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EPJ Quantum Technology

Polarization compensation method based on the wave plate group in phase mismatch for free-space quantum key distribution

Yongjian Tan, Liang Zhang, Tianxing Sun, Zhihua Song, Jincai Wu, Zhiping He

Published Feb 13, 2023• 7 citations
Publisher

Abstract

Abstract Maintaining the polarization state in communication terminals is vital for polarization-encoding free-space quantum key distribution (QKD). Wave plate group phase mismatch caused by manufacturing errors, complex environmental effects, and the working wavelength deviation can reduce the polarization compensation effect. We found in theoretical analysis, that increasing phase mismatch of wave plates leads to the compensation method failure and reduces robustness. We propose a complementary polarization compensation method, which can effectively improve the robustness. Experimental results show that this method can improve the compensation effect by 50% at a slight phase mismatch, and realize a polarization extinction ratio exceeding 250:1 at the ergodic area even if the phase deviates to 0.27π. This method is beneficial to the high-stability design of free-space QKD systems and has the potential to be applied to QKD systems operating at multiple wavelengths.

DOI
10.1140/epjqt/s40507-023-00163-4

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Abstract Maintaining the polarization state in communication terminals is vital for polarization-encoding free-space quantum key distribution (QKD). Wave plate group phase mismatch caused by manufacturing errors, complex environmental effects, and the working wavelength deviation can reduce the polarization compensation effect. We found in theoretical analysis, that increasing phase mismatch of wave plates leads to the compensation method failure and reduces robustness. We propose a complementary polarization compensation method, which can effectively improve the robustness. Experimental results show that this method can improve the compensation effect by 50% at a slight phase mismatch, and realize a polarization extinction ratio exceeding 250:1 at the ergodic area even if the phase deviates to 0.27π. This method is beneficial to the high-stability design of free-space QKD systems and has the potential to be applied to QKD systems operating at multiple wavelengths.
Quantum key distribution uses polarization coding.When using conventional single-mode fiber as the channel for quantum state transfer,optical fiber has a slight birefringence and cannot maintain the quantum state of polarization.In this case,this paper proposes a method to make polarization compensation for quantum state.Using the polarization compensation method of successive approximation,it is a promising scheme to complete the polarization compensation by rotating the combination of 2 quarter-wave plate and 1 half-wave plate successively.By measuring the values of polarization contrast ratio,it rotates the three wave plates according to the results of the measurement,until the polarization performance of the system finally meets the requirements.Experimental results show that compared with the polarization compensation method of traversal mode,the method of successive approximation can realize polarization compensation more quickly.
Prepare-and-measure quantum key distribution (QKD) schemes with polarization encoding require the minimization of polarization degradation, i.e., undesired variations in the state of polarization, in the transmitter and receiver designs. In particular, the polarization extinction ratio (PER) should be maintained as high as possible to reduce the quantum bit error rate (QBER). In this paper, we analyze the polarization degradation and its correction in the design of free-space QKD systems. The polarization of a wavefront can be degraded in two different ways: uniformly, or non-uniformly; the latter known as polarization aberrations. While uniform polarization degradation of the beam wavefront can be easily corrected with standard retardation plates, polarization aberrations are more complex and require a different correction approach. BB84 diagonal linearly-polarized states are especially affected by polarization degradation as they are reflected off certain optical elements, and its correction is essential to achieve a low QBER. We have experimentally characterized the polarization degradation in a QKD transmitter and its impact in the QBER and achieved an almost perfect correction of the diagonal linearly polarized states when choosing a specific optical design.
Abstract Polarization drift in fiber and free-space optical links is a major factor in the dynamic increase of bit error rate in polarization-coded quantum key distribution (QKD) systems. A dynamic polarization compensation method applicable to both links is a challenge. Here we propose a universally applicable real-time polarization compensation method, that the Muller parameters of the optical links are first detected using a polarization detector, and then the optimal parameters of the controller are obtained by gradient descent algorithm. Simulation results indicate advantages over current methods, with fewer waveplates, faster speed, and wider applicability for various optical links. In equivalent experiments of both satellite and fiber optical links, the average polarization extinction ratio of 27.9 dB and 32.2 dB are respectively achieved. The successful implementation of our method will contribute to the real-time polarization design of fiber and free-space QKD systems, while also contributing to the design of laser-based polarization systems.
Abstract Polarization‐dependent loss (PDL) poses a critical challenge in implementing free‐space quantum key distribution (QKD) systems. This study investigated the theoretical and experimental impact of PDL on polarization‐encoded qubits and experimentally demonstrated a method to mitigate these effects. The proposed compensation method could effectively restore the integrity of polarization states, enhancing the robustness and security of QKD systems. Specifically, a secret key rate of 94.01% could be recovered with 5 dB PDL. This study contributes to advancing scalable and secure quantum communication technologies by addressing the critical issue of PDL in QKD systems.
In polarization-encoded satellite-to-ground quantum communication, the on-board transmitter and a receiver at the optical ground station encode and decode linearly polarized quantum bits for execution of quantum key distribution (QKD) protocol. Dynamic polarization tracking and compensation is a key technique in realizing such a space based QKD system. In this work, we report the design and development of motorized waveplate based active polarization compensation system. Integrated testing and validation with Decoy-state enabled BB84 QKD testbed over ~5m free space channel and quantum link polarization extinction ratio (PER) ≥ 100:1 has been carried out successfully. Satellite motion was emulated as a continuous 1°/s polarization basis rotation at the Quantum Tx output. When active polarization compensation system was enabled, sazimuth (Az.) deviation was measured with ±1.5° accuracy. During this experiment, overall signal QBER was consistently maintained at <3.5% with secure key generation rate of ~1350kbps, proving the suitability of this scheme for realistic satellite-to-ground QKD scenarios.
Scattering of light by cloud, haze, and fog decreases the transmission efficiency of communication channels in quantum key distribution (QKD), reduces the system's practical security, and thus constrains the deployment of free-space QKD. Here, we employ wave-front shaping technology to compensate distorted optical signals in high-loss scattering quantum channels and fulfill a polarization-encoded BB84 QKD experiment. With this quantum-channel compensation technology, we achieve a typical enhancement of about 250 in transmission efficiency and build a secure communication link even considering finite key length effect, while the link is impossible to share secure keys before optimization. The method applied in the QKD system shows the potential to expand the application range of QKD systems from lossless channels to highly scattered ones and therefore enhances the deployment ability of a global quantum communication network.