Free-space continuous-variable quantum key distribution (CV-QKD) is an important technology that enables all-day quantum key distribution. Precise frame synchronization is a prerequisite for establishing a correlation between legitimate users of CV-QKD. In free-space CV-QKD, channel transmittance fluctuation caused by atmospheric turbulence increases the difficulty of synchronization. Also, as the channel transmittance is monitored in many reported experiments, the transmittance data also needs to be synchronized. We propose a novel method to solve the above problems by inserting two kinds of synchronization frames, i.e., data synchronization frames and transmittance synchronization frames. The performance of the proposed method is analyzed and Monte Carlo simulation is conducted to test its performance. The results demonstrate the feasibility and efficiency of this method. The proposed method paves the way for the realization of free-space CV-QKD.
Free-space continuous-variable quantum key distribution (CV-QKD) is an important technology that enables all-day quantum key distribution. Precise frame synchronization is a prerequisite for establishing a correlation between legitimate users of CV-QKD. In free-space CV-QKD, channel transmittance fluctuation caused by atmospheric turbulence increases the difficulty of synchronization. Also, as the channel transmittance is monitored in many reported experiments, the transmittance data also needs to be synchronized. We propose a novel method to solve the above problems by inserting two kinds of synchronization frames, i.e., data synchronization frames and transmittance synchronization frames. The performance of the proposed method is analyzed and Monte Carlo simulation is conducted to test its performance. The results demonstrate the feasibility and efficiency of this method. The proposed method paves the way for the realization of free-space CV-QKD.
Data acquisition in a continuous-variable quantum key distribution (CV-QKD) system is a necessary step to obtain secure secret keys. And the known data acquisition methods are commonly based on the assumption that the channel transmittance is constant. However, the channel transmittance in free-space CV-QKD fluctuates during the transmission of quantum signals, and the original methods are not applicable in this scenario. In this paper, we propose a data acquisition scheme based on the dual analog-to-digital converter (ADC). In this scheme, two ADCs with the same sampling frequency as the pulse repetition rate of the system and a dynamic delay module (DDM), which are used to construct a high-precision data acquisition system, eliminate the effect of transmittance fluctuation by a simple division operation of the data from the two ADCs. Simulation and proof-of-principle experimental results show that the scheme is effective for free-space channels and can achieve high-precision data acquisition under the condition of fluctuation of channel transmittance and very low signal-to-noise ratio (SNR). Furthermore, we introduce the direct application scenarios of the proposed scheme for free-space CV-QKD system and verify their feasibilities. This method is of great significance to promote the experimental realization and practical application of free-space CV-QKD.
Continuous variable quantum key distribution (CV-QKD) has emerged as a promising candidate for quantum-secure communication due to its experimentally demonstrated high key rates in fiber-optic channels. However, the feasibility of discrete modulation CV-QKD in satellite-to-ground downlinks remains an open question due to practical challenges such as high transmission loss, limited communication windows, and atmospheric turbulence. In this work, a comprehensive framework is proposed to evaluate the feasibility of discrete modulation CV-QKD by integrating orbital dynamics and atmospheric channel models, and to comprehensively analyze the influence of the parameter space on free-space discrete modulation CV-QKD. To achieve this, a free-space CV-QKD simulation platform is employed, which calculates the elevation angle and transmission distance based on precise orbital models, thereby providing a more practical assessment of the key rate for discrete modulation CV-QKD. Simulation results verify the feasibility and practicality of discrete modulation CV-QKD in satellite-based quantum communication systems. Furthermore, the critical factors influencing the key rate performance are identified, and parameter optimization strategies are proposed, providing theoretical support for realizing the future satellite-to-ground discrete modulation CV-QKD.
Frame synchronization is a crucial step in quantum key distribution (QKD) and a prerequisite for secure key distillation. Due to the extremely low signal-to-noise ratio (SNR) and fast phase drift in the continuous-variable QKD system, efficient frame synchronization is highly demanded. In this paper, a frame-synchronization scheme based on weak coherent states is proposed. This scheme uses a train of coherent-state pulses encoded by a Zadoff-Chu sequence to achieve frame synchronization. The synchronization performance is analyzed under different transmission distances, transmit powers, frequency offsets, and phase drifts through Monte Carlo simulation. Its performance is also compared with other synchronization sequences, confirming its noise-tolerance capability. An optical experiment is conducted to verify the feasibility of the synchronization scheme under low SNR (as low as $\ensuremath{-}20$ dB), and the applicable SNR range can be further reduced by increasing the sequence length. Due to its efficiency and noise tolerance, this scheme can provide technical support for weak-coherent-state-based QKD and future quantum network construction.
Continuous-variable quantum key distribution is a practical application of quantum information theory that is aimed at generation of secret cryptographic key between two remote trusted parties and that uses multi-photon quantum states as carriers of key bits. Remote parties share the secret key via a quantum channel, that presumably is under control of of an eavesdropper, and which properties must be taken into account in the security analysis. Well-studied fiber-optical quantum channels commonly possess stable transmittance and low noise levels, while free-space channels represent a simpler, less demanding and more flexible alternative, but suffer from atmospheric effects such as turbulence that in particular causes a non-uniform transmittance distribution referred to as fading. Nonetheless free-space channels, providing an unobstructed line-of-sight, are more apt for short, mid-range and potentially long-range (using satellites) communication and will play an important role in the future development and implementation of QKD networks. It was previously theoretically shown that coherent-state CV QKD should be in principle possible to implement over a free-space fading channel, but strong transmittance fluctuations result in the significant modulation-dependent channel excess noise. In this regime the post-selection of highly transmitting sub-channels may be needed, which can even restore the security of the protocol in the strongly turbulent channels. We now report the first proof-of-principle experimental test of coherent state CV QKD protocol using different levels Gaussian modulation over a mid-range (1.6-kilometer long) free-space atmospheric quantum channel. The transmittance of the link was characterized using intensity measurements for the reference but channel estimation using the modulated coherent states was also studied. We consider security against Gaussian collective attacks, that were shown to be optimal against CV QKD protocols . We assumed a general entangling cloner collective attack (modeled using data obtained from the state measurement results on both trusted sides of the protocol), that allows to purify the noise added in the quantum channel . Our security analysis of coherent-state protocol also took into account the effect of imperfect channel estimation, limited post-processing efficiency and finite data ensemble size on the performance of the protocol. In this regime we observe the positive key rate even without the need of applying post-selection. We show the positive improvement of the key rate with increase of the modulation variance, still remaining low enough to tolerate the transmittance fluctuations. The obtained results show that coherent-state CV QKD protocol that uses real free-space atmospheric channel can withstand negative influence of transmittance fluctuations, limited post-processing efficiency, imperfect channel estimation and other finite-size effects, and be successfully implemented. Our result paves the way to the full-scale implementation of the CV QKD in real free-space channels at mid-range distances.
We propose and experimentally investigate a high-speed Kramers-Kronig (KK) scheme based continuous-variable quantum key distribution (CV-QKD) system over a free-space optical (FSO) channel. The atmospheric turbulence channel is emulated by three spatial light modulators on which three randomly generated azimuthal phase patterns yielding Andrews' spectrum are recorded. The proposed KK scheme enables a low-complexity implementation, insensitivity to laser phase noise, low detection noise, and accurate channel transmittance monitoring. In addition, eight state CV-QKD protocols and 10-channels wavelength-division multiplexing (WDM) scheme are experimentally verified to obtain a high secure key rate. In our experiment, the minimum transmittance of 0.6 is required to guarantee the secure transmission, and a total SKR of 2.1 Gb/s can be obtained at the mean transmittance in weak turbulence regime.
Abstract Quantum Key Distribution (QKD) has been widely applied due to its theoretical one-time pad encryption. Typically employing laser beam as carrier, quantum states are transmitted through optical fibers or free-space channels, with the latter being more commonly adopted. However, compared to optical fibers, free-space links are more susceptible to external environmental factors, particularly atmospheric conditions. Beam propagation through the atmosphere experiences diffraction, refraction, and intensity attenuation, which can degrade key distribution efficiency and even prevent key generation at the receiver. The atmospheric turbulence poses the most significant challenge in all influencing factors, especially for the commonly encountered moderate-to-strong turbulence regimes. Therefore, in this work, we develop a phase screen model and turbulence-dominated transmittance model to comprehensively investigate the impact of various key parameters on optical beam propagation through turbulent atmospheres. Furthermore, we derive the key rate for the CV-QKD protocol using a TMSV (two-mode squeezed vacuum) state and perform numerical simulations.