Numerical simulations of free-space quantum

Abstract

Free-space quantum channels have attracted much attention due to various practical advantages. However, quantum light at the receiver stations of such channels is highly noisy—atmospheric turbulence leads to refractive index fluctuations, which in turn leads to fluctuations of the channel transmittance. These fluctuations represent a random process, whose characteristics are important for a proper understanding of quantum state transfer. Various analytical models for the probability distribution of transmittance (PDT) have been introduced in the literature; see, e.g., Refs. [1, 2]. However, other characteristics of this random process, such as time correlations and their impact on nonclassical properties of the transmitted light, have not been properly discussed. Using the sparse-spectrum model [3] of the phase screen method [4], we have provided numerical simulations of quantum light in free-space channels [5]. In particular, we have validated existing analytical models of the PDT. We have shown that the turbulence strength alone is not sufficient to determine the applicability of the models, as the aperture size should also be considered as a key factor. In addition, we proposed a new heuristic analytical model based on the beta distribution, which demonstrated superior agreement with numerical simulations over almost the entire domain of channel parameters. We further investigated the temporal behavior of the system [6] by considering the coherence time between pulses as a function of channel parameters. This study provides insight into the persistence of continuous and discrete entanglement between two pulses in turbulent atmosphere and explores the possibility of preserving nonclassicality of quantum states by means of an adaptive selection procedure based on the time interval between pulses.