Licentiate thesis defense: Bell tests and the no-signalling assumption

Bell’s theorem was originally meant for testing fundamental properties of Nature, namely local realism. However, through the years it has become a powerful device for certifying encryption security, randomness, and entanglement among other properties. Especially after the series of loophole-free violation papers at the end of 2015, these applications are becoming more and more relevant. In most of these scenarios, additional assumptions – as fair-sampling – are set forth in order to achieve the required near-optimal violations. However, it turns out that many of the experiments realised so far suffer from apparent signalling. We say “apparent” because we do not believe that the issue comes from actual communication between different measurement stations, but rather, as we show in this work, from systematic issues related to the particular experimental realisation. After making a point for the importance of correcting these errors, we identify and address some of the most common sources of signalling in a set of experiments based on single photon polarisation qubits. Finally, we report a reliable CHSH violation which is free from apparent signalling. While the core results are contained in the attached Paper I, we first provide the reader with a brief introduction to the concepts involved in this work, in addition to supplementary unpublished experimental work.