate, and achievable key rate within a newly developed quantum key distribution protocol. The flexible evaluation method, based on discretisation of the time domain, confirmed that the nested-Franson configuration enhances both the certifiable Schmidt number and overall entanglement throughput. Results indicate that moving beyond qubit-based systems to higher dimensions offers a demonstrable advantage in both key and entanglement rates, highlighting the performance of the source design and evaluation techniques.

While the entanglement-per-photon pair is below a theoretical limit, the source’s extraordinary brightness compensates for this. The authors acknowledge a limitation in not yet exploring whether enhancing entanglement per photon pair, through techniques like single-mode fibre coupling or spectral filtering, could further improve coincidence rates. Future research should investigate these possibilities, alongside potential applications in demanding quantum communication scenarios such as satellite-based quantum key distribution.