Free-Space Quantum Key Distribution

Based on the firm laws of physics rather than unproven foundations of mathematical complexity, quantum cryptography provides a radically different solution for encryption and promises unconditional security. Quantum cryptography systems are typically built between two nodes connected to each other through fiber optic. This chapter focuses on quantum cryptography systems operating over free-space optical channels as a cost-effective and license-free alternative to fiber optic counterparts. It provides an overview of the different parts of an experimental free-space quantum communication link developed in the Spanish National Research Council (Madrid, Spain).Comment: 22 pages, 15 figure

Contribuciones a las comunicaciones ópticas en espacio libre: utilización de telescopios Cherenkov como receptores y corrección de Beam Wander en comunicaciones cuánticas

.Actualmente existe un consenso en que las comunicaciones ópticas en espacio libre constituirán la próxima generación de enlaces inalámbricos de alta velocidad en varios escenarios estratégicos. Esta tesis se centra en los dos donde esta tecnología puede producir un mayor impacto: comunicaciones interplanetarias y comunicaciones cuánticas. Consecuentemente, este trabajo está estructurado en dos bloques. En el primero, se ofrece una propuesta novedosa relacionada con la utilización de telescopios Cherenkov como estaciones receptoras. Las señales que se recibirán en la Tierra desde sondas en espacio profundo en futuras implementaciones de comunicaciones ópticas serán extremadamente débiles y se requerirán nuevas estaciones terrenas para dar soporte a estos enlaces. Esta tesis analiza la viabilidad de utilizar la tecnología desarrollada para los telescopios de rayos gamma que constituirán el observatorio CTA (Cherenkov Telescope Array) en la implementación de una nueva clase de estación terrena receptora. Entre las principales ventajas que brindan estos telescopios se encuentran las mayores aperturas necesarias para superar las limitaciones de potencia que comparte la astronomía terrestre de rayos gamma y las comunicaciones ópticas desde espacio profundo. Además, el elevado número de grandes telescopios que se construirán para CTA hará posible reducir costes unitarios gracias a una producción masiva. El segundo bloque de la tesis se enmarca en las comunicaciones cuánticas en espacio libre y en particular en la distribución de clave cuántica (QKD), que se ha convertido en un nuevo paradigma en el campo de la seguridad de la información. Esta técnica ofrece una forma teóricamente segura de comunicarse a través de un canal inseguro gracias a que es posible detectar la presencia de un intruso. QKD ha demostrado ser una forma fiable de transmitir datos sensibles mediante fibra óptica. Sin embargo, su alternativa no guiada también ofrece importantes ventajas, así como nuevos retos. El más importante es la necesidad de operar bajo una fuerte turbulencia atmosférica y altos niveles de ruido de fondo durante el día. Para mitigar estos efectos normalmente se lleva a cabo un compromiso al diseñar la óptica del receptor, ya que un campo de visión estrecho mejora el rechazo al ruido de fondo, pero aumenta las pérdidas relacionadas con las turbulencias y un campo de visión amplio produce el efecto opuesto. En este segundo bloque de la tesis se propone la utilización de un sistema de corrección de turbulencias para solucionar ambas limitaciones de forma simultánea, y se analizan y experimentan las distintas estrategias para llevar a cabo la implementación e integración con el sistema de QKD.Free-space optical communication is widely regarded as the next-generation of high-speed wireless communication links in several scenarios. This thesis focuses on the two main applications where this technology can bring the most significant impact: interplanetary communications and quantum communications. Consequently, the dissertation is structured in two sections. In the first one, a novel proposal is suggested regarding to using Cherenkov telescopes as ground-station receivers. The signals that will be received on Earth from future lasercom transmitters in deep-space will be extremely weak and new ground stations will have to be developed in order to adequately support these links. This thesis addresses the feasibility of using the technology developed for the gamma-ray telescopes that will make up the Cherenkov Telescope Array (CTA) observatory in the implementation of a new kind of ground station. Among the main advantages that these telescopes provide are the much larger apertures needed to overcome the power limitation that ground-based gamma-ray astronomy and deep-space optical communication both have. Also, the large number of big telescopes that will be built for CTA will make it possible to reduce unitary costs by economy-scale production. The second section of the thesis is framed in the field of free-space quantum communications. In particular, in quantum key distribution (QKD), which has become a new paradigm in the discipline of information security. This technique offers a theoretically-secure way to communicate over an insecure channel since the presence of an eventual eavesdropper can be detected during the key transmission. QKD has proven to be a reliable way to transmit sensitive data through optical fiber. However, the free-space alternative also brings important advantages, as well as new challenges. The main challenge is the need to operate both under strong atmospheric turbulence and daylight background noise. In order to mitigate these effects, a trade-off is usually required when designing the receiver's optics, since a narrow field-of-view improves background noise rejection, but increases turbulence-related losses and a wide field-of-view produces the opposite effect. In this section of the thesis, a correction system for atmospheric turbulence is proposed to overcome both limitations at the same time, and different strategies are analyzed and experimented to carry out the implementation and integration within the QKD system.Programa Oficial de Doctorado en Ingeniería Eléctrica, Electrónica y AutomáticaPresidente: Santiago Mar Sardaña.- Secretario: David Ricardo Sánchez Montero.- Vocal: Vicente Martín Ayus

QKD from a microsatellite: the SOTA experience

The transmission and reception of polarized quantum-limited signals from space is of capital interest for a variety of fundamental-physics experiments and quantum-communication protocols. Specifically, Quantum Key Distribution (QKD) deals with the problem of distributing unconditionally-secure cryptographic keys between two parties. Enabling this technology from space is a critical step for developing a truly-secure global communication network. The National Institute of Information and Communications Technology (NICT, Japan) performed the first successful measurement on the ground of a quantum-limited signal from a satellite in experiments carried out on early August in 2016. The SOTA (Small Optical TrAnsponder) lasercom terminal onboard the LEO satellite SOCRATES (Space Optical Communications Research Advanced Technology Satellite) was utilized for this purpose. Two non-orthogonally polarized signals in the ~800-nm band and modulated at 10 MHz were transmitted by SOTA and received in the single-photon regime by using a 1-m Cassegrain telescope on a ground station located in an urban area of Tokyo (Japan). In these experiments, after compensating the Doppler effect induced by the fast motion of the satellite, a QKD-enabling QBER (Quantum Bit Error Rate) below 5% was measured with estimated key rates in the order of several Kbit/s, proving the feasibility of quantum communications in a real scenario from space for the first time.Comment: 10 pages, 14 figure

Optical communication on CubeSats - Enabling the next era in space science

CubeSats are excellent platforms to rapidly perform simple space experiments. Several hundreds of CubeSats have already been successfully launched in the past few years and the number of announced launches grows every year. These platforms provide an easy access to space for universities and organizations which otherwise could not afford it. However, these spacecraft still rely on RF communications, where the spectrum is already crowded and cannot support the growing demand for data transmission to the ground. Lasercom holds the promise to be the solution to this problem, with a potential improvement of several orders of magnitude in the transmission capacity, while keeping a low size, weight and power. Between 2016 and 2017, The Keck Institute for Space Studies (KISS), a joint institute of the California Institute of Technology and the Jet Propulsion Laboratory, brought together a group of space scientists and lasercom engineers to address the current challenges that this technology faces, in order to enable it to compete with RF and eventually replace it when high-data rate is needed. After two one-week workshops, the working group started developing a report addressing three study cases: low Earth orbit, crosslinks and deep space. This paper presents the main points and conclusions of these KISS workshops.Comment: 7 pages, 5 figures, 2 tables, Official Final Report of KISS (Keck Institute for Space Studies) workshop on "Optical communication on CubeSats" (http://kiss.caltech.edu/workshops/optcomm/optcomm.html

Design and early development of a UAV terminal and a ground station for laser communications

A free-space laser communication system has been designed and partially developed as an alternative to standard RF links from UAV to ground stations. This project belongs to the SINTONIA program (acronym in Spanish for low environmental-impact unmanned systems), led by BR&TE (Boeing Research and Technology Europe) with the purpose of boosting Spanish UAV technology. A MEMS-based modulating retroreflector has been proposed as a communication terminal onboard the UAV, allowing both the laser transmitter and the acquisition, tracking and pointing subsystems to be eliminated. This results in an important reduction of power, size and weight, moving the burden to the ground station. In the ground station, the ATP subsystem is based on a GPS-aided two-axis gimbal for tracking and coarse pointing, and a fast steering mirror for fine pointing. A beacon-based system has been designed, taking advantage of the retroreflector optical principle, in order to determine the position of the UAV in real-time. The system manages the laser power in an optimal way, based on a distance-dependent beam-divergence control and by creating two different optical paths within the same physical path using different states of polarization.Comment: 9 pages, 12 figures. Appears in Proceedings of the SPIE Security+Defence (Prague, Czech Republic, 2011