In this paper we consider the detection of noisy binary quantum optical coherent states transmitted over a turbulent medium, such as terrestrial links or satellite optical connections across the atmosphere. The propagation effect is accounted for by a random attenuation factor, which is characterized in terms of its statistical distribution. Depending on the knowledge of the propagation coefficient, error performance can vary significantly and different receiver structures show different behaviors and robustness to the channel mismatch. The performance of the quantum optimal receiver and the square root measurement (SRM) are compared, and their sensitivity to the variability of the propagation coefficient is considered. It results that the phase variability of the propagation coefficient plays a fundamental role. The optimal receiver, in the absence of instantaneous channel knowledge, performs worse than the SRM, denoting a greater sensitivity to channel estimation accuracy.
In this paper we consider the detection of noisy binary quantum optical coherent states transmitted over a turbulent medium, such as terrestrial links or satellite optical connections across the atmosphere. The propagation effect is accounted for by a random attenuation factor, which is characterized in terms of its statistical distribution. Depending on the knowledge of the propagation coefficient, error performance can vary significantly and different receiver structures show different behaviors and robustness to the channel mismatch. The performance of the quantum optimal receiver and the square root measurement (SRM) are compared, and their sensitivity to the variability of the propagation coefficient is considered. It results that the phase variability of the propagation coefficient plays a fundamental role. The optimal receiver, in the absence of instantaneous channel knowledge, performs worse than the SRM, denoting a greater sensitivity to channel estimation accuracy.
We develop an exact error probability framework for quantum free space optics (FSO) communication over atmospheric channels with turbulence and pointing errors under a physically admissible bounded transmissivity model. The proposed framework is grounded in the fundamental physical observation that, in a quantum optical channel, the random channel quantity is the transmissivity itself, namely, the fraction of transmitted photons collected at the receiver, and must therefore take values only in the physically admissible interval [0,1]. This constraint is generally violated when classical turbulence models are interpreted directly as transmissivity models in quantum settings. Accordingly, we model the turbulence-induced transmissivity as a Beta distributed random variable over (0,1), which has recently been shown to provide accurate fitting under different turbulence conditions while preserving physical admissibility. We then integrate deterministic atmospheric loss and pointing errors into a unified equivalent transmissivity model and derive exact expressions for its probability density function and moment generating function. Building upon this statistical characterization, we derive novel exact average error probability expressions for binary modulation under both quantum and classical receivers. Finally, we verify the analysis using Monte Carlo simulations, quantify the impact of turbulence and pointing errors, and demonstrate the performance advantage of quantum optimal detection in photon-limited quantum FSO channels.
This paper investigates the performance of coherent-state quantum optical wireless communication systems operating over atmospheric turbulence channels with pointing errors (PEs). The average error probability (AEP) and average achievable rate (AAR) are analyzed for on–off keying (OOK) and binary phase-shift keying (BPSK) signaling under photon-counting (PC), homodyne, and optimal Helstrom detection. The proposed framework jointly incorporates deterministic path loss, atmospheric turbulence, and PEs, and accommodates multiple statistical channel models, including Gamma-Gamma (GG), doubly inverted Gamma-Gamma (IGGG), and Fisher-Snedecor F distributions, within a unified analytical formulation. Closed-form expressions for AEP and AAR are derived via direct probability density function (PDF)-based integration of the conditional error probability, along with high-photon-number asymptotic approximations that provide insight into performance scaling behavior. Numerical results validate the analytical expressions and demonstrate the impact of PEs on system performance. The analysis further enables systematic comparison between classical and optimal quantum detection schemes and quantifies the achievable performance gains under different channel conditions. These results provide useful design insights for modulation and receiver selection in turbulent free-space optical (FSO) communication systems.
Many fundamental and applied experiments in quantum optics require transferring nonclassical states of light through large distances. In this context the free-space channels are a very promising alternative to optical fibers as they are mobile and enable to establish communications with moving objects, using satellites for global quantum links. For such channels the atmospheric turbulence is the main disturbing factor. The statistical properties of the fluctuating transmittance through the turbulent atmosphere are given by the probability distribution of transmittance (PDT). We derive the consistent PDTs for the atmospheric quantum channels by step-by-step inclusion of various atmospheric effects such as beam wandering, beam broadening and deformation of the beam into elliptic form, beam deformations into arbitrary forms. We discuss the applicability of PDT models for different propagation distances and optical turbulence strengths in the case when the receiver module has an annular aperture. We analyze the optimal detection and correction strategies which can improve the channel transmission characteristics. The obtained results are important for the design of optical experiments including postselection and adaptive strategies and for the security analysis of quantum communication protocols in freespace.
In this letter, we analyze the average probability of error of quantum modulation schemes in weakly turbulent quantum communication channels utilizing coherent states. Atmospheric turbulence-induced fading is a significant challenge that degrades the performance of free-space quantum communication systems. We derive and present closed-form expressions for the average probability of error for various detection methods: classical (shot-noise limited) single-photon detection, coherent detection, and the optimal quantum hypothesis-testing (Helstrom) detection for binary quantum modulation schemes. Our analysis focuses on the transmission of classical data modulated by quantum coherent states over a log-normal turbulent channel. Additionally, we discuss the lower bounds of the average capacities for Helstrom detection, providing comprehensive insights into the achievable information rates in realistic atmospheric conditions. The derived expressions show excellent agreement with numerical Monte Carlo simulations across a broad range of average transmitted photon numbers, offering valuable guidelines for the design and optimization of quantum communication systems.
Abstract Future quantum communication infrastructures will rely on both terrestrial and space-based links integrating high-performance optical systems engineered for this purpose. In space-based downlinks in particular, the loss budget and the variations in the signal propagation due to atmospheric turbulence effects impose a careful optimization of the coupling of light in single-mode fibers required for interfacing with the receiving stations and the ground networks. In this work, we perform a comprehensive study of the role of adaptive optics (AO) in this optimization, focusing on realistic baseline configurations of prepare-and-measure quantum key distribution, with both discrete and continuous-variable encoding, and including finite-size effects. Our analysis uses existing experimental turbulence datasets at both day and night time to model the coupled signal statistics following a wavefront distortion correction with AO, and allows us to estimate the secret key rate for a range of critical parameters, such as turbulence strength, satellite altitude and ground telescope diameter. The results we derive illustrate the interest of adopting advanced AO techniques in several practical configurations.
Abstract Since the invention of the laser in the 60s, one of the most fundamental communication channels has been the free-space optical channel. For this type of channel, a number of effects generally need to be considered, including diffraction, refraction, atmospheric extinction, pointing errors and, most importantly, turbulence. Because of all these adverse features, the free-space channel is more difficult to study than a stable fiber-based link. For the same reasons, only recently it has been possible to establish the ultimate performances achievable in quantum communications via free-space channels. Differently from previous literature, mainly focused on the regime of weak turbulence, this work considers the free-space optical channel in the more challenging regime of moderate-to-strong turbulence. This regime may occur in long-distance free-space links on the ground, in uplink to high-altitude platform systems (HAPS) and, more interestingly, in downlink from near-horizon satellites. In such a regime we rigorously investigate ultimate limits for quantum communications and show that composable keys can be extracted using continuous variable quantum key distribution. In particular, we apply our results to downlink from satellites at large zenith angles, for which not only turbulence is strong but also refraction causes non-trivial effects in terms of trajectory elongation.