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Revista Mexicana de Física

Improved performance of the cryptographic key distillation protocol of an FSO/CV-QKD system on a turbulent channel using an adaptive LDPC encoder

Josué Aarón López-Leyva, Arturo Arvizu‐Mondragón, Joel Santos‐Aguilar, Raul I. Ramos-Garcia

Published Jan 1, 2017• 4 citations
Publisher

Abstract

In this paper, a dynamical and adaptive LDPC coding scheme is proposed in order to improve the performance of the cryptographic key distillation protocol of an FSO/CV-QKD system considering the atmospheric turbulence levels that may be present in the classic channel. In this scheme, the Generator and Parity-check matrices of the encoder are modified according to the Rytov variance values estimated in the classical channel in order to improve the final secret key rate of the QKD system. The simulation results show that the final secret key was incremented 87.5 Kbps (from 52.5 Kbps to 140 Kbps) using the adaptive code rate; meaning that the information encrypted and transmitted is increased. In addition, the use of the dynamical encoder avoids the drastically reduction of the final secret key rate when the conditions of the classical channel are considered. Our proposal might be implemented based on the use of high-speed FPGA's and DSP's commercially available.

DOI
10.31349/revmexfis.63.3.268

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In this paper, a dynamical and adaptive LDPC coding scheme is proposed in order to improve the performance of the cryptographic key distillation protocol of an FSO/CV-QKD system considering the atmospheric turbulence levels that may be present in the classic channel. In this scheme, the Generator and Parity-check matrices of the encoder are modified according to the Rytov variance values estimated in the classical channel in order to improve the final secret key rate of the QKD system. The simulation results show that the final secret key was incremented 87.5 Kbps (from 52.5 Kbps to 140 Kbps) using the adaptive code rate; meaning that the information encrypted and transmitted is increased. In addition, the use of the dynamical encoder avoids the drastically reduction of the final secret key rate when the conditions of the classical channel are considered. Our proposal might be implemented based on the use of high-speed FPGA's and DSP's commercially available.
In this paper, an adaptive LDPC encoder for complete FSO/CV-QKD system using a COTS device for emulated dynamical atmospheric turbulence levels is presented. The experimental and emulation results show the maximum and minimal final secret key rates of 105 Kbps and 10 Kbps, respectively, for minimal and maximal throughput in a commercial network, 30 Mbps and 90 Mbps, respectively. Our proposal presents an adequate performance for weak and moderate atmospheric turbulence levels and a suitable option for improve the using of QKD systems.
In this paper, an adaptive low density parity check (LDPC). encoder for complete Free Space Optics/Continuous Variable-Quantum Key Distribution (FSO/CV-QKD) system using a Commercially available Off-The-Shelf (COTS) device for emulated dynamical atmospheric turbulence levels is presented. The experimental and emulation results show the maximum and minimal final secret key rates of ≈ 105 Kbps and ≈ 10 Kbps, respectively, for minimal and maximal throughput in a commercial network, 30 Mbps and 90 Mbps, respectively. Our proposal presents an adequate performance for weak and moderate atmospheric turbulence levels and a suitable option for improving the use of Quantum Key Distribution (QKD) systems.
Continuous-variable quantum key distribution (CV-QKD) has been proposed as a method for securely exchanging keys to protect against the security concerns caused by potential advancements in quantum computing. In addition to optical fibre transmission, the free-space optical (FSO) channel is an interesting channel for CV-QKD, as it is possible to share keys over this channel wirelessly. The instability of the FSO channel caused by turbulence-induced fading, however, can cause a degradation in the system's performance. One of the most important aspects of CV-QKD is the reconciliation step, which significantly impacts the performance of the CV-QKD system. Hence, rate-adaptive reconciliation is necessary for CV-QKD over FSO to combat the fluctuations in the channel and improve secret key rates (SKRs). Therefore, in this paper, we simulate the impact of discrete modulation on the reconciliation efficiency and consider the use ofddd-dimensional reconciliation withd>8d > 8d>8to mitigate this impact, improving reconciliation efficiencies by up to 3.4%. We validate our results by experimentally demonstrating CV-QKD over a turbulent FSO link and demonstrate SKR gains by up to 165%. Furthermore, we optimise the reconciliation efficiency for FSO links, achieving additional SKR gains of up to 7.6%.
Secure data transmission is essential in today's world, and quantum key distribution (QKD) is widely recognized as the technology of choice to enable information-theoretical secure symmetric key generation. While optical fibers are typically used as a transmission channel, wireless transmission offers the flexibility to reach every part of the Earth; moreover, for Earth-Satellite QKD, wireless communication naturally becomes a necessity. In this study, we deployed a fiber-wireless optical system that transmitted a 64-QAM 400 Gbps classical signal for high-rate data exchange and a 1 MHz quantum signal for QKD. The impact of turbulence in the quantum system is evaluated by transmitting single-polarization quantum states through a turbulent free-space channel and by calculating the respective intrinsic quantum bit error rate (QBER). From this, we estimate the system's secret key rate (SKR) and its dependence on turbulence. The effects of atmospheric turbulence on the optical wireless signal were emulated and investigated by using a custom-made atmospheric chamber to induce different turbulent scenarios. The atmospheric chamber allowed us to vary the turbulence in a Rytov variance range from 0.06 to 2.52. The classical signal showed an overall average reliability of 82% for the mentioned turbulence regimes. The results of the quantum signal showed that for Rytov variances up to 1.3, and under the conditions tested, secret key generation is possible, reaching a maximum SKR of 8.7×10−4\mathbf {\times 10^{-4}}×10−4bits per pulse during the produced turbulence regimes.
Abstract Free-space continuous-variable quantum key distribution (CV-QKD) links provide greater information capability and broader geographical coverage. A major challenge for the practical use of free-space quantum systems is that the signal-to-noise ratios of the channels are relatively low and are sensitive to atmospheric disturbance. The atmospheric channel has several negative characteristics that diminish the secret key rate, such as beam extinction and various turbulence effects. The effect of beam wandering on the quantum properties is mainly considered, and the probability distribution of the transmission coefficient is derived to establish the channel model. Low-density parity-check (LDPC) codes operated in the reconciliation procedure are utilized to deal with information interactivity. However, the LDPC codes need to be chosen from a range of codes, where the code rate is considered in low signal-to-noise ratio free-space links. The accumulate–repeat–accumulate code is adopted to improve the decoding threshold of a code. It is found that the multi-edge feature of extended codes is beneficial to the performance of short- and medium-length codes. And numerical results show the proposed scheme provides enhancement in the reconciliation efficiency. LDPC codes containing an additional structure that facilitates decoding could be suitable for free-space CV-QKD protocols when weak atmospheric turbulence is considered.
In continuous-variable (CV) quantum key distribution (QKD), the reconciliation efficiency and frame error rate of the information reconciliation have a significant impact on the secret key rate. For a fixed-rate multiedge-type low-density parity-check (LDPC) code, the reconciliation efficiency and frame error rate will inevitably vary due to the fluctuations of the signal-to-noise ratio (SNR) of the CV QKD system, which degrades the performance of the system. We propose a high-efficiency rate-adaptive information reconciliation scheme by combining raptorlike (RL) LDPC codes with the addition of trusted noises. We establish the model of adding trusted noises and combine it with the RL LDPC codes to optimize the secret key rate under the time-varying channel. The simulation results show that our scheme can maintain a high reconciliation efficiency of more than 94.4% and a low frame error rate within 15% fluctuation range of the SNR. Furthermore, we implement the hardware acceleration of the proposed rate-adaptive scheme on a graphics processing unit and achieve a decoding throughput of 65.5 Mbits/s by optimizing the storage of the parity-check matrix. Our results are useful for the practical CV QKD under the realistic time-varying channel.