Indefinite order in the interface of quantum mechanics and gravity

Abstract

Researchers have long been aiming to understand how the characteristics of Quantum Theory and General Relativity combine to account for regimes in their interface. One reason why this is a hard task is how differently the theories approach time and causality. For instance, causal structure in relativity is determined by the distribution of mass in spacetime while, in the quantum formalism, it is supposed to be fixed and given in advance. In this master's thesis, we discuss the notion of indefinite order, which first appears in an abstract generalization of Quantum Theory [...] where the demand for global causal structure is removed, in principle allowing cases for which the order of operations in protocols is not necessarily well defined. One epitomical example of indefinite order is the quantum switch process, which realizes a quantum superposition of orders of two operations on a target system. The quantum switch probabilities have been reproduced in experimental optical setups that are fully described in principle by quantum mechanics. Since these experiments are compatible with spacetime causal structure, this generated uncertainty about the conclusions that can be drawn from obtaining these results depending on the context. Here, we return to the initial motivations and also present how scenarios involving gravity in low energies could lead to indefinite order. This includes the formulation of a quantum switch in a quantum gravity scenario and of a quantum switch in a classical Schwarzschild metric. The switch then provides a common ground to discuss different kinds of setups. The latter proposal of a quantum switch in a classical metric is an original work that, aside from being an example of indefinite order, proposes the realization of the protocol in Earth's gravity as a test of quantum mechanics on curved spacetimes, a regime which has not yet been explored experimentally.