Thermality, causality, and the quantum-controlled Unruh-deWitt detector

Abstract

Particle detector models such as the Unruh-deWitt detector are widely used in relativistic quantum information and field theory to probe the global features of spacetime and quantum fields. These detectors are typically modeled as coupling locally to the field along a classical worldline. In this paper, we utilize a recent framework that enables us to prepare the detector in a quantum-controlled superposition of trajectories and study its response to a massless scalar field in finite-temperature Minkowski spacetime and an expanding de Sitter universe. Unlike a detector on a classical path, which cannot distinguish these spacetimes, the superposed detector can do so by acquiring nonlocal information about the geometric and causal structure of its environment, demonstrating its capability as a probe of these global properties.