Abstract

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, namely quantum systems sourcing a gravitational field and interacting gravitationally, has attracted a lot of attention, thanks to the possibility of realising these scenarios in the laboratory in the near future. When a quantum system sources a gravitational field, spacetime is fundamentally nonclassical. In this situation, also the notion of a reference frame needs to be generalised to account for quantum features of spacetime, thus realising a quantum reference frame (QRF).

In this talk, I will introduce a formalism to associate a QRF to the quantum state of a particle, and argue that QRFs can contribute to formulate physics on nonclassical spacetime, e.g. by generalising the covariance of physical laws and the Equivalence Principle. I will then discuss a recent atom-interferometric experiment [Overstreet, Asenbaum, Curti, Kim, Kasevich, Science, 375 (6577) 2022] from the perspective of different QRFs. Under some additional assumptions, among which the validity of physical laws in all QRFs, this experiment is equivalent to one in which the source of the gravitational field is in a quantum superposition.