Coherence controls the visibility of frozen disorder in an open quantum chain

Abstract

A Gaussian position distribution need not imply that frozen disorder is irrelevant. We study a resonant quantum chain where neighboring sites couple with opposite signs to auxiliary states that decay with frozen, random return probabilities. Lowering dephasing spreads the effect of each local return bias over more sites. We take dephasing to zero at a fixed, sufficiently weak disorder amplitude. On the scale of a coherent wave packet, the position distribution approaches the same Gaussian for almost every frozen sample. At larger distances and later times, it remains sample dependent and is described by diffusion in a random potential with Brownian spatial statistics. The covariance of bounded, direction-sensitive position readouts from independent trials in the same field distinguishes these regimes. Simulations resolve their separation at finite dephasing. Taking weak disorder before vanishing dephasing also reveals a different packet-scale response from a memoryless random-force model matched in clean diffusion and leading potential strength. The coherent response develops an integrable logarithmic peak near the initial site, where the reference response stays finite. It reflects the correlated duration and displacement of motion between jumps, together with the unfinished packet at measurement.