Abstract Quantum cryptography communication encompasses both quantum key distribution (QKD) and the encryption and decryption of plaintext. Continuous-variable QKD (CVQKD) leverages the fundamental principles of physics to enable legitimate parties to share secure keys, while the CVQKD-compatible quantum noise stream cipher (QNSC) exploits quantum fluctuations to further obscure the ciphertext encrypted with these keys. Integrating these two functionalities into a unified system represents a promising developmental trend. However, existing integrated architectures typically rely on multiplexing two separate optical paths and have been demonstrated exclusively over optical fibers. Here, a unified local local oscillator (LLO) quantum cryptography architecture operating over a free-space channel is proposed and experimentally demonstrated. Requiring only simple software-defined switching, our payload-efficient architecture executes both CVQKD and QNSC functionalities utilizing a single optical setup. Over an indoor free-space channel with emulated time-varying atmospheric disturbances, an asymptotic secret key rate of 35.4292 kbps is obtained at a maximum channel attenuation of 19.5054 dB for the CVQKD mode. For the QNSC mode, an encrypted image is successfully transmitted, yielding pixel accuracies of 99.58% and 96.39% under channel attenuations of 10.1690 dB and 12.0620 dB, respectively. This work validates the feasibility of integrated quantum cryptography architectures in complex environments, marking a meaningful step toward constructing payload-constrained quantum communication networks.
Abstract Quantum cryptography communication encompasses both quantum key distribution (QKD) and the encryption and decryption of plaintext. Continuous-variable QKD (CVQKD) leverages the fundamental principles of physics to enable legitimate parties to share secure keys, while the CVQKD-compatible quantum noise stream cipher (QNSC) exploits quantum fluctuations to further obscure the ciphertext encrypted with these keys. Integrating these two functionalities into a unified system represents a promising developmental trend. However, existing integrated architectures typically rely on multiplexing two separate optical paths and have been demonstrated exclusively over optical fibers. Here, a unified local local oscillator (LLO) quantum cryptography architecture operating over a free-space channel is proposed and experimentally demonstrated. Requiring only simple software-defined switching, our payload-efficient architecture executes both CVQKD and QNSC functionalities utilizing a single optical setup. Over an indoor free-space channel with emulated time-varying atmospheric disturbances, an asymptotic secret key rate of 35.4292 kbps is obtained at a maximum channel attenuation of 19.5054 dB for the CVQKD mode. For the QNSC mode, an encrypted image is successfully transmitted, yielding pixel accuracies of 99.58% and 96.39% under channel attenuations of 10.1690 dB and 12.0620 dB, respectively. This work validates the feasibility of integrated quantum cryptography architectures in complex environments, marking a meaningful step toward constructing payload-constrained quantum communication networks.
The advent of quantum computers has significantly challenged the security of traditional cryptographic systems, prompting a surge in research on quantum key distribution (QKD). Among various QKD approaches, continuous-variable QKD (CVQKD) offers superior resilience against background noise. However, the local local oscillator (LLO) CVQKD scheme faces substantial physical limitations in scenarios with high channel attenuation, and the large attenuation CVQKD remains unrealized. Bottleneck challenges include ensuring stable low-noise transmission and accurately estimating parameters under fluctuating channel conditions. We propose a continuous-mode theoretical framework that provides an analytical method for time-varying quantum systems within a free-space channel. This framework can achieve high-fidelity LLO-CVQKD in free space without the need for complex, real-time frequency locking and transmittance calibration equipment. It can also enable free-space QKD under large attenuation and high repetition frequency. Through experimental validation, we first demonstrate high-rate secure quantum key distribution over high-loss free-space channels. Specifically, we achieve asymptotic key rates of 76.366 kbps and 403.896 kbps in 25 dB attenuation free-space channels without turbulence and 21.5 dB average attenuation free-space channels with turbulence, respectively. Additionally, we confirm the feasibility of experiments on mildly turbulent atmospheric channels spanning at least 10.5 km using current equipment. Our scheme provides direct insight into constructing an integrated air-ground quantum communication network.
Quantum secure communication technology, grounded in the principles of quantum mechanics, has revolutionized traditional cryptographic methods, which predominantly rely on mathematical algorithms and logical transformations. Quantum key distribution (QKD) facilitates the secure sharing of symmetric keys between legitimate parties, ensuring the integrity and confidentiality of information in communication channels. Among various QKD schemes, continuous variable quantum key distribution (CV-QKD) has emerged as a viable alternative to the more established discrete variable quantum key distribution (DV-QKD). CV-QKD boasts several advantages, including straightforward deployment, reduced operational costs, and stable signal quality, positioning it as a significant component of the quantum communication landscape. Quantum noise stream cipher (QNSC) represents a technique that employs redundant data streams to encrypt plaintext. Notably, QNSC shares structural similarities with the optical path used in quantum key distribution, allowing for compatibility with existing QKD infrastructure. Furthermore, the keys generated through quantum key distribution ensure absolute security, enabling them to substitute the data streams produced by pseudo-random sequences traditionally used in classical encryption schemes. This substitution enhances the overall security framework of the quantum noise stream encryption approach. The quantum access network (QAN) extends quantum key distribution capabilities from point-to-point connections to point-to-multipoint configurations, thereby becoming an essential component of contemporary network architectures. By interlinking multiple end users with a network backbone, quantum access networks can seamlessly integrate with quantum key distribution protocols. This integration offers a cost-effective and scalable solution for user access to the network. Extensive experimental verifications have demonstrated the viability of downstream access networks, indicating that the merging of quantum access network structures with quantum stream encryption technologies can yield a comprehensive quantum experimental platform. On this platform, security keys are derived through quantum key distribution, which subsequently facilitates the encryption of plaintext information. Notably, experimental validations involving eight users have been successfully executed on this platform, underscoring its practical applicability and effectiveness.
Continuous-variable quantum key distribution (CVQKD) enables remote users to share high-rate and unconditionally secure secret keys while maintaining compatibility with classical optical communication networks and effective resistance against background noise. However, CVQKD experiments have been demonstrated only indoors or over short outdoor distances. Here, by developing channel-fluctuation-independent high-precision manipulation of continuous-variable quantum states, high-accuracy quantum signal acquisition and processing, and high-efficiency free-space acquisition, tracking, and pointing technology, we overcome the excess noise due to atmospheric effects especially in daylight without extra wavelength conversion and narrow-linewidth spectral filtering and demonstrate for the first time long-distance free-space quantum key distribution under the asymptotic condition over 7-km inland and 9.6-km maritime atmospheric channels with Gaussian-modulated coherent states. Given that the CVQKD system is naturally compatible with existing ground fiber telecommunication networks, it marks an essential step for realizing integrated air-ground quantum access networks with cross-domain applications.
Quantum Noise Stream Cipher (QNSC) has emerged as a physical-layer encryption technique that exploits quantum noise and non-orthogonal coherent-state modulation to secure optical communication. However, the security of QNSC relies exceedingly on the secrecy and freshness of its seed key. Quantum Key Distribution (QKD), on the other hand, provides information-theoretically secure key exchange rooted in the laws of quantum mechanics. The convergence of these two paradigms, i.e., integrated QKD-QNSC architectures, offers a compelling solution to each of their limitations. In such integrated systems, QKD continuously supplies and refreshes the secret seed key that governs QNSC modulation. Thus, governing a unified security framework that couples provably secure key establishment with high-speed quantum-enhanced physical-layer encryption. This work presents a comprehensive review of QNSC systems, examining their operating principles, security models under various attacks, and their integration with QKD systems. We analyze the security interplay between the key generation and encryption layers and survey experimental demonstrations and architectural progress toward practical deployment. Furthermore, we identify the open challenges and future research directions that must be addressed to realize fully integrated, quantum-secured optical communication networks at a practical scale.
Free-space quantum cryptography holds significant potential for enabling global quantum communication networks. This paper proposes a free-space continuous-variable quantum secret sharing (CV-QSS) protocol employing a local local oscillator (LLO). The proposed protocol simplifies the system architecture by requiring only two heterodyne detections at the dealer’s side, thereby eliminating the additional noise and complexity associated with coherent measurements at each participant’s side in conventional schemes. Theoretically, we establish a channel transmittance model that comprehensively incorporates the effects of rain extinction and atmospheric turbulence, and derive the bound of the secret key rate. Experimentally, we systematically evaluate the impact of weather conditions and atmospheric turbulence on protocol performance and conduct comparative evaluations against existing schemes. The results demonstrate that the proposed protocol outperforms the previously proposed free-space LLO-based CV-QSS protocol in terms of key rate, transmission distance, noise tolerance, and participant capacity. Furthermore, this protocol supports secret sharing among 5 participants over 75 km or up to 37 participants within 10 km, outperforming the LLO-based CV-QSS system in a fiber channel. This study validates the potential of free-space CV-QSS as a viable solution for medium-to-long-distance and multi-user quantum secret sharing.
Future quantum communication infrastructures will need to serve heterogeneous users on shared physical channels: short-range, high-throughput links favor Continuous-Variable Quantum Key Distribution (CV-QKD), while long-reach, high-loss links remain the domain of Discrete-Variable QKD (DV-QKD). Wavelength-division multiplexing (WDM) of the two protocols on a common channel would address both regimes simultaneously, but their markedly different noise sensitivities make coexistence non-trivial and, to date, experimentally untested. Here we report the first simultaneous operation of two independent CV- and DV-QKD systems on a common optical channel, using standard C-band DWDM filters at 1550.12 nm (CV) and 1545.32 nm (DV). We demonstrate joint operation on both optical fiber and a 620 m urban daylight free-space link. On fiber, the two systems exhibit the expected complementarity, crossing over at 7.56 dB of channel loss where both deliver $\sim$1.43 Mbit/s; in daylight free-space, both sustain Mbit/s key rates under time-varying atmospheric attenuation. Across all configurations we observe no measurable multiplexing-induced penalty in QBER or excess noise. These results establish hybrid CV-DV WDM as a practical building block for heterogeneous quantum communication networks, where metropolitan high-throughput users and long-reach backbone links can be served on a single physical infrastructure.