Observer-Dependent Quantum Coherence and Information Access in Accelerated and Curved-Spacetime Frameworks

Quantum coherence in relativistic settings is often described as observer dependent, yet this expression can refer to distinct changes in the definition of modes, causal accessibility, detector response, and experimentally retained information. This conceptual and analytical review examines how these mechanisms shape operational claims about phase-sensitive quantum resources for inertial, accelerated, and curved-spacetime observers. The discussion combines quantum field theory on prescribed backgrounds, detector-based interpretation of the Unruh effect, relativistic quantum information, and basis-dependent coherence quantification. Particular attention is paid to the distinction between a globally specified field state and an observer-restricted measurement record, and to the limitations of single-mode approximations for realistic wave packets. Black-hole horizons, stationary gravitational links, and nonstationary cosmological backgrounds are compared without identifying redshift, causal tracing, and physical dephasing as a single process. An operational reporting framework is proposed in which source preparation, trajectories, detector coupling, phase reference, accessible measurements, and collection probability must be specified together. The central result is a methodological conclusion: observer-dependent loss of usable coherence can be meaningful and measurable without implying that coordinate transformations alone destroy global quantum information. The article is a theoretical synthesis and reports no new simulation or experimental data.