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SPIE Conference on Quantum and Nonlinear Optics

Experimental demonstration of 5 GBaud four-state continuous-variable quantum key distribution with digital signal processing

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

Published Nov 27, 2022• 0 citations
Publisher

Abstract

In this paper, we experimentally demonstrate a 5 GBaud four-state continuous-variable quantum key distribution with digital signal processing. By employing a frequency- and polarization-multiplexing quantum key transceiver, the modulation noise and DAC quantization noise in quantum state preparation, the photo-leakage noise in co-fiber transmission, the detection noise and ADC quantization noise in polarization diversity detection can be effectively reduced for achieving an ultra-low level of excess noise. Moreover, the main polarization variation and phase noise can be accurately compensated by designing a precise digital compensation scheme including the pilot-assisted polarization and phase compensation algorithm and the data-assisted equalized compensation algorithm. Besides, the explicit asymptotic secure key rate is evaluated by using an improved semidefinite programming security analysis method, which achieves a 100 Mbps level of secure key rate within 10 km distance.

DOI
10.1117/12.2643932

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In this paper, we experimentally demonstrate a 5 GBaud four-state continuous-variable quantum key distribution with digital signal processing. By employing a frequency- and polarization-multiplexing quantum key transceiver, the modulation noise and DAC quantization noise in quantum state preparation, the photo-leakage noise in co-fiber transmission, the detection noise and ADC quantization noise in polarization diversity detection can be effectively reduced for achieving an ultra-low level of excess noise. Moreover, the main polarization variation and phase noise can be accurately compensated by designing a precise digital compensation scheme including the pilot-assisted polarization and phase compensation algorithm and the data-assisted equalized compensation algorithm. Besides, the explicit asymptotic secure key rate is evaluated by using an improved semidefinite programming security analysis method, which achieves a 100 Mbps level of secure key rate within 10 km distance.
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.
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.
Digital signal processing technology has paved the way for the realization of high-speed continuous-variable quantum key distribution systems. However, existing security proofs are limited to static digital signal processing algorithms, while practical systems rely on dynamic multiple-input multiple-output algorithms to compensate for time-varying channel impairments. Our analysis reveals that the conventional dynamic algorithm, due to its non-unitary nature, systematically underestimates the excess noise, which in turn leads to security issues and the generation of insecure keys. To close this gap, we propose a secure algorithm model, mapping the dynamic algorithm to an equivalent physical optical model whose security can be rigorously assessed. Simulations illustrate the algorithm's non-unitary property and provide a quantitative analysis of the excess noise underestimation caused by the conventional algorithm. We further experimentally validate the necessity of the proposed modeling for dynamic digital signal processing, achieving a secret key rate of 14.4 Mbps based on estimated excess noise of 0.07 shot noise unit; whereas the conventional algorithm would have dangerously overestimated the key rate to 28.2 Mbps with noise of 0.008 shot noise unit. This work provides the essential security framework for dynamic digital signal processing, overcoming a critical impediment for the development of high-performance continuous-variable quantum key distribution systems.
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.
We report a fiber-based four-state discrete modulation continuous variable quantum key distribution system based on homodyne detection. A secret key rate of 1kbit/s is achieved at a transmission distance of 30.2 km. Two factors that result in the excess noises of the quantum key distribution system are analyzed. The first is the relative phase dithering between the signal and local fields, and the second is the local field leakage into the signal field due to the scattering process that depolarizes the local field. It is found that the latter has a significant impact on the excess noise, which is the main limiting factor to the long-distance secure quantum transmission. Some protocols are also given to decrease the excess noise effectively.
We experimentally demonstrate a high-rate discretely modulated continuous-variable quantum key distribution over 80-km standard single-mode fiber with a 2.5 Gbaud, 16-symbol, two-ring constellation. With the help of well-designed digital signal processing algorithms, the excess noise of the system can be effectively suppressed. The achieved secret key rates are 49.02 Mbits/s, 11.86 Mbits/s, and 2.11 Mbits/s over 25-km, 50-km, and 80-km optical fiber, respectively, and achieve 67.4%, 70.0%, and 66.5% of the secret key rate performance of a Gaussian-modulated protocol. Our work shows that it is feasible to build a high-performance, long-distance continuous-variable quantum key distribution system with only a small constellation size.