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Nanosystems Physics Chemistry Mathematics

Increase of signal to reference ratio for phase compensation in continuous-variable quantum key distribution systems

Fedor Goncharov, B. E. Pervushin, Boris Aleksandrovich Nasedkin, R. Goncharov, D.A. Yashin, Mikhail Gellert, D.V. Sulimov, P. A. Morozova, I. M. Filipov, I.A. Adam, Vladimir Chistiakov, E.O. Samsonov, V. I. Egorov

Published Feb 28, 2023• 0 citations
Publisher

Abstract

Continuous variables quantum key distribution (CV-QKD) systems are a promising direction for quantum communications.Coherent detection, which is the basis of CV-QKD, requires taking into consideration and compensating phase distortions.Phase compensation algorithms rely on using reference pulses for phase drift estimation and correcting signal quadratures.The ratio of the number of reference pulses to that of the signal ones, affects the accuracy of the phase compensation algorithm.On the other hand, it influences the secure key rate (SKR).The paper considers the effect of the reference to signal ratio on the SKR, and proposes a modification of the phase compensation algorithm, which allows using a smaller number of references at a pulse repetition frequency close to that of the system phase noise, which results in increasing SKR.We also propose a method for estimating the phase noise in the system for selection of the optimal signal to reference ratio.

DOI
10.17586/2220-8054-2023-14-1-59-68

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Continuous variables quantum key distribution (CV-QKD) systems are a promising direction for quantum communications.Coherent detection, which is the basis of CV-QKD, requires taking into consideration and compensating phase distortions.Phase compensation algorithms rely on using reference pulses for phase drift estimation and correcting signal quadratures.The ratio of the number of reference pulses to that of the signal ones, affects the accuracy of the phase compensation algorithm.On the other hand, it influences the secure key rate (SKR).The paper considers the effect of the reference to signal ratio on the SKR, and proposes a modification of the phase compensation algorithm, which allows using a smaller number of references at a pulse repetition frequency close to that of the system phase noise, which results in increasing SKR.We also propose a method for estimating the phase noise in the system for selection of the optimal signal to reference ratio.
Continuous-variable quantum key distribution (CVQKD) with a real local oscillator (LO) is confronted with new security issues attributed to the reference pulses transmitted together with quantum signals over the insecure quantum channel. This paper proposes a method of phase attack on reference pulses of the CVQKD with real LOs. Under the phase attack, the phase drifts of reference pulses are manipulated by eavesdroppers so that the phase compensation error is increased. Consequently, the secret key rate is reduced because of the imperfect phase compensation for quantum signals. Based on the noise model of imperfect phase compensation, both the parameters of actual transmittance and actual excess noise are deduced from the phase compensation accuracy, and then they are compared with those estimated by training signals. The simulation results show that the secret key rate calculated from the theoretical parameters is consistent with the one estimated by training signals.
In Local-local Oscillator (LLO) based Continuous-Variable Quantum Key Distribution (CV-QKD), the phase reference of the transmitter and receiver, Alice and Bob, are naturally de-correlated due to their use of individual lasers. A phase reference signal is used, whose measurement is critical for estimating the phase difference and correcting the raw QKD data. We observed that asymmetry in the quadrature measurements of the shot noise-limited heterodyne detector affects the accuracy of the reference signal's phase estimation and thereby reduces the achievable transmission distance and key rate of the CV-QKD system. We quantify the effect and propose a method to counteract the effect of detection asymmetry. We also evaluate the effects of detection asymmetry using quantum optical tomography.
Abstract We propose a high-rate phase reference scheme for continuous-variable quantum key distribution (CV-QKD). In this scheme, a transmission mode has been employed in which several quantum signal pulses are accompanied by a pair of reference pulses, calibrating the measurement bases of the communicating parties. Within a certain range, and with the ratio between quantum signal pulses and reference pulses increased, the secret key rate approaches that of the traditional CV-QKD scheme (which is a scheme without phase noise). Compared with general CV-QKD schemes, which adopt the transmission mode of a quantum signal pulse plus a pair of reference pulses, this scheme has a higher spectral efficiency, enabling high-rate CV-QKD. In addition, we reduce the interference of strong reference pulses with weak quantum signal pulses by using a pair of polarization beam splitters with a dynamic polarization controller. Simulation results demonstrate that we achieve a higher secret key rate than the self-coherent phase reference scheme. This provides a pathway towards large-scale quantum communication networks.
Quantum key distribution (QKD) has been proven to be theoretically unconditionally secure. However, any theoretical security proof relies on certain assumptions. In QKD, the assumption in the theoretical proof is that the security of the protocol is considered under the asymptotic case where Alice and Bob exchange an infinite number of signals. In the continuous-variable QKD (CV-QKD), the finite-size effect imposes higher requirements on block size and excess noise control. However, the local local oscillator (LLO) CV-QKD system cannot be considered time-invariant under long blocks, especially in cases of environmental disturbances. Thus, we propose an LLO CV-QKD scheme with time-variant parameter estimation and compensation. We first establish an LLO CV-QKD theoretical model under the temporal modes of continuous-mode states. Then, a robust method is used to compensate for arbitrary frequency shift and arbitrary phase drift in CV-QKD systems with longer blocks, which cannot be achieved under traditional time-invariant parameter estimation. Besides, the digital signal processing method predicated on high-speed reference pilots can achieve a time complexity of O (1). In the experiment, the frequency shift is up to 89.05 MHz/s and phase drift is up to 3.036 Mrad/s using a piezoelectric transducer (PZT) to simulate the turbulences in the practical channel. With a signal-to-interference ratio (SIR) of −51.67 dB , we achieve a secret key rate (SKR) of 0.29 Mbits/s with an attenuation of 16 dB or a standard fiber of 80 km. This work paves the way for future long-distance field-test experiments in the finite-size regime.
In order to ensure the security of the local oscillator (LO) in continuous variable quantum key distribution (CV-QKD), the LO in CV-QKD based on the local LO (LLO) scheme no longer passes through the quantum channel. For the LLO scheme using homodyne detection, two orthogonal quadratures of reference pulse can only be measured sequentially, which, as a result, each signal pulse should be accompanied by a pair of identical reference pulses. However, due to the phase shift between two adjacent pulses, it is difficult to prepare dual reference pulses with the same phase relative to the LO. In this paper, we propose an improved method to get around this difficulty, which is demonstrated experimentally for a transmitter and a receiver connected by a 50 km single mode fiber. The phase noise measured in the experiment is 2.0×10−3rad2, which is comparable to the recent experimental results using heterodyne detection. This method simplifies the preparation of phase reference pulses and removes the obstacles for the application of homodyne detection in the LLO scheme. In addition, this method can also be used for the compensation of quadrature imbalance in heterodyne detection, then the performance of heterodyne detection can be improved.
Phase drift extremely limits the secure key rate and secure transmission distance, which is non-negligible in local oscillation continuous variable quantum key distribution (LLO CV-QKD). In order to eliminate the impact caused by phase drift, we analyze the phase noise of the system and propose a phase compensation method based on convolutional neural network (CNN). Moreover, the compensation is performed on the signal according to the estimated value of phase drift before coherent detection. In numerical simulation, we compare the performance of phase compensation methods based on CNN and Kalman filter (KF), and the results show that CNN-based phase compensation has higher accuracy and stability.