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SPIE Conference on Quantum and Nonlinear Optics

Polarization demultiplexing for CV-QKD system based on Stokes space

Yan Pan, Heng Wang, Yaodi Pi, Yun Shao, Wei Huang, Yang Li, Bingjie Xu

Published Oct 8, 2021• 3 citations
Publisher

Abstract

A polarization demultiplexing algorithm for continuous-variable quantum key distribution (CV-QKD) system based on Stokes space is proposed and experimentally demonstrated. In the CV-QKD system, the pilot tone and quantum signal is modulated on the two orthogonal states of polarization (SOP), respectively. Since the power of the pilot-tone is much higher than quantum signal, the received signals in Stokes space present a single cluster point. Therefore, the K-means algorithm is used to find the coordinate of the cluster point, and the polarization rotation angles can be obtained by the coordinate. The advantages of the proposed algorithm are fast convergence, simple computation and modulation format independence. Experimental results of 100 MHz pilot-tone-assisted Gaussian-modulated CV-QKD system with local local oscillator (LLO) are given, and the results show that the proposed algorithm split the pilot-tone and quantum signal effectively. Furthermore, experimental results show that the proposed algorithm can track SOP scrambling of ≥3141.59 rad/s without sacrificing the performance of excess noise, which is satisfying for most scenarios of the LLO CV-QKD system.

DOI
10.1117/12.2602485

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A polarization demultiplexing algorithm for continuous-variable quantum key distribution (CV-QKD) system based on Stokes space is proposed and experimentally demonstrated. In the CV-QKD system, the pilot tone and quantum signal is modulated on the two orthogonal states of polarization (SOP), respectively. Since the power of the pilot-tone is much higher than quantum signal, the received signals in Stokes space present a single cluster point. Therefore, the K-means algorithm is used to find the coordinate of the cluster point, and the polarization rotation angles can be obtained by the coordinate. The advantages of the proposed algorithm are fast convergence, simple computation and modulation format independence. Experimental results of 100 MHz pilot-tone-assisted Gaussian-modulated CV-QKD system with local local oscillator (LLO) are given, and the results show that the proposed algorithm split the pilot-tone and quantum signal effectively. Furthermore, experimental results show that the proposed algorithm can track SOP scrambling of ≥3141.59 rad/s without sacrificing the performance of excess noise, which is satisfying for most scenarios of the LLO CV-QKD system.
To reduce the influence of random channel polarization variation, especially fast polarization perturbation, for continuous-variable quantum key distribution (CV-QKD) systems, a simple and fast polarization tracking algorithm is proposed and experimentally demonstrated. The algorithm is implemented by an orthogonal pilot tone scheme, one of the pilot tones is used for estimating the polarization rotation angle, and the other one is used for compensating the polarization perturbation introduced phase noise. Meanwhile, residual effects are compensated precisely with the help of a real-valued FIR filter. In this case, the excess noise introduced by polarization perturbation is effectively suppressed. Experimental results show that the polarization scrambling rate ≥12.57 krad/s can be tracked by using the proposed algorithm, and a good estimated parameters performance is achieved. Compared to conventional polarization tracking algorithms such as the constant modulus algorithm (CMA), experimental results show that the polarization tracking capability of the proposed algorithm is significantly improved. Furthermore, much faster polarization tracking performance is evaluated by digital simulations, and the simulation results show that about 188.50 Mrad/s can be tracked by the proposed algorithm. Thus, our method provides effective technology for the practical application of fiber-based CV-QKD.
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.
The compensation process for the state of polarization (SOP) from quantum signal plays an important role in the practical implementation of continuous-variable quantum key distribution (CV-QKD). In contrast to the previous scheme using an expensive digital dynamic polarization controller, we choose a cheaper manual polarization controller interfaced with a digital algorithm based on Kalman filter to achieve compensation of the polarization loss under the condition that the SOP of the fiber is relatively stable. This paper also enumerates the details of other digital signal processing method to achieve final secret key rate. And a pilot-sequential Gaussian modulated coherent state (GMCS) continuous-variable quantum key distribution system with a local local oscillator (LLO) is experimentally implemented based on the analysis of excess noise. Finally, the experimental results show that the method of polarization loss compensation takes advantages in maintaining the stability of the final secret key rate. And a secret key rate of 110 kbps is achieved within 20 km optical fiber (with 4 dB loss) under the finite-size block of1×1061 \times {10^{6}}1×106during a more extended time in the laboratory.
We experimentally demonstrate a 25 MHz repetition rate pilot-tone-assisted Gaussian-modulated continuous-variable quantum key distribution (CVQKD) with a local local oscillator (LLO) by employing the frequency-multiplexing and polarization-multiplexing techniques. In the proposed LLO-CVQKD system, the strong pilot tone and weak quantum signal can be separated in different frequency and polarization domain for avoiding the crosstalk between them and guaranteeing different detection requirements of low-noise for quantum signal and high-saturation limitation for pilot tone. Moreover, the dominant phase noise over 25 km optical fiber channel can be compensated in real time for supporting high secure key rate of 1.61 Mbps in the asymptotic regime.