Indefinite order in the interface of quantum mechanics and gravity

• Ano de defesa: 2023
• Autor(a) principal: Bruna Larissa de Souza Sahdo e Silva
• Orientador(a): Nelson de Oliveira Yokomizo
• Banca de defesa: Pablo Lima Saldanha, Lucas Chibebe Céleri
• Tipo de documento: Dissertação
• Tipo de acesso: Acesso aberto
• Idioma: eng
• Instituição de defesa: Universidade Federal de Minas Gerais
• Programa de Pós-Graduação: Programa de Pós-Graduação em Física
• Departamento: ICX - DEPARTAMENTO DE FÍSICA
• País: Brasil
• Palavras-chave em Português: Indefinite causal order; Quantum switch; Quantum clocks; Quantum mechanics on curved spacetimes
• Link de acesso: http://hdl.handle.net/1843/58410; https://orcid.org/0000-0002-5582-5048

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 dynamical, 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. The purpose of that generalization is eliminating the incompatibility of the theory with General Relativity with respect to causality. For this, 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 by regular quantum mechanics. Since these experiments are compatible with spacetime causal structure, this generated uncertainty on what conclusions can be drawn from the realization of indefinite order depending on the context. Here, we return to the initial motivations and 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. Thus, the quantum switch will be used as a common ground for discussing differences between all 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.