Employing commercial off-the-shelf coherent optical transmission components and methods to design a continuous variable quantum key distribution (CVQKD) system is a promising trend of achieving QKD with high security key rate (SKR) and cost-effectiveness. In this paper, we explore CVQKD system based on the widely used polarization division multiplexed coherent optical transmission structure and pilot-aided digital signal processing methods. A simplified pilot-aided phase noise compensation scheme based on frequency division multiplexing (FDM) is proposed, which introduces less total excess noise than classical pilot-aided schemes based on time division multiplexing (TDM). Besides, the two schemes of training symbol (TS)-aided equalization are compared to find the optimal strategy for TS insertion, where the scheme based on block insertion strategy can provide the SKR gain of around 29%, 22%, and 15% compared with the scheme based on fine-grained insertion strategy at the transmission distance of 5 km, 25 km and 50 km, respectively. The joint optimization of pilot-aided and TS-aided methods in this work can provide reference for achieving CVQKD system with high SKR and low complexity in metropolitan-scale applications.
Employing commercial off-the-shelf coherent optical transmission components and methods to design a continuous variable quantum key distribution (CVQKD) system is a promising trend of achieving QKD with high security key rate (SKR) and cost-effectiveness. In this paper, we explore CVQKD system based on the widely used polarization division multiplexed coherent optical transmission structure and pilot-aided digital signal processing methods. A simplified pilot-aided phase noise compensation scheme based on frequency division multiplexing (FDM) is proposed, which introduces less total excess noise than classical pilot-aided schemes based on time division multiplexing (TDM). Besides, the two schemes of training symbol (TS)-aided equalization are compared to find the optimal strategy for TS insertion, where the scheme based on block insertion strategy can provide the SKR gain of around 29%, 22%, and 15% compared with the scheme based on fine-grained insertion strategy at the transmission distance of 5 km, 25 km and 50 km, respectively. The joint optimization of pilot-aided and TS-aided methods in this work can provide reference for achieving CVQKD system with high SKR and low complexity in metropolitan-scale applications.
Continuous-variable quantum key distribution (CV-QKD) with local local oscillator (LLO) is well-studied for its security and simplicity, but enhancing performance and interference resistance remains challenging. In this paper, we utilize polarization division multiplexing (PDM) to enhance spectral efficiency and significantly increase the key rate of the CV-QKD system. To address dynamic changes in the state of polarization (SOP) in Gaussian modulated coherent states (GMCS) signals due to polarization impairment effects, we designed a time-division multiplexing pilot scheme to sense and recover changes in SOP in GMCS signals, along with other digital signal processing methods. Experiments over 20 km show that our scheme maintains low excess noise levels (0.062 and 0.043 in shot noise units) and achieves secret key rates of 4.65 Mbps and 5.66 Mbps for the two polarization orientations, totaling 10.31 Mbps. This work confirms the effectiveness of PDM GMCS-CV-QKD and offers technical guidance for high-rate QKD within metropolitan areas.
We propose a pilot-multiplexed continuous-variable quantum key distribution (CVQKD) scheme based on a local local oscillator (LLO). Our scheme utilizes time-multiplexing and polarization-multiplexing techniques to dramatically isolate the quantum signal from the pilot, employs two heterodyne detectors to separately detect the signal and the pilot, and adopts a phase compensation method to almost eliminate the multifrequency phase jitter. In order to analyze the performance of our scheme, a general LLO noise model is constructed. Besides the phase noise and the modulation noise, the photon-leakage noise from the reference path and the quantization noise due to the analog-to-digital converter (ADC) are also considered, which are first analyzed in the LLO regime. Under such general noise model, our scheme has a higher key rate and longer secure distance compared with the preexisting LLO schemes. Moreover, we also conduct an experiment to verify our pilot-multiplexed scheme. Results show that it maintains a low level of the phase noise and is expected to obtain a 554-Kbps secure key rate within a 15-km distance under the finite-size effect.
Continuous-variable quantum key distribution (CVQKD) with a real local oscillator (LO) has been extensively studied recently due to its security and simplicity. In this paper, we propose a novel implementation of a high-key-rate CVQKD with a real LO. Particularly, with the help of the simultaneously generated reference pulse, the phase drift of the signal is tracked in real time and then compensated. By utilizing the time and polarization multiplexing techniques to isolate the reference pulse and controlling the intensity of it, not only the contamination from it is suppressed, but also a high accuracy of the phase compensation can be guaranteed. Besides, we employ homodyne detection on the signal to ensure the high quantum efficiency and heterodyne detection on the reference pulse to acquire the complete phase information of it. In order to suppress the excess noise, a theoretical noise model for our scheme is established. According to this model, the impact of the modulation variance and the intensity of the reference pulse are both analysed theoretically and then optimized according to the experimental data. By measuring the excess noise in the 25km optical fiber transmission system, a 3.14Mbps key rate in the asymptotic regime proves to be achievable. This work verifies the feasibility of the high-key-rate CVQKD with a real LO within the metropolitan area.
A high-speed Gaussian-modulated continuous-variable quantum key distribution (CVQKD) with a local local oscillator (LLO) is experimentally demonstrated based on pilot-tone-assisted phase compensation. In the proposed scheme, the frequency-multiplexing and polarization-multiplexing techniques are used for the separate transmission and heterodyne detection between quantum signal and pilot tone, guaranteeing no crosstalk from strong pilot tone to weak quantum signal and different detection requirements of low-noise for quantum signal and high-saturation limitation for pilot tone. Moreover, compared with the conventional CVQKD based on homodyne detection, the proposed LLO-CVQKD scheme can measure X and P quadrature simultaneously using heterodyne detection without need of extra random basis selection. Besides, the phase noise, which contains the fast-drift phase noise due to the relative phase of two independent lasers and the slow-drift phase noise introduced by quantum channel disturbance, has been compensated experimentally in real time, so that a low level of excess noise with a 25 km optical fiber channel (with 5 dB loss) is obtained for the achievable secure key rate of 7.04 Mbps in the asymptotic regime and 1.85 Mbps under the finite-size block of 107.
We experimentally demonstrated a high-performance multi-protocol continuous-variable quantum key distribution (CV-QKD) using one transceiver. The demonstrated multi-protocol CV-QKD system can simultaneously distribute 5 subcarriers with Gaussian, 4QAM, 16QAM, 64QAM, and 1024QAM modulation protocols based on orthogonal-frequency-division-multiplexing (OFDM) technology. Moreover, the modulation variance and probability distribution factor of 5 subcarriers are carefully optimized to maximize their respective SKRs. Besides, we innovatively design an effective OFDM-based DSP scheme and a precise dual-stage phase noise compensation scheme to realize a low excess noise for each subcarrier operating at a 2 GHz symbol rate. Thus, the multi-protocol CV-QKD achieves multiple SKRs of 48.79, 7.58, 23.67, 32.93, and 36.78 Mbps over a typical transmission distance of 50 km within a single transmission. Therefore, our work provides a practical solution to achieve highly efficient, interoperable, flexible, and secure QKDs for high-speed quantum secure communication.
Dual-polarization division multiplexing (DPDM) is considered to be a potential method to boost the secure key rate (SKR) of the continuous-variable quantum key distribution (CV-QKD) system. In this article, we propose a pilot alternately assisted local local oscillator (LLO) CV-QKD scheme based on multi-dimensional multiplexing, where time division multiplexing and frequency division multiplexing are combined with dual-polarization multiplexing techniques to dramatically isolate the quantum signal from the pilot tone. We establish a general excess noise model for the LLO CV-QKD system to analyze the influence mechanism of various disturbances (e.g., time-domain diffusion, frequency-domain modulation residual, and polarization perturbation) on the key parameters, such as the channel transmittance and excess noise. Specifically, the photon leakage noise from the reference path to the quantum path and that between quantum signals with two different polarization paths are simultaneously analyzed in the dual-polarization LLO CV-QKD scheme for the first time. Furthermore, a series of simulations are established to verify the performance of the proposed scheme. The results show that the maximal isolation degree achieves 84.0 dB~90.4 dB, and the crosstalk between pilot tones and quantum signals can be suppressed to a very small range. By optimizing the system parameters (e.g., modulation variance and repetition frequency), the SKR with 12.801 Mbps@25 km is achieved under the infinite polarization extinction ratio (PER) and 30 dB residual ratio of the frequency modulation in the nanosecond-level pulse width. Moreover, the performance of the proposed DPDM CV-QKD scheme under relatively harsh conditions is simulated; the results show that the SKR with 1.02 Mbps@25 km is achieved under a relatively low PER of 17 dB with the nanosecond-level pulse width and 20 dB residual ratio of the frequency modulation. Our work lays an important theoretical foundation for the practical DPDM LLO CV-QKD system.