The ever-growing demands of modern telecommunication systems in terms of data rates as well as the impending threat of the increasing computing power of modern computers, make the secure transmission of data an essential requirement and thus a very active field of study. Quantum key distribution (QKD) allows for the exchange of cryptographic keys whose security level does not depend on the complexity of a mathematical algorithm but rather relies on exploiting the properties of quantum mechanics cite{scarani2009}. Depending on the protocol, the key bits will be encoded either on the superposition of modes of individual photons, such as polarization modes, as is the case for the discrete variable protocols (DV) or they will be encoded into the quadratures of a very low flux electromagnetic field as it happens in the continuous variable protocols (CV). While offering security levels unattainable by classical means, QKD protocols in their terrestrial implementation are severely limited in distance reaching only several hundred kilometers because of the exponential attenuation suffered by fiber-transmitted signals. Since the amplification methods of classical optical communications repeaters are not compatible with a signal that is quantum in nature, and because of the current lack of technological maturity regarding quantum repeaters, satellite relays present an interesting alternative for the establishment of secure intercontinental quantum links. A study by Dequal et al. upon which a part of the present study is based on, examines the possibility of performing a continuous variable key exchange between a satellite and a ground station by proposing a modeling of the propagation channel accounting for the effects of beam wandering, a fluctuating atmospheric transmission and a fixed loss due to single mode fiber coupling. It is as an in-depth continuation of this analysis that this simulation study was initially developed. Taking into account in particular the effects of propagation through the turbulent atmosphere on the spatial coherence of the optical signal, as well as expanding on the protocols taken into account. Adaptive optics (AO) are able to partially correct some of the aforementioned propagation effects. A typical AO system consists of a feedback loop containing elements capable of measuring and correcting wavefront aberrations in real time and we will focus our efforts in analyzing the effect of such a system in the performance of several protocols of quantum key distribution under different scenarios.

Publication

Quantum Key Distribution through atmospheric turbulence : secure satellite-to-ground links

The ever-growing demands of modern telecommunication systems in terms of data rates as well as the impending threat of the increasing computing power of modern computers, make the secure transmission of data an essential requirement and thus a very active field of study. Quantum key distribution (QKD) allows for the exchange of cryptographic keys whose security level does not depend on the complexity of a mathematical algorithm but rather relies on exploiting the properties of quantum mechanics cite{scarani2009}. Depending on the protocol, the key bits will be encoded either on the superposition of modes of individual photons, such as polarization modes, as is the case for the discrete variable protocols (DV) or they will be encoded into the quadratures of a very low flux electromagnetic field as it happens in the continuous variable protocols (CV). While offering security levels unattainable by classical means, QKD protocols in their terrestrial implementation are severely limited in distance reaching only several hundred kilometers because of the exponential attenuation suffered by fiber-transmitted signals. Since the amplification methods of classical optical communications repeaters are not compatible with a signal that is quantum in nature, and because of the current lack of technological maturity regarding quantum repeaters, satellite relays present an interesting alternative for the establishment of secure intercontinental quantum links. A study by Dequal et al. upon which a part of the present study is based on, examines the possibility of performing a continuous variable key exchange between a satellite and a ground station by proposing a modeling of the propagation channel accounting for the effects of beam wandering, a fluctuating atmospheric transmission and a fixed loss due to single mode fiber coupling. It is as an in-depth continuation of this analysis that this simulation study was initially developed. Taking into account in particular the effects of propagation through the turbulent atmosphere on the spatial coherence of the optical signal, as well as expanding on the protocols taken into account. Adaptive optics (AO) are able to partially correct some of the aforementioned propagation effects. A typical AO system consists of a feedback loop containing elements capable of measuring and correcting wavefront aberrations in real time and we will focus our efforts in analyzing the effect of such a system in the performance of several protocols of quantum key distribution under different scenarios.