Local measurements in relativistic quantum information : localization and signaling

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Παπαγεωργίου, Μαρία-Ευτυχία

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In this thesis, we study some foundational aspects of detector models in quantum field theory (QFT) related to signaling and localization, and we analyze certain frictions with relativistic causality. We characterize the spatiotemporal information that can be extracted from the field using various detector models in different regimes and we define a signaling estimator, based on quantum metrology, that can be used to quantify how much signaling can be transmitted reliably through the quantum field. We analyze ‘impossible measurements’ scenarios in which the microcausality condition in QFT is not sufficient for blocking superluminal signaling between multiple detectors coupled to the field. Further, since QFT does not admit a straightforward particle or field ontology, we ask: what do detectors detect? We answer this question by interpreting the detector’s response in different regimes, for single-particle wavepacket states or coherent states of the field. In the weak coupling regime, we demonstrate in detail how detector models can be used to save particle-like phenomenology, related to the phenomenon of resonance and ‘time-of-arrival’. In the strong coupling regime, we demonstrate how a continuous pointer variable can get correlated with smeared field time-averages. Finally, adapting the formalism of the quantum Brownian motion, we develop an improved field-detector interaction model that is exactly solvable and can be used to characterize the weak, strong and intermediate regime. Apart from an improved description of field measurements and resonance, this models clearly demonstrates the modulation of particle-field duality by a single tunable parameter (the coupling strength), which is a novel feature that is in principle experimentally accessible.