Flow of local sensitivity in a spin chain coupled to a bosonic bath

Abstract

We study how local sensitivity to an encoded parameter, captured by the quantum Fisher information, flows between a spin chain, a coupled bosonic bath, and their quantum correlations, where we treat the bath as a full many-body quantum system beyond the Lindbladian approximation. We analyze how the coupling symmetry determines the destination of the departed sensitivity directly at the level of the Hamiltonian. We consider an excitation-exchanging Jaynes--Cummings coupling, which preserves the total number of excitations, and a spin-excitation-conserving Holstein coupling. We find that the Holstein coupling leaves the bath with no first-order information about the phase and stores the lost sensitivity entirely in spin--bath correlations, whereas the Jaynes--Cummings coupling passes this sensitivity to the bath, in full for a single excitation. The bath spectrum then governs whether the sensitivity ever returns to the spins. It revives fully and periodically when the coupled-system frequencies share a common period, but when those frequencies disperse the departed share dephases across the bath modes and never returns. We note that a transported metrological register loses more sensitivity than its arrival fidelity implies.