Phd in PHYSICS

Thesis title: What can low energy quantum systems teach us about space and time?

Quantum gravity is generally seen as a high energy physics field. However, recent progress in quantum control of matter means we might be able to perform tests of quantum gravity predictions using slow-moving masses. Interest in these kinds of proposals has renewed a conversation between the quantum gravity, quantum information, and quantum foundations communities. This thesis presents original work on topics at the intersection of these three fields. Part of the thesis is concerned with studies of low energy tests of quantum gravity, and beyond. One such test aims at detecting entanglement between masses in a superposition as a result of gravitational interaction. We detail a quantum optics simulation of this effect and report on the results. We then derive from first principles the quantum phases involved in this quantum gravity experiment. We obtain an exact formula that improves on the current literature, as it takes into account relativistic retardation and can be computed for general trajectories and arbitrary number of particles. We also propose an experiment to test the discreteness of time. This test involves a single mass in a superposition of paths in a weak gravitational field. We assess its experimental feasibility and find it in a similar range as the low energy quantum gravity experiments proposed in the literature. The rest of the thesis focuses on more conceptual aspects of space and time. We study the origin of the time-orientation of the operational formulations of quantum mechanics, which is in tension with the time-reversal symmetry of the rest of fundamental physics. We argue that the formalism is time-oriented, not because the fundamental physics is time-oriented, but because it is designed to study situations created by time-oriented macroscopic systems. Finally, we explore the consequences of admitting relative facts in the ontology of quantum mechanics, as proposed by the relational interpretation of quantum mechanics, and as hinted by a recent no-go theorem. We argue in particular that this clarifies the measurement problem and Wigner's friend-type scenarios, and allows to explain the emergence of the shared macroscopic world in which humans operate.