Abstract: A first step towards scenarios at the interface of quantum theory and gravity is to consider a quantum superposition of semiclassical spacetimes, such as the one, arguably, sourced by a gravitating object in superposition. But what does it mean that a gravitational source is in a superposition of “different” locations? How can one compare points across the different manifolds? Due to the diffeomorphism invariance of general relativity, there is a priori no notion of the “same” or “different” points across the branches. In fact, there are multiple ways of relating points or events on the different spacetimes. Here, we make this concrete by using coincidences of four scalar fields to construct a comparison map between all spacetimes in superposition, which allows us to determine whether a system or an event is located at the “same” or “different” points across the branches. Different choices of scalar fields can be understood as different instantiations of quantum reference frames (QRFs), connecting the construction to recent research in the field of quantum foundations. As an explicit application of this formalism, we explore how the localisation of events is relative to the choice of QRF and formulate a quantum generalisation of Einstein’s famous hole argument. The latter calls into question the metaphysical meaning of not just spacetime points but also the identification between spacetime points across manifolds in superposition. Finally, we discuss the implications of QRF changes for indefinite causal order, the locality of interaction, and potential empirical consequences in interference experiments.

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Titel: Security proof of QKD protocols via entropic uncertainty relations

Abstract: The field of Quantum Key Distribution (QKD) has attracted a lot of attention as one of the quantum technologies that not only addresses one of the most pressing problems in secret communication, but has also shown promising progress on the implementation side. A major advantage of QKD protocols is their provable security. This talk will introduce the concepts of entanglement-based QKD protocols and show a proof technique that uses entropic uncertainty relations as one of the main ingredients. Furthermore, I will show some results on how the use of Rényi entropies can increase the provably secure key rates.