Continuous variable quantum key distribution with on-chip light Sources
Fig. 1 (a). Schematic of the optical layer of the LLO-CVQKD system with on-chip III-V/Si3N4 external cavity laser. The integrated system contains two parties, Alice and Bob, as the transmitter and receiver. Alice's sid·e consists of an isolator (Iso), two attenuators (ATT), an amplitude modulator (AM), a beamsplitter (BS), an IQ-modulator (IQ MOD), and a DC supply. Bob's side consists of two polarization controllers (PC) and an integrated coherent receiver (ICR). The photon detectors (PDs) are used for optical power monitoring. The inset shows the schematic structure of external cavity lasers (ECLs). (b). Photograph of the packaged ECLs. The RSOA is butt-coupled to an extension chip. The laser is supplied with electric current via wires. The on-chip light is measured and used in the QKD system using an optical fiber. (c). Microscope photo of the ECL. The footprint of the external chip is 2.4×1.27 mm2.
Fig. 2 (a). Optical spectrum of two lasers before (blue) and after (orange) adjustment. (b). Histogram of the measured beat frequency. (When measuring the change of the beat frequency, we use a data volume of 5 Mbits per frame and a sampling frequency of 10 Gbps to measure the frequency offset of the beat frequency. After analyzing the 800 frames of data, we found that the beat frequency of more than 70% of the frames is below 40 MHz, which sufficiently satisfies the demand of the system.) c. Frequency noise spectrum of the signal laser.
The continuous variable quantum key distribution system (CV-QKD) based on Gaussian modulated coherent state protocol (GMCS protocol, also known as GG02 protocol) has many characteristics such as complete theoretical security proof, high secure key rate, compatibility with existing optical communication architecture, etc. It is one of the promising routes to build a high-performance quantum key distribution network in the future.
The emerging quantum photonics technology makes it possible to realize a highly stable, portable, scalable, miniaturized and low-cost on-chip continuous variable quantum key distribution system and quantum communication network terminal that can be flexibly deployed on a large scale. Although the CV-QKD system based on the off chip light source and the transmitted local oscillator (TLO) scheme has been preliminarily verified on the integrated chip platform, the integration of high-performance on-chip light sources suitable for the CV-QKD system has always been a challenging problem since the GG02 protocol was proposed.
The continuous variable quantum key distribution system based on the locally local oscillator (LLO) scheme can ensure that the quantum signal reaches the Quantum noise limit detection under the condition of long-distance transmission, and significantly eliminate the signal crosstalk through more robust local oscillator light, finally realizing a high-performance CV-QKD system that can support long-distance secure key distribution. It is an effective way to overcome the long-distance transmission limitations and practical security issues caused by local oscillator crosstalk signal light in TLO schemes. However, the integration of the LLO-CVQKD system imposes stricter requirements on the light source: (1) Precise wavelength tunability to meet the alignment of the central wavelengths of two independent lasers; (2) Sufficient optical output power to ensure Quantum noise limit detection; (3) Narrow linewidth and low noise to ensure non trusted noise suppression and secure key generation. Therefore, owning two efficient on-chip light sources with narrow line width, low noise, well tunability, and sufficient power remains a challenge, which is crucial for achieving high-performance CV-QKD systems and further promoting the deployment of integrated quantum communication networks.
To solve the above problems and achieve the integration of quantum light sources in the CV-QKD system, the research group led by Prof. Gui-hua Zeng from Shanghai Jiao Tong University cooperating with Prof. Lin-jie Zhou from Shanghai Jiao Tong University, proposed a compact and high-performance III-V/Si3N4 external cavity laser (ECL) with the characteristics of high output, narrow linewidth, wide wavelength tunability and high SMSR, which can overcome the above problems from a mechanism perspective. Finally, a complete LLO CV-QKD experimental system with a secure transmission distance exceeding 100 kilometers was implemented.
The relevant research results were published in Photonics Research, Volume. 11, Issue 4, 2023 (Lang Li, Tao Wang, Xinhang Li, Peng Huang, Yuyao Guo, Liangjun Lu, Linjie Zhou, Guihua Zeng. Continuous-variable quantum key distribution with on-chip light sources[J]. Photonics Research, 2023, 11(4): 504).
This work proposes and realizes two high-performance III-V silicon nitride external cavity chip tunable quantum light sources for CV-QKD, which have the characteristics of the highest output power (up to 47.3 mW), wide tuning range (up to 73 nm), narrow linewidth (as low as 1.6 kHz), and high side mode suppression ratio(up to 75 dB) among the existing quantum integrated light sources for QKD. It can simultaneously meet the strict requirements of CV-QKD chip system for quantum light source in three aspects: reaching the Quantum noise limit detection, central wavelength alignment of non homologous lasers, and untrusted excess noise suppression.
In addition, the team solved the practical security issues faced by the existing integrated CV-QKD system and the photon crosstalk of the local oscillator light in the TLO scheme by utilizing LLO solutions. Based on this laser, the team has achieved the first high-performance LLO CV-QKD system based on integrated on-chip light sources, with a secure key rate of 0.75 Mbps under 50 kilometers on standard commercial optical fibers and a secure transmission distance of over 100 kilometers on standard commercial optical fibers. This result marks a breakthrough in building a fully integrated CV-QKD system and paves the way for establishing a reliable and efficient ground quantum communication network.
连续变量密钥分发光源首次集成
图1（a）基于片上III-V/Si3N4外腔激光器（ECL）实现的LLO-CVQKD光学系统方案图；（b）激光器蝶形管壳封装照片；（c）激光芯片显微镜照片，芯片的面积为2.4×1.27 mm2
图2（a）1545 nm波长处的L-I曲线；（b）通过驱动两个微环和移相器进行连续频率调谐的叠加输出光谱；（c）SMSR大于75 dB的单模激光谱；（d）波长调谐过程中ECL激光光谱叠加图
基于高斯调制相干态协议（GMCS协议,也称GG02协议）的连续变量量子密钥分发系统(CV-QKD)具备理论安全性证明完备、高码率及与现有光通信架构兼容等诸多特点，是构建未来高性能量子密钥分发网络极具前景的路线之一。
新兴的量子光子学技术为实现大规模灵活部署的高稳定、可便携、可拓展、小型化和低成本的芯片化连续变量量子密钥分发系统及量子通信网络终端提供可能。尽管基于片外光源和随路本振光同传（TLO）方案的CV-QKD系统已经在集成芯片平台得到初步验证，但自GG02协议提出以来，能适用于CV-QKD系统的高性能片上光源的集成一直是挑战性问题。
基于本地本振（LLO）方案的连续变量量子密钥分发系统可以保证量子信号在长距离传输条件下达到量子噪声极限探测，并通过更鲁棒的本振光显著消除信号的串扰，最终实现能支持长距离安全密钥分发的高性能CV-QKD系统，是克服TLO方案中因本振光串扰信号光所导致的长距离传输受限和实际安全性问题的有效途径。但LLO-CVQKD系统的集成对光源提出所需满足的更加严苛的要求：(1)精确的波长可调性，以满足两个独立激光器中心波长的对准；(2)足够的光输出功率，确保量子噪声极限探测；(3)窄线宽和低噪声，以保证非可信过噪声抑制和安全密钥的生成。因此，拥有两个线宽窄、低噪声、可调性好、同时功率充足的高效片上光源仍然具有挑战性，这对于实现高性能CV-QKD系统从而进一步推动集成化量子通信网络的部署至关重要。
为解决以上问题并实现CV-QKD系统中的量子光源集成，上海交通大学曾贵华教授课题组和上海交通大学周林杰教授课题组合作，提出一种具有高输出、窄线宽、宽波长可调性和高SMSR的特点的紧凑高性能III-V∕Si3N4外腔激光器（ECL），能从机理上克服上述难题，最终实现了一个安全传输距离超过100公里的完整LLO CV-QKD实验系统。相关研究成果发表于Photonics Research 2023年第4期。
本工作提出并实现了两个用于CV-QKD的高性能的III-V氮化硅外腔片上可调谐量子光源，它在现有用于QKD的量子集成光源中具有最高的输出功率（高达47.3 mW）、宽调谐范围（高达73 nm）、窄线宽（低至1.6 kHz）、高边模抑制比（高达75 dB）的特性，能同时满足CV-QKD芯片系统对量子光源提出的达到量子噪声极限检测、非同源激光器的中心波长对准以及非可信过噪声抑制三大方面严苛要求（图1、2所示）。
此外，该团队通过利用本地本振方案，解决了现有集成CV-QKD系统面临的实际安全问题和随路本振方案中本振光的光子串扰。基于该款激光器，该团队实现了在标准商业光纤上50公里下，系统的安全密钥率达到0.75 Mbps，安全传输距离可以超过100公里的首个基于集成片上光源的高性能本地本振CV-QKD系统。该结果标志着在构建全集成 CV-QKD系统方面取得了突破，并为建立可靠高效的地面量子通信网络铺平了道路。