Peng Huang

Continuous-variable quantum key distribution (CVQKD) can allow remote users to share high-rate and unconditionally secure secret keys with capabilities of well compatibility with classical optical communication networks and effective resistance against background noise. We overcome the excess noise due to atmospheric effects especially in daylight without extra wavelength conversion and spectral filtering, and demonstrate for the first time all-day free-space quantum key distribution over 7 km in an urban atmosphere and 9.6 km in a marine atmosphere with Gaussian-modulated continuous variables. This achieved distribution distance of secure quantum secret keys is well beyond the effective thickness of the aerosphere, hence presenting a possible alternative way for realizing satellite-based quantum cryptography communication in daylight. Moreover, given that the CVQKD system is naturally compatible with existing ground fibre telecommunication networks, it marks an essential step for realizing integrated air-ground quantum access networks with cross-domain applications.

We propose a discrete-modulated coherent-state basis-encoding quantum key distribution (DMCS-BE-QKD) protocol, where the secret keys are encoded in the random choice of two measurement bases and it only needs simple binary sequence error correction. We analyze the secret key rate of DMCS-BE-QKD protocol under collective attacks in the linear Gaussian channel. The results show that DMCS-BE-QKD can greatly enhance the ability to tolerate the channel loss and excess noise compared to the original DMCS-CVQKD protocol. Finally, a proof-of-principle experiment is conducted under a 50.5 km optical fiber to verify the feasibility of DMCS-BE-QKD.

Bypassing the use of quantum coherent source and active modulations, passive-state-preparation (PSP) continuous-variable quantum key distribution (CVQKD) with thermal source provides a solution of high-speed on-chip modulators. However, the field experiment of free-space PSP CVQKD has still not been realized due to the lack of efficient excess noise suppression techniques via high-loss free-space channels. Here, we realize the PSP CVQKD field test over an urban free-space channel with record-breaking attenuation from -12.24 dB to -15.59 dB. Specifically, a novel scheme is proposed to reduce excess noise from PSP, and efficient quantum coherence detection alongside advanced digital signal processing algorithms is developed to achieve low-noise synchronized raw data acquisition. The secure keys are successfully generated, with statistical summation values of 0.85 kbps during the day and 1.52 kbps at night.

Privacy amplification is a critical component in quantum key distribution (QKD) to eliminate eavesdropper information and distill unconditional secure keys. In this paper, a privacy amplification algorithm using two-dimensional cellular automata (2D CA) for quantum key distribution (QKD) is presented. The proposed algorithm decreases the computation complexity and increases the processing speed. Unlike conventional Toeplitz hash algorithms, the proposed algorithm utilizes the inherent parallelism of 2D CA to enable simultaneous multi-bit confusion through cyclic row shifts and XOR operations. And we prove that the 2D CA-based algorithm is a universal hash family and satisfies the principle of privacy amplification. The results of NIST randomness test and avalanche test show that the proposed algorithm has great randomness performance. Finally, we implement the proposed algorithm in field-programmable gate array (FPGA). The experimental results on a Xilinx Artix-7 FPGA demonstrate that our scheme achieves high throughput and significantly reduces hardware resource consumption.

The advent of quantum computers has significantly challenged the security of traditional cryptographic systems, prompting a surge in research on quantum key distribution (QKD). Continuous-variable QKD (CVQKD) resists noise well, but the local local oscillator (LLO) CVQKD has limits in high-attenuation channels. Bottleneck challenges include ensuring stable low-noise transmission and accurately estimating parameters under fluctuating channel conditions. We propose a LLO-CVQKD scheme that combines the main quantum system with an auxiliary quantum system, featuring time-varying parameter compensation and time-varying channel transmittance estimation capabilities. 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 403.896 kbps in 21.5 dB average attenuation free-space channels with turbulence at a 1 GHz repetition rate.

Quantum networks revolutionize the way of information transmission and are an essential step in building a quantum internet. Generally, the information capacity per user-channel in a quantum network drastically decreases with the increase of network capacity, making it difficultly scale to large-user scenarios. To break this limit, we propose a network capacity-independent quantum network (NCI-QN) that maintains constant information capacity per user-channel regardless of network scale, overcoming the scalability bottleneck in conventional quantum networks. The architecture employs a multi-mode time-frequency framework, with theoretical analysis extending PLOB and Holevo bounds to network scenarios to establish capacity independence. Experimentally, we demonstrate a 19-user NCI-QN using optical frequency combs in quantum key distribution, achieving a record 8.75 Gbps composable finite-size secure key rate.

The quantum internet has the potential to enable applications that are fundamentally unattainable with classical internet technologies. One of its most notable applications is the quantum key distribution (QKD) network, which enables two distant nodes to establish a secure cryptographic key based on the principles of quantum mechanics. However, the heavy reliance on interference in existing QKD protocols undermines the robustness of both the system and the corresponding network infrastructure. We propose an interference-free quantum network architecture based on a Kramers-Kronig receiver. Specifically, we introduce a continuous-variable QKD protocol employing direct detection without the need for interference, wherein the quadrature components are recovered via the Kramers-Kronig relation. Building upon this foundation, we extend the protocol to continuous-variable quantum access networks, thereby demonstrating the enhanced robustness and cost-effectiveness afforded by interference-free detection. Experimental results indicate that each user within the access network can achieve a secret key rate of 200 kbit/s using only a single photodetector and without the inclusion of interference structures. This approach offers a promising direction for constructing interference-free quantum networks and represents a significant step toward the realization of a large-scale quantum internet.