Partial projected ensembles reveal slow tilt-constrained information spreading

Abstract

Projected ensembles reveal information about quantum many-body dynamics beyond the reduced density matrix. Here we investigate how spatial constraints affect the emergence of this information in a kicked Ising chain with a spatially varying longitudinal field. We consider partial projected ensembles, obtained by measuring a remote region while leaving an intervening buffer unobserved. Our numerical results reveal a pronounced contrast between nearly ballistic correlation onset in the ergodic regime and strongly delayed onset in the tilt-constrained regime, corroborated by quantum mutual information. Despite these different onset scales, the conditional-state fluctuations decrease approximately exponentially with buffer length in both regimes. Full projected ensembles additionally exhibit slow relaxation and persistent measurement-basis dependence in their higher moments. We establish an information-theoretic characterization of the connected second moment and use a solvable dephasing model with effective-basis measurements to illustrate the separation of phase accumulation and coherence loss. Within this model, a factorially suppressed coupling envelope yields sublogarithmic spreading consistent with the finite-distance onset trends. Our results identify partial projected ensembles as probes of the distinction between the time required for correlations to develop and the information that remains accessible under partial observation.