Qiss

I will sketch the toolbox and achievements of modern lattice quantum gravity, aka solving the full 4D Lorentzian gravitational path integral, using causal dynamical triangulations (CDT). It provides us with a long-sought window on physics in a near-Planckian regime, where geometry is highly nonclassical and not describable in terms of standard tensor calculus. With concrete answers to longstanding issues of how to properly implement diffeomorphism invariance, causal structure and Wick rotation, we are now focusing on interesting, approach-independent questions on the nature of nonperturbative quantum gravity and its observables. Based on results on the dynamical emergence of a 4D universe with de Sitter properties from first principles, early-universe cosmology may be our best bet in terms of quantum gravity phenomenology.

Quantum networks are the center of many of the recent advances in quantum science, not only leading to the discovery of new properties in the foundations of quantum theory but also allowing for novel communication and cryptography protocols. It is known that networks beyond that in the paradigmatic Bell’s theorem imply new and sometimes stronger forms of nonclassicality. We will review some recent experiments addressing non-classicality in different network structures.

Understanding the fundamental nature of gravity at the interface with quantum theory is a major open question in theoretical physics. Recently, the study of gravitating quantum systems, for instance a massive quantum system prepared in a quantum superposition of positions and sourcing a gravitational field, has attracted a lot of attention: experiments are working towards realising such a scenario in the laboratory, and measuring the gravitational field associated to a quantum source is expected to give some information about quantum aspects of gravity. However, there are still open questions concerning the precise conclusions that these experiments could draw on the nature of gravity, such as whether experiments in this regime will be able to test more than the Newtonian part of the gravitational field. In this talk, I will show that a static mass in a quantum state gives rise to effects that cannot be reproduced using the Newton potential nor with a known classical model of gravity. These effects can in principle be measured by performing an interference experiment, and are independent of graviton emission.
Identifying stronger quantum aspects of gravity than those reproducible with the Newton potential is crucial to prove the nonclassicality of the gravitational field and to plan a new generation of experiments testing quantum aspects of gravity in a broader sense than what proposed so far.