Relativistic quantum Otto engine: Instant work extraction from a quantum field

In this study, we carry out a non-perturbative approach to a quantum Otto engine, employing an Unruh-DeWitt particle detector to extract work from a quantum Klein-Gordon field in an arbitrary globally hyperbolic curved spacetime. We broaden the scope by considering the field in any quasi-free state, which includes vacuum, thermal, and squeezed states. A key aspect of our method is the instantaneous interaction between the detector and the field, which enables a thorough non-perturbative analysis. We demonstrate that the detector can successfully extract positive work from the quantum Otto cycle, even when two isochoric processes occur instantaneously, provided the detector in the second isochoric process receives a signal from the first interaction. This signaling allows the detector to release heat into the field, thereby the thermodynamic cycle is completed. As a demonstration, we consider a detector at rest in flat spacetime and compute the work extracted from the Minkowski vacuum state.Comment: 26 pages, 5 figures;(v2) Added some references in Introduction and added a note at the end of Conclusion; (v3) Added a figure in Fig.

Correlation harvesting in the presence of Unruh and Hawking effects

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

Mutual information harvested by uniformly accelerated particle detectors

We investigate the mutual information harvesting protocol for two uniformly accelerated particle detectors. We numerically show that, while a single detector responds as if it is immersed in a thermal bath, the quantum mutual information between two accelerating detectors behaves differently than that of two inertial detectors in a thermal bath. This is due to the fact that while the Wightman function along the trajectory of a single uniformly accelerating detector is the same as that of as a detector in a thermal bath, a pair of detectors in the same respective cases will have different Wightman functions.Comment: 9 pages, 4 figure

Harvesting mutual information from BTZ black hole spacetime

We investigate the correlation harvesting protocol for mutual information between two Unruh-DeWitt detectors in a static BTZ black hole spacetime. Here, the effects coming from communication and change in proper separation of the detectors are set to be negligible so that only a black hole affects the extracted mutual information. We find that, unlike the entanglement harvesting scenario, harvested mutual information is zero only when a detector reaches an event horizon, and that although the Hawking effect and gravitational redshift both affect the extraction of mutual information, it is extreme Hawking radiation that inhibits the detectors from harvesting.Comment: 10 pages, 4 figures; v2: fixed typo

Correlation harvesting between particle detectors in uniform motion

We investigate the correlation harvesting protocol using two Unruh-DeWitt particle detectors moving along four classes of uniformly accelerated trajectories categorized by Letaw: linear, catenary, cusped, and circular motions. For each trajectory, two types of configurations are carried out: one possesses a stationary (time-translation invariant) Wightman function and the other is nonstationary. We find that detectors undergoing linear, catenary, and cusped motions gain fewer correlations in the nonstationary configurations compared to those in stationary configurations. Detectors in circular motion have similar behavior in both configurations. We discuss the relative suppression of correlation harvesting due to high acceleration for each case. Remarkably we find that under certain circumstances detectors in both linear and circular states of motion can harvest genuine (non-communication assisted) entanglement even though they are in causal contact.Comment: 16 pages, 7 figure

Extraction of entanglement from quantum fields with entangled particle detectors

We consider two initially entangled Unruh-DeWitt particle detectors and examine how the initial entanglement changes after interacting with a quantum scalar field. As initially nonentangled detectors extract entanglement from the field, entangled detectors also can gain more entanglement so long as they are weakly correlated at the beginning. For initially sufficiently entangled detectors, only degradation takes place. We then apply our analysis to a gravitational shockwave spacetime and show that a shockwave could enhance the initial entanglement of weakly entangled detectors. Moreover, we find that this enhancement can occur for greater detector separations than in Minkowski spacetime.Comment: 13 pages, 6 figure

Tripartite Entanglement Extraction from the Black Hole Vacuum

The first investigation of tripartite entanglement harvesting in the vicinity of a black hole is carried out. Working in the context of a static Ba\~{n}ados-Teitelboim-Zanelli (BTZ) black hole spacetime we find that it is possible to harvest tripartite entanglement in regions where harvesting of bipartite entanglement is known to be impossible due to intense Hawking radiation. In these situations, it implies that the harvested entanglement is of the Greenberger-Horne-Zeilinger (GHZ) type.Comment: 12 pages, 8 figures; (v2) matched to the final versio

Channel capacity of relativistic quantum communication with rapid interaction

In this work we study nonperturbatively the transmission of classical and quantum information in globally hyperbolic spacetimes, where the communication channel is between two qubit detectors interacting with a quantized massless scalar field via delta-coupling interaction. This interaction approximates very rapid detector-field interaction, effectively occurring at a single instant in time for each detector. We show that when both detectors interact via delta-coupling, one can arrange and tune the detectors so that the channel capacity is (at least) as good as the quantum channel constructed nonperturbatively using \textit{gapless detectors} by Landulfo [PRD 93, 104019]. Furthermore, we prove that this channel capacity is in fact optimal, i.e., both nonperturbative methods give essentially the same channel capacity, thus there is a sense in which the two methods can be regarded as equivalent as far as relativistic quantum communication is concerned.Comment: 11+4 pages, 1 figure; Revtex4-2; v2: fixed typos and clarify Section II; v3: fixed more typos and minor mistakes, updated to match published version; v4: fixed notational issue with the original Eq. (32