Quantum communication systems support unique applications in the form of distributed quantum computing, distributed quantum sensing, and several cryptographic protocols. The main enabler in these communication systems is an efficient infrastructure that is capable to transport unknown quantum states with high rate and fidelity. This feat requires a new approach to communication system design which efficiently exploits the available physical layer resources, while respecting the limitations and principles of quantum information. Despite the fundamental differences between the classic and quantum worlds, there exist universal communication concepts that may proven beneficial in quantum communication systems as well. In this survey, the distinctive aspects of physical layer quantum communications are highlighted in a attempt to draw commonalities and divergences between classic and quantum communications. More specifically, we begin by overviewing the quantum channels and use cases over diverse optical propagation media, shedding light on the concepts of crosstalk and interference. Subsequently, we survey quantum sources, detectors, channels and modulation techniques. More importantly, we discuss and analyze spatial multiplexing techniques, such as coherent control, multiplexing, diversity and MIMO. Finally, we identify synergies between the two communication technologies and grand open challenges that can be pivotal in the development of next-generation quantum communication systems.

翻译：量子通信系统以分布式量子计算、分布式量子传感及多种密码协议等形式支持独特应用。这些通信系统中的主要使能技术是一种能够以高速率和高保真度传输未知量子态的高效基础设施。这一壮举需要一种新的通信系统设计方法，该方法能有效利用可用的物理层资源，同时遵循量子信息的限制与原理。尽管经典世界与量子世界存在根本差异，但仍存在一些普适的通信概念，这些概念也可能在量子通信系统中被证明是有益的。本综述重点阐述了物理层量子通信的独特方面，试图梳理经典通信与量子通信之间的共性与差异。具体而言，我们首先概述了不同光传播介质上的量子信道与用例，阐明了串扰和干扰的概念。随后，我们综述了量子源、探测器、信道及调制技术。更重要的是，我们讨论并分析了空间复用技术，如相干控制、复用、分集与MIMO。最后，我们识别了这两种通信技术之间的协同效应，以及可能对下一代量子通信系统发展至关重要的重大开放挑战。