We propose and experimentally investigate a high-speed Kramers-Kronig (KK) scheme based continuous-variable quantum key distribution (CV-QKD) system over a free-space optical (FSO) channel. The atmospheric turbulence channel is emulated by three spatial light modulators on which three randomly generated azimuthal phase patterns yielding Andrews' spectrum are recorded. The proposed KK scheme enables a low-complexity implementation, insensitivity to laser phase noise, low detection noise, and accurate channel transmittance monitoring. In addition, eight state CV-QKD protocols and 10-channels wavelength-division multiplexing (WDM) scheme are experimentally verified to obtain a high secure key rate. In our experiment, the minimum transmittance of 0.6 is required to guarantee the secure transmission, and a total SKR of 2.1 Gb/s can be obtained at the mean transmittance in weak turbulence regime.
We propose and experimentally investigate a high-speed Kramers-Kronig (KK) scheme based continuous-variable quantum key distribution (CV-QKD) system over a free-space optical (FSO) channel. The atmospheric turbulence channel is emulated by three spatial light modulators on which three randomly generated azimuthal phase patterns yielding Andrews' spectrum are recorded. The proposed KK scheme enables a low-complexity implementation, insensitivity to laser phase noise, low detection noise, and accurate channel transmittance monitoring. In addition, eight state CV-QKD protocols and 10-channels wavelength-division multiplexing (WDM) scheme are experimentally verified to obtain a high secure key rate. In our experiment, the minimum transmittance of 0.6 is required to guarantee the secure transmission, and a total SKR of 2.1 Gb/s can be obtained at the mean transmittance in weak turbulence regime.
A high-speed four-state continuous-variable quantum key distribution (CV-QKD) system, enabled by wavelength-division multiplexing, polarization multiplexing, and orbital angular momentum (OAM) multiplexing, is studied in the presence of atmospheric turbulence. The atmospheric turbulence channel is emulated by two spatial light modulators (SLMs) on which two randomly generated azimuthal phase patterns yielding Andrews' spectrum are recorded. The phase noise is mitigated by the phase noise cancellation (PNC) stage, and channel transmittance can be monitored directly by the D.C. level in our PNC stage. After the system calibration, a total SKR of >1.68 Gbit/s can be reached in the ideal system, featured with lossless channel and free of excess noise. In our experiment, based on commercial photodetectors, the minimum transmittances of 0.21 and 0.29 are required for OAM states of 2 (or -2) and 6 (or -6), respectively, to guarantee the secure transmission, while a total SKR of 120 Mbit/s can be obtained in case of mean transmittances.
We experimentally demonstrate an eight-state continuous-variable quantum key distribution (CV-QKD) over atmospheric turbulence channels. The high secret key rate (SKR) is enabled by 4-D multiplexing of 96 channels, i.e., six-channel wavelength-division multiplexing, four-channel orbital angular momentum multiplexing, two-channel polarization multiplexing, and two-channel spatial-position multiplexing. The atmospheric turbulence channel is emulated by a spatial light modulator on which a series of azimuthal phase patterns yielding Andrews' spectrum are recorded. A commercial coherent receiver is implemented at Bob's side, followed by a phase noise cancellation stage, where channel transmittance can be monitored accurately and phase noise can be effectively eliminated. Compared to four-state CV-QKD, eight-state CV-QKD protocol potentially provides a better performance by offering higher SKR, better excess noise tolerance, and longer secure transmission distance. In our proposed CV-QKD system, the minimum transmittances of 0.24 and 0.26 are required for OAM states of 2 (or -2) and 6 (or -6), respectively, to guarantee the secure transmission. A maximum SKR of 3.744 Gb/s is experimentally achievable, while a total SKR of 960 Mb/s can be obtained in case of mean channel transmittances.
We experimentally demonstrate a RF-assisted four-state continuous-variable quantum key distribution (CV-QKD) system in the presence of turbulence. The atmospheric turbulence channel is emulated by two spatial light modulators (SLMs) on which two randomly generated azimuthal phase patterns are recorded yielding Andrews’ azimuthal phase spectrum. Frequency and phase locking are not required in our system thanks to the proposed digital phase noise cancellation (PNC) stage. Besides, the transmittance fluctuation can be monitored accurately by the DC level in this PNC stage, which is free of post-processing noise. The mean excess noise is measured to be 0.014, and the maximum secret key rate of >20Mbit/s can be obtained with the transmittance of 0.85, while employing the commercial PIN photodetectors.
In this invited paper, a high-speed continuous-variable quantum key distribution (CV-QKD) system is investigated in the presence of atmospheric turbulence, which is enabled by employing the wavelength, polarization, and orbital angular momentum (OAM) states as degrees of freedom. Four-state discrete modulation is used for the key shaping for the convenience of easy generation, as well as to enhance the reconciliation efficiency. The polarization insensitive atmospheric turbulence channel is emulated by two spatial light modulators (SLMs) on which randomly generated azimuthal phase patterns yielding Andrews' spectrum are recorded. The phase noise is mitigated by the phase noise cancellation (PNC) stage, and transmittance fluctuation is monitored directly by the D.C. component in the PNC stage. After system calibration, a total secret key rate (SKR) of 240 Mb/s can be reached in the presence of atmospheric turbulence.
Optical communications have been widely deployed all over the world. In state-of-the-art optical communication systems, security and high capacity are the most important factors to be considered. As the rapid development of quantum computing, traditional computational complexity based encryption faces unprecedented challenges. To guarantee the unconditional security, quantum key distribution (QKD) has been proposed and now is attracting increasing attentions. Classical optical communications have been exponentially growing for decades. The telecommunications industry is always in favor of high-speed, long-haul, and cost-efficient optical transports. Lots of advanced techniques have been applied to meet the requirements, e.g., multi-dimensional multiplexing, advanced modulation formats, coherent detection, forward error correction (FEC) coding, constellation shaping, and adaptive coding. In this dissertation, I will investigate continuous-variable QKD (CV-QKD) protocols to enable unconditional security and study the reliable classical advanced optical transmission systems. In a dynamic fiber-optic transmission system, the system performance can be improved by the proposed self-adaptive coding approach. We also experimentally and numerically compare the mutual information (MI) performances of regular/probabilistically shaped/geometrically shaped (PS/GS)-8/16/32 quadrature amplitude modulation (QAM) formats. Therefore, the industry can select the best shaping scheme accordingly in a specific situation. Furthermore, we propose a novel hybrid PS/GS scheme, which can minimize non-Gray mapping penalty as well as be tolerant to the implementation penalty. In addition, free-space optical (FSO) communication systems are also studied here. We emulate the outdoor atmospheric turbulence by indoor turbulence emulator, which is based on split-step beam propagation method (SSBPM). Turbulence mitigation methods, including adaptive optics (AO), channel coding, spatial offset, and Huffman coding, are proposed and experimentally demonstrated to enable high-speed FSO communications. At the end, we propose novel four-state and eight-state CV-QKD systems, where the phase noise can be effectively mitigated, and power fluctuation caused by turbulence can be accurately monitored. Thereafter, beyond 1 Gb/s CV- QKD systems can be realized by controlling phase noise, leveraging reconciliation efficiency, monitoring power fluctuation, and multiplexing quantum channels.