Université Libre de Bruxelles (ULB)

## Talks by Esteban Castro Ruiz

## Relative subsystems and quantum reference frame transformations.

Esteban Castro Ruiz
Université Libre de Bruxelles (ULB)

Transformations between reference frames play a crucial role in our understanding of physical processes. In practice, reference frames are realised by physical systems, which are standardly treated as classical. However, assuming that every physical system is ultimately quantum, it is interesting to ask how a theory of transformations wrt quantum reference frames would look like, and what implications it would have for our description of spacetime.

## Time reference frames and gravitating quantum clocks

Esteban Castro Ruiz
Université Libre de Bruxelles (ULB)

The standard formulation of quantum theory relies on a fixed space-time metric determining the localisation and causal order of events. In general relativity, the metric is influenced by matter, and it is expected to become indefinite when matter behaves quantum mechanically. Here we explore the problem of operationally defining events and their localisation in the presence of gravitating quantum systems. We develop a framework for "time reference frames," in which events are defined in terms of quantum operations with respect to a quantum clock.

## Entanglement of quantum clocks through gravity

Esteban Castro Ruiz
Université Libre de Bruxelles (ULB)

In general relativity, the picture of space–time assigns an ideal clock to each world line. Being ideal, gravitational effects due to these clocks are ignored and the flow of time according to one clock is not affected by the presence of clocks along nearby world lines. However, if time is defined operationally, as a pointer position of a physical clock that obeys the principles of general relativity and quantum mechanics, such a picture is, at most, a convenient fiction.