We calculate the magnitude of cross phase modulation (XPM) induced by classical channels, and analyze its impact on quantum secure key rate. The results show that the XPM induced by classical signals is small and its impact on the quantum key distribution can be neglected.
In this paper, the crosstalk-induced impact of coexisting dense-wavelength-division-multiplexed (DWDM) channels on continuous-variable quantum key distribution (CV-QKD) in practical communication environment is investigated. We analyze the nonlinear effect resulting from classical channels on the transmitted local oscillator in CV-QKD, while various existing studies mainly focus on the crosstalk-induced excess noise. We demonstrate that the generally ignored phenomenon of four-wave mixing in excess noise analysis is a visible factor causing local oscillator fluctuation characterized by the statistical properties of the power evolution of local oscillator. Besides, the impact of stimulated Raman scattering on local oscillator is derived in the same theoretical approach and experimentally investigated. Finally, by establishing a trusted model for local oscillator fluctuation we perform the security analysis to constrain the threshold of fluctuation. All of the above analysis should be taken into account as a supplementary for evaluating the performance of CV-QKD with coexisting DWDM system and optimal allocation of classical and quantum channels.
In continuous-variable quantum key distribution (CV-QKD), the key information are encoded on quadratures of the optical field, which are measured via balanced homodyne detector (BHD). The bandwidth of the BHD is one of key parameters for precise characterization of quantum states. We establish a theoretical model to analyze the impact of the BHD bandwidth and signal modulation patterns on the channel parameters estimation of CV-QKD systems. Based on the proposed model, the secure key rate of a practical CV-QKD system under different BHD bandwidths and signal modulation patterns are investigated. Our results show that insufficient BHD bandwidth will result in wrong estimate of the transmission loss and excess noise, which significantly affects the performance of CV-QKD systems. Given the BHD bandwidth, there exists an optimal signal repetition rate that maximizes the secure key rate. The BHD bandwidth requirement of the QKD system increases with the transmission distance for large duty cycle pulse. Furthermore, the root raised-cosine pulse signal modulation performs better than the square pulse signal modulation in general.
Abstract Continuous-variable quantum key distribution (CV-QKD) provides a theoretical unconditionally secure solution to distribute symmetric keys among users in a communication network. However, the practical devices used to implement these systems are intrinsically imperfect, and, as a result, open the door to eavesdropper attacks. In this work, we study the impact of transmitter stage imperfections on the performance and security of a Discrete Modulated (DM) CV-QKD system using M-symbol Quadrature Amplitude Modulation (M-QAM) and Amplitude and Phase Shift Keying (M-APSK) coupled with Probabilistic Constellation Shaping (PCS). Assuming two different modulation stage topologies, we first deform the constellations and then evaluate the secure key rate achievable with the deformed constellation. The presented results show that, due to the erroneously estimated channel parameters, non-monitored imbalances greatly reduce the system's performance, with situations where Bob and Alice estimate that no secure bits can be obtained while the real value of the key rate is still positive. Our results show the importance of monitoring these constellation imbalances and show that the optimal constellation may vary depending on the degree of device imperfection.
Quantum key distribution (QKD) leverages the principles of quantum mechanics to enable secure secret key exchange. Continuous-variable QKD (CV-QKD), utilizing coherent states and coherent detection, has emerged as a promising method, easily integrable into existing telecom networks. In this work, we study, through numerical simulation, the coexistence of a CV-QKD channel with a classical channel capable of up to 1 Gb/s. We first analyze the performance of the quantum channel when the classical channel emits a pure tone, by varying the frequency gap between both channels and adjusting their power ratios. Then, by maintaining a fixed electrical separation of 500 MHz, we study QPSK modulation with root-raised-cosine filtering in the classical channel. In this scenario, for several signal-to-noise ratios, we reduce crosstalk by optimizing filter properties such as roll-off factor, symbol rate and number of filter taps.
Continuous-variable quantum key distribution (CV-QKD) with a real local oscillator (LO) experiences a phase attack security challenge due to the reference pulses transmitted along with quantum signals over an unsecure quantum channel. In this paper, a phase attack on reference pulses of CV-QKD with a real LO is investigated. The performance of the discrete-modulated (DM) CV-QKD under this attack is analyzed analytically and by simulation, and compared with the Gaussian-modulated (GM) CV-QKD. It is found that such an attack by an eavesdropper's, significantly reduce the transmission distance for the GM based CV-QKD, whereas, the DM based CV-QKD shows a high tolerance to high phase noise. Based on phase attack monitoring, a new adaptive modulation scheme is proposed for CV-QKD to adaptively switch between GM and DM so as to maintain the optimum secret key rate.