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International Conference on Optical Communications and Networks (ICOCN)

An Orthogonal Dual-Polarization and Time Multiplexing Scheme of Continuous-Variable Quantum Key Distribution

Shuai Zhang, Yan Pan, Heng Wang, Yun Shao, Tao Zhang, Jinlu Liu, Yaodi Pi, Ting Ye, Wei Huang, Yang Li, Bingjie Xu

Published Jul 31, 2023• 2 citations
Publisher

Abstract

In this article, we propose a time and orthogonal polarization division multiplexing scheme for local local oscillator continuous-variable quantum key distribution (LLO CV-QKD) system. According to the influence mechanism of time-domain diffusion and polarization disturbance on the key parameters, such as the channel transmittance and excess noise, of the studied system, we established a dual-polarization theoretical model and verified its validity by simulations. Here, the pulse width at the nanosecond level and polarization extinction ratio (PER) are considered for optimizing the performance of secure key rate (SIKR) quantitatively. For transmitting over 25 km optical fiber, the required PER should be above 22 dB when the pulse width is fixed to lns. Moreover, the optimal repetition frequency is obtained. In this case, 5.25 Mbps SKR can be achieved under the lns pulse width and 25dB PER, considering the $10^{11}$ finite-size block. Our work provides a systematic and theoretical framework for the study of improving the SKR of the dual-polarization CV-QKD system.

DOI
10.1109/icocn59242.2023.10236340

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In this article, we propose a pilot alternately assisted scheme of orthogonal dual-polarization and time multiplexing for the local local oscillator continuous-variable quantum key distribution (LLO CV-QKD). Our scheme utilizes time multiplexing and dual-polarization multiplexing techniques to dramatically isolate the quantum signal from the pilot light. To analyze the influence mechanism of time-domain diffusion and polarization perturbation on the key parameters, such as the channel transmittance and excess noise, of the studied system, a general LLO excess noise model based on polarization extinction ratio (PER) and time-domain pulse extinction ratio (TER) is established. We mainly focus on the photon-leakage noise from the reference path to the quantum signal path, which is first analyzed in the dual polarization LLO regime. Furthermore, we conduct a series of simulations to verify the proposed dual polarization and time multiplexing model. Results show that it maintains a low level of excess noise and a secure key rate (SKR) of 10.25 Mbps@25km can be obtained under the finite-size effect. We achieved 0.93Mbps@25km SKR under a relatively low PER of 17 dB in the nanosecond level pulse width. Our work greatly extends the application scenarios of the dual-polarization division multiplexing CV-QKD system and provides a theoretical and representative framework for the study of improving the performance of the dual-polarization CV-QKD system.
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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 experimentally demonstrated a sub-Mbps key rate Gaussian-modulated coherent-state (GMCS) continuous-variable quantum key distribution (CV-QKD) over a 100-km transmission distance. To efficiently control the excess noise, the quantum signal and the pilot tone are co-transmitted in the fiber channel based on wideband frequency and polarization multiplexing methods. Furthermore, a high-accuracy data-assisted time domain equalization algorithm is carefully designed to compensate the phase noise and polarization variation in low signal-to-noise ratio. The asymptotic secure key rate (SKR) of the demonstrated CV-QKD is experimentally calculated to be 7.55 Mbps, 1.87 Mbps, and 0.51 Mbps over a transmission distance of 50 km, 75 km, and 100 km, respectively. The experimentally demonstrated that the CV-QKD system significantly improves the transmission distance and SKR compared to the state-of-art GMCS CV-QKD experimental results, and shows the potential for long-distance and high-speed secure quantum key distribution.
Quantum key distribution (QKD) has been proven to be theoretically unconditionally secure. However, any theoretical security proof relies on certain assumptions. In QKD, the assumption in the theoretical proof is that the security of the protocol is considered under the asymptotic case where Alice and Bob exchange an infinite number of signals. In the continuous-variable QKD (CV-QKD), the finite-size effect imposes higher requirements on block size and excess noise control. However, the local local oscillator (LLO) CV-QKD system cannot be considered time-invariant under long blocks, especially in cases of environmental disturbances. Thus, we propose an LLO CV-QKD scheme with time-variant parameter estimation and compensation. We first establish an LLO CV-QKD theoretical model under the temporal modes of continuous-mode states. Then, a robust method is used to compensate for arbitrary frequency shift and arbitrary phase drift in CV-QKD systems with longer blocks, which cannot be achieved under traditional time-invariant parameter estimation. Besides, the digital signal processing method predicated on high-speed reference pilots can achieve a time complexity of O (1). In the experiment, the frequency shift is up to 89.05 MHz/s and phase drift is up to 3.036 Mrad/s using a piezoelectric transducer (PZT) to simulate the turbulences in the practical channel. With a signal-to-interference ratio (SIR) of −51.67 dB , we achieve a secret key rate (SKR) of 0.29 Mbits/s with an attenuation of 16 dB or a standard fiber of 80 km. This work paves the way for future long-distance field-test experiments in the finite-size regime.
In a commercial fiber-based quantum key distribution system, the local and signal optical fields are transmitted through long distance fibers by using time division multiplexing and polarization multiplexing. The state of polarization of the optical field is inevitably disturbed by random birefringence of the standard single-mode fiber caused by external complex environments. This drift of the state of polarization significantly affects the balanced homodyne detection results and the secret key rate. Therefore, the key technology of the dynamic polarization control unit is crucial for the system in a large-scale commercial application. We theoretically analyze and prove that the polarization control unit only needs the combination of two degrees of freedom when considering the result of an arbitrary polarization extinction ratio at the receiver of the system. To overcome the influence of polarization variations, we propose a chaotic monkey algorithm based on Bayesian parameter estimation method and implement intelligence algorithm on field programmable gate array (FPGA) hardware under pulsed light with an integral-type detector for the dynamic polarization control unit. The simulation results show that the optimal combination is four degrees of freedom and the optimal prior distribution is an exponential distribution among various distributions in the dynamic polarization control unit. According to the simulation results, the experimental results show that the achieved polarization extinction ratio is over 30 dB and the average time of polarization control is 400 μs for a single random polarization scrambling. By combining the dynamic polarization control unit with the system, we demonstrate the continuous variable quantum key distribution (CV-QKD) under a continuous polarization scrambling scope of 0－2 krad/s and verify its effectiveness. In addition, the methods presented will improve the performance of the system and expand the range of applications even under strong external disturbance.