Side channel effects such as temporal disparity and intensity fluctuation of photon pulses caused by random bit generation with multiple laser diodes in high-speed polarization-based BB84 quantum key distribution (QKD) systems can be eliminated by increasing DC bias current condition. However, background photons caused by the spontaneous emission process under high DC bias current degrade the performance of the QKD systems. In this study, we investigated, for the first time, the effects of spontaneously emitted photons on the system performance in a high-speed QKD system at a clock rate of 400 MHz. Also, we further show improvements of system performance without side channel effects by utilizing temporal filtering technique with real-time FPGA signal processing.
Side channel effects such as temporal disparity and intensity fluctuation of photon pulses caused by random bit generation with multiple laser diodes in high-speed polarization-based BB84 quantum key distribution (QKD) systems can be eliminated by increasing DC bias current condition. However, background photons caused by the spontaneous emission process under high DC bias current degrade the performance of the QKD systems. In this study, we investigated, for the first time, the effects of spontaneously emitted photons on the system performance in a high-speed QKD system at a clock rate of 400 MHz. Also, we further show improvements of system performance without side channel effects by utilizing temporal filtering technique with real-time FPGA signal processing.
Most polarization-based BB84 quantum key distribution (QKD) systems utilize multiple lasers to generate one of four polarization quantum states randomly. However, random bit generation with multiple lasers can potentially open critical side channels that significantly endangers the security of QKD systems. In this paper, we show unnoticed side channels of temporal disparity and intensity fluctuation, which possibly exist in the operation of multiple semiconductor laser diodes. Experimental results show that the side channels can enormously degrade security performance of QKD systems. An important system issue for the improvement of quantum bit error rate (QBER) related with laser driving condition is further addressed with experimental results.
High-speed is one development tendency of the practical and commercial quantum key distribution (QKD) systems, and the gain-switched semiconductor laser (GSSL) and Sagnac interferometer have been popular components for high-speed commercial polarization-based BB84 QKD systems thanks to their stability, compactness and low cost. However, due to the finite extinction ratio (ER) of the GSSL, the time-dependent source side-channel, which is theoretically presented in [Phys. Rev. A106(6): 062618 (2022)10.1103/PhysRevA.106.062618], also exists in these QKD systems. In this article, we experimentally investigate this side-channel in the polarization-based BB84 QKD system. The results show an obvious correlation in the output polarization states between the weak leakage light and its adjacent pulses, which will cause information to leak. More importantly, we have proposed several countermeasures, retested the side channel, and developed a mathematical framework to quantitatively assess the impact of the time-dependent side channel and these countermeasures on the secure key rate. Finally, we offer a trade-off analysis that considers defense effectiveness, pulse impact, and cost for various countermeasures in QKD. This provides a practical recommendation for improving the security of QKD systems.
One of the challenges of implementing free-space quantum key distribution (QKD) systems working in daylight is to remove unwanted background noise photons from sunlight. Elaborate elimination of background photons in the spectral, temporal, and spatial domains is an indispensable requirement to decrease the quantum bit error rate (QBER), which guarantees the security of the systems. However, quantitative effects of different filtering techniques and performance optimization in terms of the secure key rate have not been investigated. In this study, we quantitatively analyze how the performance of the QBER and the key rates changes for different combinations of filtering techniques in a free-space BB84 QKD system in daylight. Moreover, we optimize the conditions of filtering techniques in order to obtain the maximum secure key rate.
Improvement of QKD performance, particularly on key generation rate, has been required to meet current network traffic. A high-speed QKD system should be equipped with low loss receivers with high visibility, highly efficient photon detectors with small dark count probability, and a stable clock synchronization system with low stray light to the quantum signals, because the key generation rate is determined by photon detection rate and error rate. High speed post processing is also required to improve key generation through-put. A solution for these issues will be discussed.
In 1946, C. E. Shannon proved that One-Time Pad is truly unbreakable [1].However, stringent conditions pose difficulties such as the key distribution problem which limited its practicability.Fortunately, public key cryptosystem widely used today was developed to solve the key distribution problem.Its security is based on the assumption that an adversary has limited computational power to factor a number with large prime factors.With the increase in computational power and technological advancement, this security may one day be compromised.In contrast, quantum key distribution (QKD) offers a platform for secure key distribution and everlasting secrecy.Unlike conventional cryptography, quantum key distribution's security is governed by the laws of quantum mechanics.The basic principle of quantum key distribution is to encode classical binary bit information onto the properties of quantum states such as the polarisation of a photon.Because of the quantum no-cloning theorem, an eavesdropper is unable to simply duplicate these photons.Moreover, by intercepting these photons, the eavesdropper will leave detectable trace which will reveal its presence.In 1989, the first experimental demonstration of QKD using the polarisations of single photons based on the BB84 protocol occurred through 32cm of air [2].Since then, QKD over hundreds of kilometres of optical fibre Abstract iv and free space have been reported [3][4][5].Key generation rate of a few megahertz have also been demonstrated [6][7][8].However, photons travelling through optical fibres are subjected to random polarisation drifts.Moreover, clock drifts often causes inaccuracy during the detection of these photons.If these performance issues were not addressed, the reliability and availability of QKD systems and their cryptographic keys will be affected.Therefore, polarisation recovery schemes have been implemented to mitigated polarisation drifts [9][10][11][12][13][14].However, these schemes often limit the transmission distance, slow down or even disrupt the key generation process for polarisation recovery.These limitations lead to the proposed development of a polarisation-encoded QKD system based on an adaptive polarisation state monitoring and recovery scheme that adapts the system to the existing polarisation drift condition in the transmission link to enhance its reliability and availability.On the other hand, current high-speed single-photon detection schemes are often designed to work with idealised parameters such as fixed gating rate and operating temperature.Therefore, such schemes are unable to accommodate changes in gating frequency induced by clock drifts which results in the reduction of detection efficiency.Hence, a proposed robust high-speed singlephoton avalanche diode with tunable sinusoidal gate frequency was developed to mitigate the effect caused by clock drift in order to maintain the detection efficiency over varying operating conditions.
Abstract Quantum light sources emitting triggered single photons or entangled photon pairs have the potential to boost the performance of quantum key distribution (QKD) systems. Proof-of-principle experiments affirmed these prospects, but further efforts are necessary to push this field beyond its current status. In this work, we show that temporal filtering of single-photon pulses enables a performance optimization of QKD systems implemented with realistic quantum light sources, both in experiment and simulations. To this end, we analyze the influence of temporal filtering of sub-Poissonian single-photon pulses on the expected secret key fraction, the quantum bit error ratio, and the tolerable channel losses. For this purpose, we developed a basic QKD testbed comprising a triggered solid-state single-photon source and a receiver module designed for four-state polarization coding via the BB84 protocol. Furthermore, we demonstrate real-time security monitoring by analyzing the photon statistics, in terms of g(2)(0), inside the quantum channel by correlating the photon flux recorded at the four ports of our receiver. Our findings are useful for the certification of QKD and can be applied and further extended for the optimization of various implementations of quantum communication based on sub-Poissonian quantum light sources, including measurement-device-independent schemes of QKD as well as quantum repeaters. Our work represents an important contribution towards the development of QKD-secured communication networks based on quantum light sources.