Physical Layer Aspects of Quantum Communications: A Survey

TL;DR

This survey analyzes the physical-layer building blocks of quantum communications, comparing them to classical wireless systems and identifying how classical techniques (e.g., MIMO, WDM, adaptive optics) can be re-purposed for quantum links. It provides a comprehensive review of quantum sources, channels, detectors, and modulation across DV and CV paradigms, as well as media-specific issues in fiber, free space, and integrated photonics. The authors discuss advanced concepts such as quantum coherent control, quantum MIMO, and bosonic-codes (e.g., Cat and GKP) that enable interfacing DV and CV systems, while outlining open challenges in interference (Hong–Ou–Mandel), frequency-selective fading, and channel-state assessment. Overall, the paper argues for leveraging mature classical techniques to accelerate quantum-network development, while highlighting uniquely quantum mechanisms necessary for robust, scalable quantum communications.

Abstract

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.

Figures (11)

• Figure 1: The general structure of a quantum MIMO channel. Preprocessing and post-processing are required to take full advantage of the higher dimensions offered by the MIMO setup. DV to CV and CV to DV blocks might be required only in some select scenarios, e.g., when connecting DV quantum nodes while transmitting CV modes over the channel.
• Figure 2: Paper Structure
• Figure 3: Markovian cross-talk acting on different gates that are spacelike separated.
• Figure 4: Non-Markovian cross-talk acting on different gates that are timelike separated.
• Figure 6: Crosstalk in six-core fiber. Signals transmitted through neighboring cores at the same wavelength can crosstalk with each other, as illustrated with cores $C_1$ and $C_2$.