Conditional Gaussian Preparation with Retained Archives

Can reversible quantum dynamics prepare a subsystem for measurement while retaining the physical archives of its earlier history? This paper provides an explicit mathematical construction and identifies the conditions under which such preparation succeeds. An exact, smooth split–copy–return protocol couples a Gaussian writer to a finite bank of position archives. The analysis shows why preparation of the writer alone is insufficient: singular archive populations and exact retained copies can preserve its fine information. A positive preparation theorem instead uses regularity and moment conditions on the complete original conditional law, retaining every archive coordinate and its correlations without imposing an upper bound on the probability density.A worked example with 102 position coordinates gives a joint conditional preparation error below 0.000010052. Under the stated assumptions, the same bound applies to a subsequent one-use labelled projective measurement with an arbitrary finite internal reference system. A separate physical receiver copies the writer’s earlier outcome and preserves it during the specified holding dynamics. The paper includes complete proofs, counterexamples, robustness estimates, and an analysis of reference systems and postselection. Its results concern conditional subsystem preparation and a one-use measurement within a controlled canonical-current model. Physical realization of the controls and independent justification of the admitted initial statistical laws remain separate requirements.