Spatial-Mode Diversity and Multiplexing for Continuous Variables Quantum Communications

We investigate the performance of continuous-variable (CV) quantum communication systems employing diversity schemes to mitigate the effects of realistic channel conditions, including Gaussian lossy channels, fading, and crosstalk. By modeling the transmittivity of the channel as a log-normal distribution, we account for the stochastic nature of fading. We analyze the impact of both post-processing amplification at the receiver and pre-amplification at the transmitter on the fidelity of the communication system. Our findings reveal that diversity schemes provide significant advantages over single-channel transmission in terms of fidelity, particularly in conditions of strong fading and high thermal background noise. We also explore the effect of crosstalk between channels and demonstrate that a noticeable advantage persists in scenarios of strong fading or thermal noise. For CV-QKD, we show that diversity can outperform multiplexing in terms of average secret key rate, revealing a diversity advantage over multiplexing in some regimes.