Quantum-based security in optical fibre networks

Electronic communication is used everyday for a number of different applications. Some of the information transferred during these communications can be private requiring encryption and authentication protocols to keep this information secure. Although there are protocols today which provide some security, they are not necessarily unconditionally secure. Quantum based protocols on the other hand, can provide unconditionally secure protocols for encryption and authentication. Prior to this Thesis, only one experimental realisation of quantum digital signatures had been demonstrated. This used a lossy photonic device along with a quantum memory allowing two parties to test whether they were sent the same signature by a single sender, and also store the quantum states for measurement later. This restricted the demonstration to distances of only a few metres, and was tested with a primitive approximation of a quantum memory rather than an actual one. This Thesis presents an experimental realisation of a quantum digital signature protocol which removes the reliance on quantum memory at the receivers, making a major step towards practicality. By removing the quantum memory, it was also possible to perform the swap and comparison mechanism in a more efficient manner resulting in an experimental realisation of quantum digital signatures over 2 kilometres of optical fibre. Quantum communication protocols can be unconditionally secure, however the transmission distance is limited by loss in quantum channels. To overcome this loss in conventional channels an optical amplifier is used, however the added noise from these would swamp the quantum signal if directly used in quantum communications. This Thesis looked into probabilistic quantum amplification, with an experimental realisation of the state comparison amplifier, based on linear optical components and single-photon detectors. The state comparison amplifier operated by using the wellestablished techniques of optical coherent state comparison and weak subtraction to post-select the output and provide non-deterministic amplification with increased fidelity at a high repetition rate. The success rates of this amplifier were found to be orders of magnitude greater than other state of the art quantum amplifiers, due to its lack of requirement for complex quantum resources, such as single or entangled photon sources, and photon number resolving detectors

Enhanced Uplink Quantum Communication with Satellites via Downlink Channels

In developing the global Quantum Internet, quantum communication with low-Earth-orbit satellites will play a pivotal role. Such communication will need to be two way: effective not only in the satellite-to-ground (downlink) channel but also in the ground-to-satellite channel (uplink). Given that losses on this latter channel are significantly larger relative to the former, techniques that can exploit the superior downlink to enhance quantum communication in the uplink should be explored. In this work we do just that - exploring how continuous variable entanglement in the form of two-mode squeezed vacuum (TMSV) states can be used to significantly enhance the fidelity of ground-to-satellite quantum-state transfer relative to direct uplink-transfer. More specifically, through detailed phase-screen simulations of beam evolution through turbulent atmospheres in both the downlink and uplink channels, we demonstrate how a TMSV teleportation channel created by the satellite can be used to dramatically improve the fidelity of uplink coherent-state transfer relative to direct transfer. We then show how this, in turn, leads to the uplink-transmission of a higher alphabet of coherent states. Additionally, we show how non-Gaussian operations acting on the received component of the TMSV state at the ground station can lead to even further enhancement. Since TMSV states can be readily produced in situ on a satellite platform and form a reliable teleportation channel for most quantum states, our work suggests future satellites forming part of the emerging Quantum Internet should be designed with uplink-communication via TMSV teleportation in mind

Wireless Microwave Quantum Communication

228 p.Esta Tesis explora los límites en la aplicación de microondas cuánticas propagantes para comunicación y sensórica cuántica, así como el diseño de nuevos aparatos y protocolos para combatir estas limitaciones.Nos servimos de estados cuánticos Gaussianos para teleportación cuántica e iluminación cuántica, y estudiamos cómo mejorar estos protocolos utilizando destilación de entrelazamiento y purificación parcial, respectivamente. La Tesis se centra en la distribución de entrelazamiento por el aire, y sigue los pasos de generación de estados dentro de un criostato, adaptación de impedancias entre el criostato y elaire con una nueva generación de antenas coplanares, y propagación por el aire, en el marco actual de las tecnologías de microondas. También tratamos las dificultades producidas por pérdidas y medidas ineficientes, y exploramos una extensión hacia comunicación cuántica entre satélites, donde analizamos los efectos de la difracción y las turbulencias, especialmente cómo estas últimas afectan a las señales en el rango óptico. Concluimos con el estudio de la teleportación de información cuántica en una red de área local cuántica. En resumen, esta Tesis contribuye al desarrollo de la comunicación inalámbrica con microondas en el régimen de microondas, estudiando sus limitaciones y cómo vencerlas. Aun así, aún se trata de una tecnología emergente, y queda mucho trabajo por hacer para que llegue a ser competitiva