Recently, the time synchronization via optical fiber, (e.g., two-way fiber time transfer technique, and round-trip time transfer technique) is more and more attractive, due to its higher precision and longer distance. However, because this method is based on the assumption that the forward and backward propagation delays of one optical fiber are nearly the same, it may fail to achieve time synchronization due to link asymmetry. To resolve this problem, we propose a time filtering method to detect link asymmetry and mitigate time asynchronization caused by link asymmetry. Specifically, we create an array to record four time parameters, i.e., (1) round-trip time, (2) time offset between Server and Client, (3) the difference of round-trip time, and (4) the difference of time offset. The four time parameters are calculated for each synchronization period. The third and fourth parameters are used to detect link asymmetry, and the second parameter is used to compensate the symmetry of the transmission delay, and minimize the time asynchronization. We simulate our method on a two-way fiber time synchronization system with OptiSystem. Simulation results show that our method can quickly detect link asymmetry within 1 second, and achieves nanoseconds time synchronization in the presence of link asymmetry.
The phase compensation with high accuracy is one of the key technologies in continuous variable quantum key distribution (CVQKD) system, which directly influences the secure key rate and transmission distance. However, traditional phase compensation method cannot accurately estimate the phase drift due to the additive noise introduced by coherent detector. In this paper, we propose a new phase compensation method based on mean denoising, where a training sequence is designed for estimating phase drift in the transmitter (Alice) and an average of the multi-points in the training sequence is estimated to remove the influence of additive noise. Simulation results show that the compensation accuracy of the proposed method can reach 0.9932, which is 20% better than that based on traditional method. Our method can significantly reduce the influence of additive noise, and improve the system performance by controlling excess noise in phase compensation process.
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.
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.
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