Conventional continuous-variable quantum key distribution systems typically rely on discrete optical components, which have limited integration, are bulky and expensive. To overcome these drawbacks, photonic integrated circuit technology is a viable alternative that promises to increase the system integration. However, variable optical attenuators compatible with established photonic integration processes have stability difficulties that limit the performance of the system. This work provides a chip-based variable optical attenuator based on Mach-Zehnder interferometer structure. By using an unbalanced interferometer arm, the sensitivity of the optical attenuation coefficient to environmental fluctuations can be reduced, ensuring the high-precision control required for continuous-variable quantum key distribution systems. Our scheme will facilitate the implementation of a reliable and highly stable chip-based continuous-variable quantum key distribution system.
In this paper, we propose a multi-rate and multi-protocol CV-QKD scheme based on the orthogonal-frequencydivision- multiplexing (OFDM) technology. The proposed OFDM-based multi-carrier CVQKD scheme only requires one transmitter and one receiver to realize QKD with different modulation protocols and different key rates in one communication. More importantly, the multiple subcarriers with different modulation protocols have different excess noise tolerances in the same transmission channel, which can achieve the flexible QKD service even in long-distance and high-disturbance fiber channel. In order to verify the proposed scheme, 5 subcarriers with QPSK, 64QAM, 256QAM, 1024QAM and Gaussian modulation protocols are evaluated by the SDP and no-switch Gaussian security analysis method at different transmission distances. The simulation results show the proposed OFDM-based multi-carrier scheme allows various QKDs with different modulation protocols and different key rates in one communication. Moreover, according to the obtained 5 SKRs, we can choose the optimal modulation protocol of the subcarriers to meet different needs of quantum network operators. In addition, the scheme also can choose much more subcarriers and different symbol rates to flexibly achieve the QKD in different quantum secure communication scenarios. Therefore, the proposed scheme changes the modulation protocol, subcarrier number and symbol rate to achieve the interoperability, flexibility and compatibility.
The source noise in plug-and-play continuous variable quantum key distribution (CV-QKD) system plays a crucial role in determining the secret key rate and transmission distance. In general, the source noise is considered untrusted and fully controlled by Eve, which is because the laser travels through the unsecure channel before being modulated. However, this may overestimate the key information stolen by Eve leading to an underestimation of the key rate share between the legal communication parties. Here, we use a beam-splitter with signal attenuation to model the source noise combined with source monitoring scheme to characterize the source imperfection in the plug-and-play CV-QKD system. We show that the performance of the plug-and-play CV-QKD can be significantly improved under the above scheme compared to the untrusted source model. Our numerical simulation results also show that the plug-and-play CV-QKD with source monitoring has a key generation rate close to that of a trusted source under the same simulation parameters.
In this paper, a multi-carrier Gaussian modulated continuous variable quantum key distribution (CV-QKD) scheme has been proposed based on orthogonal frequency division multiplexing (OFDM) for distributing multiplexing independent secret keys encoded on N subcarriers within a single fiber channel. However, the performance of the system will be significantly influenced by the extra modulation noise in the multi-carrier quantum state preparation. Therefore, a modulation noise model is analyzed in more compact for multi-carrier Gaussian modulated CV-QKD system. Specifically, the gain imbalance and quadrature skew in IQ modulation and the third-order intermodulation effect in N subcarrier modulation are systematically analyzed in the OFDM-based multi-carrier CV-QKD with Gaussian modulation. That is, the IQ imbalance noise and the intermodulation noise are modeled as the modulation noise of the multi-carrier Gaussian modulated CV-QKD system. Moreover, the secure performances of the multi-carrier Gaussian modulated CVQKD are evaluated based on the proposed modulation noise model. Besides, the simulation results show the SKRs are greatly increased by N independent quantum state preparation, which indicates that the multi-carrier CV-QKD system gets rid of the asymptotic SKR limit of single-carrier CV-QKD system for future high-rate CV-QKD deployment in broadband access network.
In this article, we propose a pilot alternately assisted scheme of orthogonal dual-polarization and time multiplexing for the local local oscillator continuous-variable quantum key distribution (LLO CV-QKD). Our scheme utilizes time multiplexing and dual-polarization multiplexing techniques to dramatically isolate the quantum signal from the pilot light. To analyze the influence mechanism of time-domain diffusion and polarization perturbation on the key parameters, such as the channel transmittance and excess noise, of the studied system, a general LLO excess noise model based on polarization extinction ratio (PER) and time-domain pulse extinction ratio (TER) is established. We mainly focus on the photon-leakage noise from the reference path to the quantum signal path, which is first analyzed in the dual polarization LLO regime. Furthermore, we conduct a series of simulations to verify the proposed dual polarization and time multiplexing model. Results show that it maintains a low level of excess noise and a secure key rate (SKR) of 10.25 Mbps@25km can be obtained under the finite-size effect. We achieved 0.93Mbps@25km SKR under a relatively low PER of 17 dB in the nanosecond level pulse width. Our work greatly extends the application scenarios of the dual-polarization division multiplexing CV-QKD system and provides a theoretical and representative framework for the study of improving the performance of the dual-polarization CV-QKD system.
For a high-speed and secure continuous-variable quantum key distribution (CV-QKD) system, privacy amplification (PA) plays an important role. To reduce the finite size effect, the input length of PA should be at least on the order of 10^8, 10^9, 10^10 when the transmission distance is about 50km, 80km, 100km, respectively. This leads to high computation complexity and large storage demand of the data, which is unfriendly to field programmable gate array (FPGA) implementation for its limited resource. In addition, the limited IO speed of Double Data Rate Synchronous Dynamic Random Access Memory (DDR SDRAM) restricts the implementation performance of PA. In this paper, we propose an effective way to access data based on DDR to improve the performance of PA. As a result, the reading time from DDR can be reduced, and it can eliminate the effect of the limited IO speed of DDR, so that PA can perform with multiple code-words. This can make full use of the resource of FPGA and increase the execution speed of PA. Besides, combining with the proposed method, an easier algorithm is used to decrease the complexity of calculations. Based on these methods, we realize PA with Toeplitz matrix based on FPGA and the experimental throughput is about 288Mbps when the input length is about 100Mbits.
The throughput of error correction is one of the main bottlenecks of high-speed continuous variable quantum key distribution (CV-QKD) post-processing, which directly restricts the practical secret key rates (SKR). Implementing the decoder of low-density parity-check (LDPC) codes based on FPGA in limited precision can improve the decoding throughput significantly. In this paper, a high-throughput decoder architecture with limited precision for quasi-cyclic LDPC (QC-LDPC) codes is proposed. In particular, decoding of two typical LDPC codes, with code rates 0.2 and 0.1, for CV-QKD have been implemented on a commercial FPGA. The clock operates at 100 MHZ and the throughput of 1.44 Gbps and 0.78 Gbps is achieved, respectively, which can support 71.89 Mbps and 9.97 Mbps real-time SKR under transmission distance of 25 km and 50 km, respectively. The proposed architecture paves the way for high-rate real-time CV-QKD deployment in secure metropolitan area network.
The trusted phase noise model for continuous-variable quantum key distribution protocol with a real local oscillator (LLO CVQKD) has been established recently, which can lead to a better quantum key distribution (QKD) performance by moving part of the phase noise from the untrusted channel-added noise to the trusted detector-added noise. However, the calibration of the trusted phase noise is related to the intensity of the phase-reference pulse, which can be used by the eavesdropper to hack the QKD system. Here, we present a polarization attack scheme against the phase-reference pulse. In practical LLO CVQKD systems, only a part of the phase-reference pulses are used to measure and compensate for the polarization drift of the signal pulses due to the limitation of polarization measurement. We show that Eve can manipulate the polarization of the unmeasured part of the phase-reference pulses to control the trusted phase noise. Simulations show that improving the polarization measurement ratio to 100% or monitoring the phase-reference pulse intensity in real time is necessary to guarantee the security of the practical LLO CVQKD system.
KEYWORDS: Polarization, Digital signal processing, Continuous variable quantum key distribution, Quantum signals, Modulation, Quantum key distribution, Analog to digital converters
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.
The throughput of error correction is the main bottleneck of continuous variable quantum key distribution (CV-QKD) postprocessing. Implementing the decoder of low-density parity-check (LDPC) codes based on FPGA with limited precision can improve the decoding throughput significantly. However, the limited precision on FPGA results in the existence of residual error-bits after decoding, which lowers the secret key rate and restricts the application of high-rate real-time CVQKD system. In this paper, an efficient decoding scheme is proposed to erase the residual error-bits and decrease the frame errors rate (FER), where the decoding process into two stages and some values of initial Log Likelihood Ratio (LLR) are adjusted according to the proposed principles before starting the second-stage decoding. For the rates 0.2 and 0.1 LDPC codes, numerical results demonstrate that the proposed decoding scheme decreases the FER obviously and the throughputs of 152.47Mbps and 88.32Mbps are achieved, which can be applied to support high-speed CV-QKD system under transmission distance of 25km and 50km respectively.
Continuous-variable quantum key distribution (CV-QKD) offers the advantages of high secret key rates in metropolitan areas. Optimization of modulation variance is an efficient method to improve the secret key rate of CVQKD system. However, in practical CV-QKD system, inevitable slight parameter fluctuation could occur after the modification of modulation variance, and controlling the modulation variance with arbitrary accuracy is also difficult. In this paper, we propose a two-step optimization for practical CV-QKD. The first step is to determine the optimal working state by combining the modulation variance optimization with error correction matrix optimization. The second step is to optimize the rate-adaptive reconciliation parameters to compensate the loss of secret key rate caused by inaccuracy modulation variance. Our results show that the secret key rate can be improved by 17.8% in comparison to one-step optimization method. Our method can be conveniently applied to CV-QKD protocol with homodyne and heterodyne detection, which will pave the way to the deployment of high stable and high performance for CV-QKD.
KEYWORDS: Polarization, Signal to noise ratio, Digital signal processing, Modulation, Detection and tracking algorithms, Oscillators, Modulators, Continuous wave operation, Telecommunications, Signal processing
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.
In continuous-variable quantum key distribution system with a true local oscillator (LLO CV-QKD), part of the phase noise associated with the coherent detector and the phase-reference intensity can be considered as trusted because which can be locally calibrated at the receiver’s side. The trusted phase noise model can significantly improve the noise tolerance of the system since the phase noise is the major excess noise. However, the transmission of the phase-reference pulse through the insecure quantum channel in the LLO CV-QKD system may leave rooms for the eavesdropper to mount attacks. Here, we propose a practical and flexible phase-reference intensity attack scheme using a phaseinsensitive amplifier to amplify the intensity of the phase-reference pulse. In this case, the eavesdropper can compromise the security of the LLO CV-QKD system severely by lowering the trusted part of the phase noise to compensate her increased attack on the signal pulse while the total excess noise is unchanged. We simulate the secure key rate with respect to the transmission distance to show that precisely monitoring the instantaneous intensity of the phase-reference pulse in real time is of great importance to guarantee the security of the LLO CV-QKD system.
In this paper, a high-rate Gaussian-modulated coherent-state (GMCS) continuous-variable quantum key distribution (CV-QKD) scheme with a local local oscillator is experimentally demonstrated. The transmission of quantum signal and pilot tone in optical fiber adopts frequency and polarization multiplexing technology. By optimizing frequency bandwidth, modulation variance and intensity of the pilot tone, the CVQKD system is demonstrated at different metropolitan distance, and the secure key rate of 13.53Mbps, 8.24Mbps, 5.39Mbps 3.66Mbps and 2.55Mbps over transmission distance of 5km, 10km, 15km, 20km and 25km are obtained, respectively.
In this paper, a frequency-shifted-assisted continuous variable quantum key distribution with local local oscillator (LLOCVQKD) scheme is proposed based on Gaussian modulated coherent state. In the proposed scheme, the quantum signal and pilot tone can be completely isolated in frequency domain by frequency-shifting quantum optical carrier, so that the crosstalk from strong pilot tone to weak quantum signal can be effectively eliminated compared with our former pilottone scheme based on CS-DSB modulation. Moreover, an improved phase noise compensation scheme based on pilottone- assisted phase calibration and adaptive phase rotation is proposed for eliminating the dominate phase noise without the help of any training sequences, which promotes the blocks of the quantum key. Besides, a low level of excess noise is experimentally obtained for supporting the secure key rate of 7.15 Mbps over secure transmission distance of 25 km, verifying the simple and high-rate LLO-CVQKD.
We build a new two-way fiber time transfer technique (TWFTT) simulation model, and simulate controllable asymmetric attack of delay and attenuation. Both asymmetric attacks can make the clock in the remote module slower by attacking the 1pps signal from local to remote side. Otherwise, the clock will be faster. In this paper, the asymmetric delay attack can linearly control the synchronization error from 0 to 300 ps. The asymmetric attenuation attack can adjust the synchronization error from 0 ~ 302.7 ps with the controllable attenuation from 0 to 2.8 dB. Moreover, we find that the time interval counter change greatly when the system is attacked. The research has a significant meaning in defense of such asymmetric attacks.
Privacy amplification (PA) is an essential process for high-speed and real-time implementation of a continuous-variable quantum key distribution (CV-QKD) system. This work focuses on the improvement of the performance of PA, and we realize PA with Toeplitz matrix and accelerate it using fast Fourier transform (FFT) on graphic processing unit (GPU). Based on the architectural feature of FFT, we adjust its form of input length and obtained an average speed of PA about 2Gbps with input length ranges from 1Mbits to 128Mbits, which is length-adaptable to satisfy various requirements of CV-QKD systems at different transmission distances. Furthermore, we test this work with different compress ratios of PA, which can also achieve a high implementation speed around 2Gbps. With the method used in this paper, the requirements of PA for the high-speed and real-time CV-QKD system can be entirely satisfied.
Throughput of error correction is the bottleneck of the postprocessing for continuous-variable quantum key distribution system. In this paper, a shuffled iterative decoding method is proposed to reduce the number of iterations for error correction. For three typical code rate, i.e., 0.1, 0.05, 0.02, our results show that the maximum decoding speed is up to 72.86 Mbits/s, 53.96 Mbits/s and 42.45 Mbits/s, respectively, which significantly improves the real-time processing capacity of continuous-variable quantum key distribution system.
In this paper, a novel experimental preparation scheme of Gaussian modulated coherent state (GMCS) in continuous variable quantum key distribution (CVQKD) system is proposed based on dual-drive Mach-Zehnder modulator (DDMZM). The experimental implementation of the proposed GMCS preparation scheme only depends on a DDMZM instead of an AM and a PM in conventional CVQKD, which simplifies the experimental setup and reduces the costs of the CVQKD system. Moreover, the sum-difference signals of the Rayleigh distribution and uniform distribution are applied on two parallel electrodes of the DDMZM, respectively, getting rid of the accurate time-delay alignment between the AM and the PM in conventional Gaussian modulation scheme. Besides, the measurement method of the prepared GMCS is experimentally demonstrated based on heterodyne detection, and both quadrature (X and P) are simultaneously measured to verify the proposed GMCS preparation scheme.
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