水下量子通信的数值模拟及误码率分析

Simulation and QBER Analysis of Underwater Quantum Communication

水下量子密钥分配可以为水下通信提供绝对安全的保密手段。本文采用蒙特卡洛方法,结合海水信道的光学性质和光子的量子特性模拟了光子在海水中的传输过程,研究其衰减和偏振特性,计算了接收到的光子数随接收端口径、视场角和传输距离的变化,从保真度的角度分析散射光的偏振变化情况,并结合背景光的影响分析了水下量子通信误码率。结果表明,水下量子通信理论上可以实现百米量级的安全通信。

Underwater quantum key distribution（QKD）can provide absolute security for underwater communication.In this paper,we study the attenuation and depolarization of photons propagating underwater with Monte Carlo method,based on the Mie scattering theory and the vector Radioactive transport theory and combined with the optical properties of ocean and the quantum properties of photons.Firstly,we set up an ocean channel modeling.The ocean water in our model is considered as the mixture of pure seawater and some kinds of particles meeting a certain particle size distribution,and the effects of ocean on the photons that we mainly consider are absorption and scattering.The absorption will mainly affect the number of detected photons and the scattering will affect both the number and the polarization states of the detected photons.Secondly,we simulate the transmission of photons in the clearest ocean when the attenuation coefficient is 0.03/m.Because bit rate and quantum bit error rate（QBER）are two important parameters in QKD,we then investigate the varying of the number of detected photons,which will directly affect the bit rate,with the distance,aperture and field of view（FOV）of the detector,and we analyze the depolarization of the detected photons,which will directly affect the QBER.Finally,we study the QBER of underwater QKD when background noise is considered.Our simulation results show that,because of the effect of water,the transmission distance is limited at a level of 100 meters,where the number of detected photons will keep around 4.9% of the initial one,which is close to the theoretical result,5%.When the distance is 60 m,the number of detected photons increase by at most about 8% as the aperture of the detector increase from 4cm to 50 cm and by at most about 1% as the FOV increase from 174 mrad to522mrad.When the aperture of the detector is 20 cm and the FOV is 174 mrad,the fidelity of detected photons keeps close to 1,which means that the polarization states of most of the detected photons keep unchanged or change a little.When we consider the effect of background noise on the QBER under the condition that the aperture of the detector,FOV and the irradiance is 5cm,174 mrad and 1×10-6 Wm2 separately,the QBER is 10% at the distance of 60 meter,which is absolutely secure for any kind of attack,and 24.4% at the distance of 105 meter,which is able to resist intercept and re-send attack.Our conclusion is that when background noise is weak it is theoretically feasible to have absolutely secure underwater QKD up to about 60 min the clearest ocean and secure against simple intercept and re-send attacks up to more than 100 min the same type of ocean water.