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Free-space optical (FSO) communication has attracted significant interest recently. This technology can potentially complement or be an integral part of next-generation networks. FSO links provide several advantages compared to conventional radio frequency links, including higher data rates, license-free spectrum, and power efficiency. Furthermore, they could be used to access users in remote areas where optical fiber communications are unavailable.Here, we present a simulation framework for modeling, designing, and analyzing classical and quantum communication systems over terrestrial and satellite free-space optical links. We address different FSO use cases in terrestrial, ground-tosatellite, satellite-to-ground, and inter-satellite links using direct-and coherent detection schemes. For the FSO channel modeling, we discuss two methods. The first approach considers atmospheric scintillation, pointing errors, the Doppler effect, attenuation due to the beam diffraction, and scintillation-induced divergence. The second method captures the wavefront of the optical beam using the phase screens technique, which provides a more detailed description of the signal propagation. Additionally, we provide essential details about system-level simulations to analyze and optimize the entire link performance. Finally, we discuss the simulation environment for designing quantum-key distribution (QKD) systems as an FSO use case.Using this simulation framework, we investigate the performance of several different FSO application examples: a terrestrial link with spatial diversity receivers, inter-satellite communication from low Earth to geostationary orbit, and a polarization-encoded BB84 with decoy states QKD over a satellite downlink.

Quantum imaging can beat classical resolution limits, imposed by the diffraction of light. In particular, it is known that one can reduce the image blurring and increase the achievable resolution by illuminating an object by entangled light and measuring coincidences of photons. If an n-photon entangled state is used and the nth-order correlation function is measured, the point-spread function (PSF) effectively becomes $$\sqrt{n}$$ n times narrower relatively to classical coherent imaging. Quite surprisingly, measuring n-photon correlations is not the best choice if an n-photon entangled state is available. We show that for measuring (n − 1)-photon coincidences (thus, ignoring one of the available photons), PSF can be made even narrower. This observation paves a way for a strong conditional resolution enhancement by registering one of the photons outside the imaging area. We analyze the conditions necessary for the resolution increase and propose a practical scheme, suitable for observation and exploitation of the effect.

We demonstrate how an existing simulation framework for modelling classical optical systems can be utilized for simulations of weak-coherent QKD links.

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