Classical free-space optical (FSO) communication promises massive data throughput rates relative to traditional wireless technologies - an attractive outcome now being pursued in the context of satellite-ground, inter-satellite and deep-space communications. The question we investigate here is: how can we minimally alter classical FSO systems, both in infrastructure and in energy input, to provide some element of quantum communication coexisting with classical communications? To address this question, we explore additional Gaussian displacements to classical FSO encoding on the satellite, determining the minimum signal requirements that will meet given specifications on the combined classical and quantum communications throughput. We then investigate whether enhanced quantum-based amplifiers embedded in receivers, which have proven advantageous in standalone quantum communication, can enhance our combined classical-quantum communication throughput. We show how this is indeed the case, but only at the cost of some additional receiver complexity, relative to standalone quantum communications. This additional complexity takes the form of an additional beamsplit-ter and two heterodyne detectors at the receiver. Our results illustrate a viable pathway to realising quantum communication from classical FSO systems with minimal design changes.
Classical free-space optical (FSO) communication promises massive data throughput rates relative to traditional wireless technologies - an attractive outcome now being pursued in the context of satellite-ground, inter-satellite and deep-space communications. The question we investigate here is: how can we minimally alter classical FSO systems, both in infrastructure and in energy input, to provide some element of quantum communication coexisting with classical communications? To address this question, we explore additional Gaussian displacements to classical FSO encoding on the satellite, determining the minimum signal requirements that will meet given specifications on the combined classical and quantum communications throughput. We then investigate whether enhanced quantum-based amplifiers embedded in receivers, which have proven advantageous in standalone quantum communication, can enhance our combined classical-quantum communication throughput. We show how this is indeed the case, but only at the cost of some additional receiver complexity, relative to standalone quantum communications. This additional complexity takes the form of an additional beamsplit-ter and two heterodyne detectors at the receiver. Our results illustrate a viable pathway to realising quantum communication from classical FSO systems with minimal design changes.
Catering for the emerging satellite-based free-space optical (FSO) communication networks, exploiting polarization encoding via Stokes operators, we propose a novel simultaneous quantum-classical communications (SQCC) protocol that enables the coexistence of secured quantum communications and high-throughput classical communications under minimal alterations in both infrastructure and energy input. Relative to the conventional SQCC protocol, our new protocol provides superior real-world practicability by eliminating the need for a separate local oscillator, allowing for the simple readout of both quantum and classical information using direct detection, and minimizing the undesirable interplay between the quantum and classical parts of communication. We provide a detailed description, along with theoretical and numerical analyses on the feasibility and performance of our protocol, illustrating a promising pathway towards the practical and effective realization of combined classical-quantum communications.
Catering to emerging satellite-based free-space optical (FSO) communication networks and exploiting polarization encoding via Stokes operators, we propose a novel simultaneous quantum-classical communications (SQCC) protocol. The protocol enables the coexistence of secure quantum communications and high-throughput classical communications with minimal alterations in both the infrastructure and the energy input. Compared to the conventional SQCC protocol, our new approach provides superior practicality in the real world, eliminates the need for a separate local oscillator, and allows for the simple readout of both quantum and classical information using direct detection. The protocol also minimizes the undesirable interplay between the quantum and the classical parts of communication. We provide a detailed mathematical formulation of the protocol, along with theoretical and numerical analysis of its performance, illustrating a promising path to practical and effective realization of combined classical-quantum communications.
Free-Space Optical (FSO) communication is a technology that enables ultra-high-rate data exchange and will be crucial in enabling secure Quantum Key Distribution (QKD) over long distances. However, FSO communication systems are comparatively complex technologies, comprising a variety of subsystems that all have to work together in a coordinated way. Hence, assessing the feasibility of establishing a (classical or quantum) link in a given communication scenario is a difficult task in itself. Here we introduce QCalc, a software tool that aims at easing and streamlining this process, allowing the user to keep track of all the relevant parameters in a link budget. This software computes the total transmissivity, and thus, the signal intensity observed at the receiver for any given parameters of the transceivers and the FSO communication channel. The estimated signal intensity can then be used to compute either the link budget in a classical communication scenario or the secure key rate of a QKD protocol.
Satellite QKD/FSO systems, which facilitate quantum key distribution (QKD) over free-space optical (FSO) links between satellites and ground stations, present a promising pathway toward achieving global security in upcoming sixth-generation (6G) wireless communications. Our study focuses on a superior type of these systems, the satellite EB/CV-QKD/FSO, which utilizes the continuous-variable (CV) method for quantum state representation and the entanglement-based (EB) scheme for QKD implementation. We propose the use of optical phase-shift keying (QPSK) signaling and dual-threshold/heterodyne detection (DT/HD) receivers to bolster the reliability and feasibility of satellite EB/CV-QKD/FSO systems. Closed-form expressions for key system performance metrics are derived using improved channel modeling. Numerical results are presented to showcase the effects of channel impairments on the system performance. We also provide recommendations for optimal system setup parameters, aiming to enhance performance.
Integrated millimeter-wave (mmWave) and free-space optical (FSO) communication systems offer a complementary solution for secure and high-speed data transmission. In this hybrid system, mmWave serves as the classical channel and FSO operates as a quantum channel for quantum key distribution (QKD), creating a multi-layered communication infrastructure. This dual channel faces significant challenges, including vulnerability to atmospheric attenuation, blockage, beam misalignment, and high sensitivity to environmental factors such as weather conditions, turbulence, and obstacles, which impact their reliability and performance. Taking into account these challenges, the integration of mmWave and FSO enables robust applications such as secure 6G networks, satellite-to-ground communications, and urban connectivity, where mmWave ensures reliability during FSO disturbances. By combining classical high-speed data transfer with the quantum security of QKD, this hybrid architecture opens the road for next-generation secure and high-capacity networks.
Born about a century ago, Quantum Mechanics has revolutionized the description and the interpretation of Physics at sub-microscopic level. In the last decades, due to the influence of mathematical and engineering research fields, Quantum Mechanics has given birth to related research areas like Quantum Computation, Quantum Information and Quantum Communication. With the discovery of the laser, and later the development of fiber optics and satellite networks, Quantum Communication and Quantum Optics seems to have a natural field of application in Communication Systems. Despite this, the interest in this technology and studies for communication purpose has been overshadowed by the great results in communication networks achieved in the last decades with classical paradigms. However, due to the increasing demand of communication data rate, system designers are now looking at Quantum Mechanics for new and more performanting solutions in communication purposes. Early theoretical studies on Quantum Discrimination Theory and Quantum Information predict better performance for Communication Systems that take advantage of the quantum laws. In addition, Quantum Mechanics provides the deepest description of the physical phenomena, and there are scenarios where a quantum model fits best, as in in deep space communications, where the received signal is really weak, or in a satellite networks, where we are interested in strongly reducing the power of transmitted signals, possibly without sacrificing performance significantly. However, if on one side Quantum Communication Theory promises great gains in the performance of communication systems, on the other hand it fails to describe how to implement physical devices that reach these ultimate limits. So far, only a few architectures achieving these performances are known, and only for simple modulation formats. We are interested in the scenario of optical communications, where the message transmitted is encoded in a sequence of coherent states. Transmitter devices for coherent modulation are well known and consist in laser pulse generators. Instead, receiver implementations working at the quantum limit performance limit are yet to be found. In this Thesis I deal with different topics in the quantum transmission scenario. First, I review existing classical (suboptimal) and quantum (suboptimal and optimal) receiver schemes for the binary coherent modulation. I present a new formulation of the optimal scheme known as Dolinar Receiver with the multiple copies problem, focusing on the information gained during the measurement. Second, I analyze the binary communication in a noisy environment, studying the error probability and the capacity of the binary channel induced. Given the description of the quantum channel, I optimize both the transmitted quantum states and the measurement operators employed in the communication. Third, I consider the Pulse Position Modulation, that is particularly suitable for space and satellite communication due to its simplicity of implementation and high capacity. I review some known suboptimal receivers, and I propose a receiver scheme which approaches the limit performance predicted with quantum theory outperforming the existing schemes. To sum up the results of this Thesis, in order to approach the limit performance predicted by Quantum Mechanics, an optimization is always necessary to exceed the classical effects and trigger the quantum phenomena. In particular, the information gained during the measurement plays an important role, for example in the definition of adaptive receivers. In this Thesis both these aspects have been deeply investigated.