Quantum communication is a holy grail to achieve secure communication among a set of partners, since it is provably unbreakable by physical laws. Quantum sensing employs quantum entanglement as an extra resource to determine parameters by either using less resources or attaining a precision unachievable in classical protocols. A paradigmatic example is the quantum radar, which allows one to detect an object without being detected oneself, by making use of the additional asset provided by quantum entanglement to reduce the intensity of the signal. In the optical regime, impressive technological advances have been reached in the last years, such as the first quantum communication between ground and satellites, as well as the first proof-of-principle experiments in quantum sensing. The development of microwave quantum technologies turned out, nonetheless, to be more challenging. Here, we will discuss the challenges regarding the use of microwaves for quantum communication and sensing. Based on this analysis, we propose a roadmap to achieve real-life applications in these fields.
Quantum communication is a holy grail to achieve secure communication among a set of partners, since it is provably unbreakable by physical laws. Quantum sensing employs quantum entanglement as an extra resource to determine parameters by either using less resources or attaining a precision unachievable in classical protocols. A paradigmatic example is the quantum radar, which allows one to detect an object without being detected oneself, by making use of the additional asset provided by quantum entanglement to reduce the intensity of the signal. In the optical regime, impressive technological advances have been reached in the last years, such as the first quantum communication between ground and satellites, as well as the first proof-of-principle experiments in quantum sensing. The development of microwave quantum technologies turned out, nonetheless, to be more challenging. Here, we will discuss the challenges regarding the use of microwaves for quantum communication and sensing. Based on this analysis, we propose a roadmap to achieve real-life applications in these fields.
The thesis establishes a link between theoretical foundations and practical applications in the emerging field of propagating quantum microwaves. Although the concrete focus of the main results lies in specific quantum communication and sensing protocols, the thesis also gives a self-contained introduction to quantum parameter estimation and Gaussian quantum continuous variables, justifying the theoretical results used. Motivated, firstly, by the compatibility between superconducting circuits (a promising quantum computing platform), and the microwave frequency range; and, secondly, by the transparency window of the atmosphere to these frequencies, the work contains two concrete contributions to the fields of microwave quantum sensing and communication: a novel protocol that uses frequency entanglement to measure the first order dependence in frequency of the reflectivity of an object; and a large investigation on different aspects of entanglement degradation due to loss and atmospheric absorption in the context of continuous-variable entanglement distribution for the task of quantum teleportation in open-air. The dissertation includes, as a last chapter, an article that reviews the state-of-the-art in quantum microwave technology, and proposes different research lines, including quantum communication between Earth-based stations and satellites, quantum radar, direct dark matter detection, and exploration of the quantum properties of the cosmic microwave background. This more speculative, yet rigorous chapter closes with a roadmap for possible future research lines in the field of propagating quantum microwaves, that can serve as an outlook of the thesis.
This Thesis explores the limits in the application of propagating quantum microwaves for quantum communication and quantum sensing, as well as the design of new devices and protocols to fight these limitations. We take advantage of Gaussian quantum states for quantum teleportation and quantum illumination, and studies how these protocols can be improved using entanglement distillation and partial purification, respectively. The Thesis is centered around open-air entanglement distribution, and it follows the steps of state generation inside the cryostat, impedance matching between the cryostat and the open air with a new generation of coplanar antennae, and open air propagation, in the limited framework of current microwave technology. We also address the limitations produced by losses and measurement inefficiencies, and explore the extension to satellite quantum communications. There, we analyze the effects of diffraction and turbulence, studying how the latter affects signals in the optical regime as well. We conclude by studying the teleportation of quantum information in a quantum local area network. To sum up, this Thesis contributes to the development of wireless quantum communications in the microwave regime, studying its technological limitations and how to overcome them. Nevertheless, quantum technologies working in this frequency range are still emergent and plenty of work must be accomplished in order to make them competitive.
Quantum communications has emerged as an important paradigm for achieving secure fast communications. Significant technological breakthroughs have been achieved in this area over the years using quantum optics. In contrast, quantum microwaves are recently beginning to receive increased attention, due in part to the potential for an easier integration with current microwave communications infrastructure. This paper explores the generation, propagation and detection of quantum microwaves as processes required for open-air communication. It presents a brief description of the theory and provides a summary of recent progress in these operations. The paper ends by highlighting challenges regarding the practical realization of this technology.
Microwave technology plays a central role in current wireless communications, including mobile communication and local area networks. The microwave range shows relevant advantages with respect to other frequencies in open-air transmission, such as low absorption losses and low-energy consumption, and in addition, it is the natural working frequency in superconducting quantum technologies. Entanglement distribution between separate parties is at the core of secure quantum communications. Therefore, understanding its limitations in realistic open-air settings, especially in the rather unexplored microwave regime, is crucial for transforming microwave quantum communications into a mainstream technology. Here, we investigate the feasibility of an open-air entanglement distribution scheme with microwave two-mode squeezed states. First, we study the reach of direct entanglement transmission in open air, obtaining a maximum distance of approximately 500 m with parameters feasible for state-of-the-art experiments. Subsequently, we adapt entanglement distillation and entanglement swapping protocols to microwave technology in order to reduce the environment-induced entanglement degradation. The employed entanglement distillation helps to increase quantum correlations in the short-distance low-squeezing regime by up to $46\mathrm{%}$, and the reach of entanglement increases by $14\mathrm{%}$ with entanglement swapping. Importantly, we compute the fidelity of a continuous-variable quantum teleportation protocol using open-air-distributed entanglement as a resource. Finally, we adapt this machinery to explore the limitations of quantum communication between satellites, where the impact of thermal noise is substantially reduced and diffraction losses are dominant.
Abstract The field of propagating quantum microwaves is a relatively new area of research that is receiving increased attention due to its promising technological applications, both in communication and sensing. While formally similar to quantum optics, some key elements required by the aim of having a controllable quantum microwave interface are still on an early stage of development. Here, we argue where and why a fully operative toolbox for propagating quantum microwaves will be needed, pointing to novel directions of research along the way: from microwave quantum key distribution to quantum radar, bath-system learning, or direct dark matter detection. The article therefore functions both as a review of the state-of-the-art, and as an illustration of the wide reach of applications the future of quantum microwaves will open.
Quantum communication and quantum sensing, which leverage the unique characteristics of quantum systems, enable information-theoretically secure communication and high-precision measurement of physical quantities. However, they both face numerous challenges on the path to practical application. Artificial intelligence (AI), as a major technological advancement in current scientific landscape, offers powerful data processing and analytical capabilities, providing new ideas and methods for optimizing and enhancing quantum communication and sensing systems. In quantum communication, AI techniques have greatly improved the performance and security of quantum key distribution, quantum memory, and quantum networks through parameter optimization, real-time feedback control, and attack detection. In quantum sensing, quantum sensing technology enables ultra-high sensitivity detection of physical quantities such as time and magnetic fields. The introduction of AI has opened up new avenues for achieving high-precision and high-sensitivity quantum measurements. With AI, sensor performance is optimized, and measurement accuracy is further enhanced through data analysis. This paper also analyzes the current challenges in using AI to empower quantum communication and sensing systems, such as implementing efficient algorithm deployment and system feedback control under limited computational resources, and addressing complex task environments, dynamically changing scenarios, and multi-task coordination requirements. Finally, this paper discusses and envisions future development prospects in this field.