Relativistic Quantum Metrology Beyond the X-State Ansatz: Single- and Two-Qubit Estimation of Phase and Charging Time

Relativistic motion introduces decoherence that limits quantum parameter estimation. Earlier treatments of bipartite Unruh-DeWitt detectors have often restricted the reduced state to the X-state form. We consider single-qubit and isolated-ancilla two-qubit probes whose reduced state is not restricted to this form, moving along a trajectory that combines uniform acceleration with a transverse velocity. The quantum Fisher information for an initially encoded phase and for a charging time across the three velocity regimes is investigated. The two parameters respond to acceleration in distinct ways, and the isolated ancilla improves both estimations over different portions of the parameter space. Phase estimation remains nearly unaffected by the transverse velocity of the probe, whereas at short drive durations, the charging-time QFI reverses with acceleration. The separable regime that follows entanglement loss is not devoid of metrological value: both parameters can still be estimated, and the discord of the reduced state remains nonzero. These results point to quantum correlations beyond entanglement as a resource for parameter estimation in accelerated probes.