Quantum Detectors: the Unruh Effect and Temporal Quantum Correlations

This thesis investigates quantum detectors as operational probes of relativistic quantum field theory and temporal quantum correlations. On the relativistic side, it addresses the Unruh effect from the perspective of quantum detectors, focusing on the characterization of their response and its relation to thermality. The first original contribution concerns the analysis of finite-size quantum detectors, whose spatial extension gives rise to nonlocal effects and supports an operational notion of temperature beyond the pointlike approximation. In collaboration with P. Zanghì and P. Solinas, the author demonstrates that the Unruh effect can be recovered for extended detectors in specific regimes and for suitable probe sizes. In these cases, the notion of Unruh temperature can be generalized to account for the detector’s spatial delocalization. A second original result, in collaboration with P. Zanghì and P. Solinas, is the development of a realistic detector model based on a charged particle in relativistic circular motion driven by a magnetic field, revealing an ultrarelativistic excitation plateau with potential experimental relevance. On the side of temporal correlations, the thesis moves beyond Leggett–Garg inequalities by introducing a quantum non-demolition measurement protocol that allows for the reconstruction of temporal quasi-probabilities. Their negativity is shown to provide a necessary and sufficient witness of macrorealism violation, thereby overcoming the limitations of inequality-based approaches. A central original contribution of this work, developed in collaboration with P. Solinas and D. Melegari, is the demonstration that the proposed protocol achieves the same efficiency as standard Leggett–Garg inequalities and, in certain cases, can even outperform them. Overall, the work establishes quantum detectors as a unified operational framework for the investigation of the Unruh effect and non-classical temporal statistics.