Correlation harvesting in the presence of Unruh and Hawking effects

Abstract

Quantum field theory (QFT) in curved spacetime is a study of quantum fields under the influence of the relativistic motion of particles or spacetime curvature. The famous outcomes of this subject are the Unruh and Hawking effects. The Unruh effect claims that a uniformly accelerating atom (people in the community tend to use a model called the Unruh-DeWitt (UDW) particle detector, which is a two-level quantum system coupled to a quantum field) thermalizes even though an inertial observer sees no particles. That is, an acceleration motion excites the internal degree of freedom of the atom in such a way that the atom experiences as if it is immersed in a thermal bath. The Hawking effect is a phenomenon where a black hole radiates thermal quanta. If one puts a UDW detector outside an event horizon, then it also perceives thermality. Both the Unruh and Hawking effects show thermality, which is the core theme of this thesis.

In recent years, a protocol called entanglement harvesting has attracted great interest. Entanglement harvesting utilizes multiple UDW detectors to extract (or ‘harvest’) entanglement pre-existed in a quantum field. The extracted entanglement is influenced by the geometry of spacetime and the trajectories of UDW detectors. One can also extract other types of correlations, and so we collectively call this the correlation harvesting protocol. In this thesis, we examine how thermal effects influence the ability of correlation harvesting. In a previous study, the case of two inertial UDW detectors coupled to a thermal quantum field was investigated. It was shown that as the temperature of the field increases, the extracted entanglement between the detectors decreases while the quantum mutual information (the total correlations including classical and quantum correlations) increases. Since a single detector in uniform acceleration motion or hovering near a black hole experiences thermality as if it is immersed in a thermal quantum field, it is natural to ask if harvested correlations also behave in the same manner. In contrast, we show that (i) the Unruh temperature of uniformly accelerating detectors prevents the detectors from extracting any correlations at the high temperatures, i.e., even the quantum mutual information vanishes at the extreme Unruh temperatures; (ii) high black hole temperatures also prevent the detectors from harvesting correlations, and this is no exception even for tripartite entanglement; and (iii) freely falling detectors in a black hole spacetime are less affected by this, and they have no trouble extracting correlations from the field even when detectors are causally disconnected by an event horizon.