Quantum Key Distribution System Research Articles

Autonomic end-to-end quality-of-service assurance over QKD-secured optical networks

Quantum key distribution (QKD) provides future-proof security for data communications over optical networks. Currently, sophisticated QKD systems are developed and the scale of QKD-secured optical networks (QKD-ONs) becomes larger. Given the complex network conditions and dynamic end-to-end security services in QKD-ONs, autonomic management and control becomes a promising paradigm to support end-to-end quality-of-service (QoS) assurance in an efficient and stable way without requiring human intervention. Hence, to enable and utilize the autonomic functionalities over QKD-ONs for realizing the end-to-end QoS assurance becomes a challenge. This work enhances the software defined networking (SDN) technique to tackle this challenge because SDN can add programmability and flexibility for QKD-ON’s management and control. A new architecture of SDN-based QKD-ONs supporting autonomic end-to-end QoS assurance is designed, where a knowledge engine with autonomic control loops is developed in the SDN controller. We present the autonomic end-to-end QoS assurance procedure, and the cross-layer collaborative QoS assurance (CLC-QA) strategy for implementing the autonomic functionalities in the network level over QKD-ONs. We also establish an experimental testbed of SDN-based QKD-ONs supporting autonomic end-to-end QoS assurance, and perform the numerical simulation to verify our proposed approaches. Experimental results demonstrate that our presented approaches can achieve the millisecond-level overall latency of 337 ms and 618 ms, during the first and second autonomic adjustment without human intervention in case of the autonomic QoS protection. Moreover, the CLC-QA strategy is evaluated under different traffic loads by being compared with the baseline strategy without cross-layer collaboration. It can improve 22.5% protection success ratio and save 5.7% average key consumption.

Advanced Secure Communication: Exploring Quantum Key Distribution, the BB84 Method

One promising way to use quantum information to secure everyday communication is through the distribution of quantum keys. By exchanging a secret key, the Quantum Key Distribution (QKD) approach allows two parties to communicate securely.BB84 protocol is among the most well-known QKD protocols. In this protocol, qubits are exchanged via a quantum channel between the sender and the receiver. This enables them to produce a shared key that is impenetrable to eavesdroppers and illustrate the fundamental ideas of QKD using current simulations and implementations. The results of this study demonstrate that the BB84 protocol is a highly secure QKD technique that has been investigated in great detail and used in a variety of contexts. Additionally, over the enhancements made to the BB84 protocol such as the use of advanced error correction techniques and decoy states to increase its security and usability is discussed. With an emphasis on the BB84 protocol in secure communication technologies, this study offers an extensive analysis of QKD systems overall.

Chip-Based Electronic System for Quantum Key Distribution

Quantum Key Distribution (QKD) has garnered significant attention due to its unconditional security based on the fundamental principles of quantum mechanics. While QKD has been demonstrated by various groups and commercial QKD products are available, the development of a fully chip-based QKD system, aimed at reducing costs, size, and power consumption, remains a significant technological challenge. Most researchers focus on the optical aspects, leaving the integration of the electronic components largely unexplored. In this paper, we present the design of a fully integrated electrical control chip for QKD applications. The chip, fabricated using 28 nm CMOS technology, comprises five main modules: an ARM processor for digital signal processing, delay cells for timing synchronization, ADC for sampling analog signals from monitors, OPAMP for signal amplification, and DAC for generating the required voltage for phase or intensity modulators. According to the simulations, the minimum delay is 11ps, the open-loop gain of the operational amplifier is 86.2 dB, the sampling rate of the ADC reaches 50 MHz, and the DAC achieves a high rate of 100 MHz. To the best of our knowledge, this marks the first design and evaluation of a fully integrated driver chip for QKD, holding the potential to significantly enhance QKD system performance. Thus, we believe our work could inspire future investigations toward the development of more efficient and reliable QKD systems.

A multi-mode free-space delay interferometer with no refractive compensation elements for phase-encoded QKD protocols

We demonstrate a compensation-free approach to the realization of multi-mode delay interferometers, mainly for use in phase-encoded quantum key distribution (QKD). High interference visibility of spatially multi-mode beams in unbalanced Michelson or Mach–Zehnder interferometers with a relatively wide range of delays is achieved by the appropriate choice of the transverse size of the beam. We provide a simple theoretical model that gives a direct connection between the visibility of interference, the delay and the beam parameters. The performed experimental study confirms our theoretical findings and demonstrates measured visibility of up to 0.95 for a delay of 2 ns. Our approach’s simplicity and robust performance make it a practical choice for the implementation of QKD systems, where a quantum signal is received over a multi-mode fiber. The important application of such a configuration is an intermodal QKD system, where the free-space atmospheric communication channel is coupled into a span of the multi-mode fiber, delivering the spatially distorted beam to the remote receiver with minimal coupling loss.

Telecom‐Band Spontaneous Parametric Down‐Conversion in AlGaAs‐on‐Insulator Waveguides

AbstractWidespread commercial adoption of telecom‐band quantum key distribution (QKD) will require fully integrated, room‐temperature transmitters. Implementing highly efficient spontaneous parametric down‐conversion (SPDC) on a platform that offers co‐integration of the pump laser has been an outstanding challenge. Here, using such a platform based on AlGaAs‐on‐insulator waveguides, telecom‐band SPDC (and SHG, second harmonic generation) is reported with exceedingly large efficiencies of 26 GHz generated pairs per mW over a 7 THz bandwidth, which would saturate the usable photon‐flux for a 70‐channel wavelength‐multiplexed QKD system at merely 1.7 mW of pump laser power.

Continuous-variable quantum key distribution over 28.6 km fiber with an integrated silicon photonic receiver chip

Quantum key distribution, which ensures information-theoretically secret key generation, is currently advancing through photonic integration to achieve high performance, cost reduction, and compact size, thereby facilitating the large-scale deployment. Continuous-variable quantum key distribution is an attractive approach for photonic integrations due to its compatibility with off-the-shelf optical communication devices. However, its chip-based systems have encountered significant limitations primarily related to the shot-noise-limited receiver design, which demands low noise, wide bandwidth, high clearance, and well stability. Here, we report the implementation of a real local oscillator continuous-variable quantum key distribution system with an integrated silicon photonic receiver chip. Thanks to the well-designed chip-based homodyne detectors with a bandwidth up to 1.5 GHz and a clearance up to 7.42 dB, the transmission distance of the system has been extended to 28.6 km, achieving a secret key generation rate of Mbps level. This technological advancement enables the quantum key distribution systems with photonic integrated receivers to achieve the coverage in both access network scenarios and short-distance metropolitan interconnections, paving the way for the development of the next-generation quantum key distribution networks on a large scale.

Application of Big Data Analysis and Intelligent Algorithm in Power System Operation Optimization

The power communication system provides powerful technical support for realizing the intelligent operation and information management of the power grid and improving the operation efficiency and power supply quality of the power grid. Quantum key distribution (QKD) is considered one of the most promising technologies for commercialization. QKD uses a single photon to encrypt data to produce a more secure and reliable password. This paper intends to study the hierarchical, centralized control architecture of power dispatching based on quantum essential supply (QKD). The performance indexes of MDI-QKD under symmetric and asymmetric conditions were studied by local optimization. The optimal key formation rate of the algorithm is analyzed. From the perspective of quantum critical utilization, a quantum key utilization scheme for grid backbone dispatching service is proposed. The dynamic adjustment test of multi-node time slot and service key update rate is carried out. Experiments show that the MDI scheme can effectively improve the effectiveness of a multi-node QKD system. Thus, the security of data transmission of the core business of power dispatching data networks can be ensured to the greatest extent. AMDI can effectively reduce the transmission timeout of low-priority data streams because the delay of high-priority data streams reaches the proportion. It can be an excellent solution to the power system and the password requirements.

Highly stable power control for chip-based continuous-variable quantum key distribution system.

Quantum key distribution allows secret key generation with information theoretical security. It can be realized with photonic integrated circuits to benefit the tiny footprints and the large-scale manufacturing capacity. Continuous-variable quantum key distribution is suitable for chip-based integration due to its compatibility with mature optical communication devices. However, the quantum signal power control compatible with the mature photonic integration process faces difficulties on stability, which limits the system performance and causes the overestimation of a secret key rate that opens practical security loopholes. Here, a highly stable chip-based quantum signal power control scheme based on a biased Mach-Zehnder interferometer structure is proposed, theoretically analyzed, and experimentally implemented with standard silicon photonic techniques. Simulations and experimental results show that the proposed scheme significantly improves the system stability, where the standard deviation of the secret key rate is suppressed by an order of magnitude compared with the system using traditional designs, showing a promising and practicable way to realize a highly stable continuous-variable quantum key distribution system on chip.

Countermeasure against blinding attack for single-photon detectors in quantum key distribution

Quantum key distribution (QKD), rooted in quantum mechanics, offers information-theoretic security. However, practical systems open security threats due to imperfections, notably bright-light blinding attacks targeting single-photon detectors. Here, we propose a concise, robust defense strategy for protecting single-photon detectors in QKD systems against blinding attacks. Our strategy uses a dual approach: detecting the bias current of the avalanche photodiode (APD) to defend against continuous-wave blinding attacks, and monitoring the avalanche amplitude to protect against pulsed blinding attacks. By integrating these two branches, the proposed solution effectively identifies and mitigates a wide range of bright light injection attempts, significantly enhancing the resilience of QKD systems against various bright-light blinding attacks. This method fortifies the safeguards of quantum communications and offers a crucial contribution to the field of quantum information security.

Simplified decoy-state quantum key distribution based on planar lightwave circuit integration module

The BB84 protocol with decoy states for quantum key distribution (QKD) has generated significant interest due to its theoretical unconditional security. Presently, the most of QKD systems demonstrated rely on bulk optics. Nevertheless, in upcoming large-scale commercial availability, it is imperative to have highly integrated, consistently performing, and cost-effective systems. Photonic integrated circuits(PICs) based on silica planar lightwave circuits (PLCs) play a crucial role in QKD due to its mature manufacturing process, excellent stability, low loss, and high integration level. We develop a simplified BB84 time-bin QKD experiment system that can significantly improve the key rate because it does not require time basis decoding. And two silica PLC asymmetric Mach-Zehnder interferometer (AMZI) module are used with a high-precision temperature control system (± 0.01 °C). Three quantum states |ψ0〉 , |ψ1〉 and |ψ+〉 are encoded and decoded with the module. The system achieves an estimated key rate about 3.4 Mbps using an analytical model with one decoy state at a clock repetition rate of 1.25 GHz. Finally, we experimentally obtain a key rate of 531 kbps over a 20 km single-mode fiber (SMF) at a clock repetition rate of 156.25 MHz.

Novel intelligent blind information reconciliation for LDPC codes in quantum key distribution systems

Information reconciliation is an important step in quantum key distribution (QKD). As a good channel coding technology, LDPC (low-density parity-check) code perform well in information reconciliation. In this paper, we investigate a new approach, namely blind information reconciliation. In our approach, after the state distribution is carried out by Alice and the measurements are taken by Bob, Alice can select an appropriate LDPC encoder (parity-check matrix) from the predefined candidate pool according to the estimated QBER (quantum bit-error-rate). Bob can use the expectation–maximization (EM) estimator and the maximum average log-likelihood ratio (LLR) detector to blindly identify parity-check matrices associated with not only different code-rates but also different codeword-lengths Alice employed. Moreover, puncturing and shortening can also be invoked to adjust the code-rate slightly to improve the error-correction performance. Monte Carlo simulations have demonstrated that our proposed new blind information-reconciliation scheme can successfully identify the parity-check matrix even under high QBER conditions, reduce the interactive communication-rounds, and improve the reconciliation efficiency in comparison with the conventional rate-adaptive LDPC coding scheme without QBER estimation.

Real-time polarization compensation method in quantum communication based on channel Muller parameters detection

Polarization drift in fiber and free-space optical links is a major factor in the dynamic increase of bit error rate in polarization-coded quantum key distribution (QKD) systems. A dynamic polarization compensation method applicable to both links is a challenge. Here we propose a universally applicable real-time polarization compensation method, that the Muller parameters of the optical links are first detected using a polarization detector, and then the optimal parameters of the controller are obtained by gradient descent algorithm. Simulation results indicate advantages over current methods, with fewer waveplates, faster speed, and wider applicability for various optical links. In equivalent experiments of both satellite and fiber optical links, the average polarization extinction ratio of 27.9 dB and 32.2 dB are respectively achieved. The successful implementation of our method will contribute to the real-time polarization design of fiber and free-space QKD systems, while also contributing to the design of laser-based polarization systems.

Eurasian-scale experimental satellite-based quantum key distribution with detector efficiency mismatch analysis.

The Micius satellite is the pioneering initiative to demonstrate quantum teleportation, entanglement distribution, quantum key distribution (QKD), and quantum-secured communications experiments at the global scale. In this work, we report on the results of the 600-mm-aperture ground station design which has enabled the establishment of a quantum-secured link between the Zvenigorod and Nanshan ground stations using the Micius satellite. As a result of a quantum communications session, an overall sifted key of 2.5 Mbits and a total final key length of 310 kbits have been obtained. We present an extension of the security analysis of the realization of satellite-based QKD decoy-state protocol by taking into account the effect of the detection-efficiency mismatch for four detectors. We also simulate the QKD protocol for the satellite passage and by that validate our semi-empirical model for a realistic receiver, which is in good agreement with the experimental data. Our results pave the way to the considerations of realistic imperfection of the QKD systems, which are important in the context of their practical security.

Numerical study of single-layer and interlayer grating polarizers based on metasurface structures for quantum key distribution systems

Polarization is an important property of light, which refers to the direction of electric field oscillations. Polarization modulation plays an essential role for polarization encoding quantum key distribution (QKD). Polarization is used to encode photons in the QKD systems. In this work, visible-range polarizers with optimal dimensions based on resonance grating waveguides have been numerically designed and investigated using the COMSOL Multiphysics Software. Two structures have been designed, namely a single-layer metasurface grating (SLMG) polarizer and an interlayer metasurface grating (ILMG) polarizer. Both structures have demonstrated high extinction ratios, ~1.8·103 and 8.68·104, and the bandwidths equal to 45 and 55 nm for the SLMG and ILMG, respectively.

High-rate multiplexed entanglement source based on time-bin qubits for advanced quantum networks

Entanglement distribution based on time-bin qubits is an attractive option for emerging quantum networks. We demonstrate a 4.09-GHz repetition rate source of photon pairs entangled across early and late time bins separated by 80 ps. Simultaneous high rates and high visibilities are achieved through frequency multiplexing the spontaneous parametric down conversion output into eight time-bin entangled channel pairs. We demonstrate entanglement visibilities as high as 99.4%, total entanglement rates up to 3.55×106 coincidences/s, and predict a straightforward path towards achieving up to an order of magnitude improvement in rates without compromising visibility. Finally, we resolve the density matrices of the entangled states for each multiplexed channel and express distillable entanglement rates in ebit/s, thereby quantifying the trade-off between visibility and coincidence rates that contributes to useful entanglement distribution. This source is a fundamental building block for high-rate entanglement-based quantum key distribution systems or advanced quantum networks.

Continuous-variable quantum key distribution system: Past, present, and future

Quantum key distribution provides secure keys with information-theoretic security ensured by the principle of quantum mechanics. The continuous-variable version of quantum key distribution using coherent states offers the advantages of its compatibility with telecom industry, e.g., using commercial laser and homodyne detector, is now going through a booming period. In this review article, we describe the principle of continuous-variable quantum key distribution system; focus on protocols based on coherent states, whose systems are gradually moving from proof-of-principle lab demonstrations to in-field implementations and technological prototypes. We start by reviewing the theoretical protocols and the current security status of these protocols. Then, we discuss the system structure, the key module, and the mainstream system implementations. The advanced progresses for future applications are discussed, including the digital techniques, system on chip, and point-to-multipoint system. Finally, we discuss the practical security of the system and conclude with promising perspectives in this research field.

The impact of spot‐size on single‐photon avalanche diode timing‐jitter and quantum key distribution

AbstractIn free‐space implementations of Quantum key distribution (QKD), the wide adoption of near‐Infrared wavelengths has led to the common use of silicon single‐photon avalanche diodes (Si‐SPAD) for receiver systems. While the impacts of some SPAD properties on QKD have been explored extensively, the relationship of spot‐size and spatial position on the full instrumental response and thus quantum bit error rate (QBER) has been studied little. Changes in spot size and spatial position can result from atmospheric turbulence and pointing and tracking errors. Here, An empirical analysis of that relationship is presented utilising a large active area, 500 μm, free‐space coupled Si‐SPAD designed for free‐space QKD. A baseline full‐width at half‐maximum timing jitter of 182 ps and a QBER contribution of 0.1 % for a 1 GHz clock frequency QKD system and 100 ps time‐gating window are reported. The impacts of spot‐size and spatial position can increase the QBER to over 0.3%. The link between the spot‐size and timing jitter will allow the understanding of tolerancing for the alignment of Si‐SPADs within free‐space QKD receiver systems—an important factor in designing properly engineered practical systems and the equipment needed to compensate for atmospheric turbulence and pointing and tracking.

Practical high-dimensional quantum key distribution protocol over deployed multicore fiber

Quantum key distribution (QKD) is a secure communication scheme for sharing symmetric cryptographic keys based on the laws of quantum physics, and is considered a key player in the realm of cyber-security. A critical challenge for QKD systems comes from the fact that the ever-increasing rates at which digital data are transmitted require more and more performing sources of quantum keys, primarily in terms of secret key generation rate. High-dimensional QKD based on path encoding has been proposed as a candidate approach to address this challenge. However, while proof-of-principle demonstrations based on lab experiments have been reported in the literature, demonstrations in realistic environments are still missing. Here we report the generation of secret keys in a 4-dimensional hybrid time-path-encoded QKD system over a 52-km deployed multicore fiber link forming by looping back two cores of a 26-km 4-core optical fiber. Our results indicate that robust high-dimensional QKD can be implemented in a realistic environment by combining standard telecom equipment with emerging multicore fiber technology.

Low-complexity adaptive reconciliation protocol for continuous-variable quantum key distribution

In continuous-variable quantum key distribution systems, reconciliation is a crucial step that significantly affects the secret key rate (SKR). The rateless protocol based on Raptor codes can achieve high reconciliation efficiency at low signal-to-noise ratios (SNRs). However, the high complexity of low-density parity-check (LDPC) codes used for the precoding in Raptor codes limits the speed of reconciliation. In this paper, we propose an adaptive reconciliation protocol by modifying Raptor codes. The length of random binary sequences is increased because we remove the LDPC precoding that adds redundancy. The modified Raptor codes reduce the complexity of encoding with better performance. The proposed protocol gives a reconciliation efficiency higher than 98.1% in the SNR below −20 dB and maintains a certain SKR in long-distance transmission.

A fast and efficient algorithm for time synchronization in satellite quantum key distribution

Time synchronization is one of the most crucial issues that must be addressed in developing quantum key distribution (QKD) systems. It not only lets the transmitter and the receiver to assign a sequence number to each event and then do correct basis reconciliation, but also allows to increase signal-to-noise ratio. Time synchronization in satellite communications is especially complicated due to such factors as high loss, signal fading and Doppler effect. In this work, a simple, efficient and robust algorithm for time synchronization is proposed. It was tested during experiments on QKD between Micius, the world’s first quantum communications satellite, and an optical ground station located in Russia. The obtained synchronization precision lies in the range from 467 to 497 ps. The authors compare their algorithm for time synchronization with the previously used methods. The proposed approach can also be applied to terrestrial QKD systems.