Continuous-Variable Quantum Communication over Free-Space Lossy Channels

Abstract

This thesis analyses continuous-variable (CV) entanglement distribution over atmospheric-fading channels dominated by turbulence-induced beam wandering, and its application to entanglement-based CV quantum key distribution (QKD). The first contribution of this thesis is to quantify Gaussian entanglement between two ground stations generated via various quantum communication schemes, where communication occurs via a satellite over two independent atmospheric channels. In one scheme the quantum complexity remains largely on the ground, with the satellite acting as a reflector. On the receiving ground station we apply classical and quantum post-selection, demonstrating the better performance of classical post-selection in enhancing the Gaussian entanglement between the stations relative to quantum post-selection. Then, by considering CV-QKD based on the Gaussian entanglement between the ground stations, we present the first quantitative assessment of key rates in the scenario of reflection off the satellite. We compare the Gaussian entanglement produced by this scheme with two other schemes, in which quantum complexity is added to the satellite, illustrating the tradeoff between space-based quantum complexity and the Gaussian entanglement. The second contribution is to determine CV-QKD key rates between two ground stations resulting from a measurement-device independence (MDI) protocol which utilizes Gaussian states. In this MDI protocol the measurement device is a satellite communicating with the ground stations over two independent atmospheric channels. The MDI protocol we study is useful since we show that key rates between the stations are possible even when the satellite is held by an adversary. The security of the protocol is confirmed through an equivalent entanglement swapping scheme. The third contribution is to determine entanglement-generation rates and quantum key rates via non-Gaussian transfer over atmospheric channels. Utilizing a Kraus representation of the channel, we present the first quantitative assessment of non Gaussian entanglement evolution through atmospheric channels. Most of the non-Gaussian states we transmit over the channel are created just-in-time via non-Gaussian operations on input Gaussian states that would otherwise be used directly in the channel. In such an operational scenario we show sending the incoming Gaussian state directly over the channel would be the best option in terms of the entanglement-generation rate.