High-speed and high-performance polarization-based quantum key distribution system without side channel effects caused by multiple lasers
Temporal disparity and intensity fluctuation of photon pulses in a polarization-based QKD system with and without side channel effects.
Quantum key distribution (QKD), whose security is guaranteed by the laws of quantum physics, provides unconditionally secure key distribution between two distant parties. The fundamental principles of the security are based on the properties of the quantum states, such as the no-cloning theorem and superposition. However, this theoretically proven unconditional QKD security is sometimes threatened because of hidden implementation loopholes in practical systems.
The most common configuration of the free-space QKD system implemented with multiple semiconductor lasers can also be insecure because of the potential side channel effects of time and intensity fluctuations in the output photon pulses, especially when it is operated in high-speed conditions. To eliminate such side channel effects, laser sources must be biased at a high direct current (DC) close to the lasing threshold level. However, this approach unavoidably degrades the performance of the QKD system because of spontaneously emitted noise photons from laser diodes. The aforementioned side channel effects and the performance of the QKD system with and without the side channel effects are reported in Photonics Research, Volume 6, No. 3, 2018 (H. Ko et al., High-speed and high-performance polarization-based quantum key distribution system without side channel effects caused by multiple lasers).
In the study, the researchers from QKD research group at Electronics and Telecommunications Research Institute in South Korea propose the temporal filtering technique as a key approach to alleviate the unavoidable performance degradation caused by the noise photons, which are spontaneously emitted by laser diodes biased at a high DC. This approach eliminates photon detections in unwanted temporal regions. Using this elaborate temporal filtering technique, the QKD performances of the quantum bit error rate and the secure key rate are significantly improved, even at a high DC bias current.
Dr. Youn, the QKD research group leader, believes that this work will become a key reference to achieve both security and high performance in QKD systems implemented with multiple lasers, especially for high-speed operation. Further studies can focus on the effects of multiple lasers on the performance of the QKD system in diverse situations, such as decoy systems, daylight operation, ultra-high-speed systems, and the potential loopholes caused by a high DC bias current.
在无多激光器引起的边信道效应情况下的高速高性能偏振量子密钥分发系统
偏振量子密钥分发系统中光子脉冲的时间差异和强度波动等边信道效应消除前后对比
量子密钥分发可以为分隔两地的双方分配绝对安全的密钥，而该技术的安全性是由量子力学原理保障的。安全性的基本原理基于量子态的性质，如不可克隆定理和叠加性。然而，量子密钥分发虽然在理论上被证明具有绝对的安全性，但实际上仍有可能受到潜在实施漏洞的威胁。
由于输出光子脉冲的时域和强度波动等潜在边信道效应，由多个半导体激光器实现的自由空间量子密钥分发系统这一最常见配置也可能是不安全的，特别是当其高速运行时。为消除这些边信道效应，激光源必须用接近激光阈值水平的大直流电流来偏置。然而，由于激光二极管会自发辐射噪声光子，这种方法不可避免地会降低量子密钥分发系统的性能。关于上述边信道效应及在有无该效应两种情况下量子密钥分发系统的性能，可见Photonics Research 2018年第6卷第3期文章（H. Ko et al., High-speed and high-performance polarization-based quantum key distribution system without side channel effects caused by multiple lasers）。
在该项研究中，韩国电子通信研究院量子密钥分发研究小组提出采用时域滤波技术作为缓解噪声光子带来的不可避免的性能下降问题的关键方法(噪声光子由大直流电流偏置的激光二极管自发辐射产生)。该方法可以避免在非目标时间窗口中进行光子检测。通过这种精细的时域滤波技术，即使在大直流偏置电流下，量子密钥分发系统的性能——如量子比特差错率和安全密钥率，也能得到显著改善。
量子密钥分发研究小组负责人Youn博士认为，该研究将成为多激光器量子密钥分发系统（特别是高速运行时）同时实现安全性和高性能的关键技术。下一步他们将重点研究多种情况下多激光器对量子密钥分发系统性能的影响，例如：诱骗系统、日间运行、超高速系统以及由大直流偏置电流引起的潜在漏洞等。