Quantum Information Copy Time: Exact Receiver Discrimination in Diffusive Exclusion Dynamics

When can a distant receiver reliably distinguish two locally prepared quantum states?Neither an operator-spreading front nor a locality bound answers this question without speci-fying the receiver and the required discrimination threshold. We define quantum informationcopy time through the Helstrom trace distance and distinguish optimal readout from exper-imentally restricted measurements.For the ensemble-averaged diagonal quantum symmetric simple exclusion process, wederive an exact finite-volume expression for the optimal discrimination advantage of anyfinite receiver following a one-site perturbation of a Bernoulli product state. Reversibilityidentifies the optimal occupation-weighted test and implies an exact two-site no-gain identityat half filling. The same microscopic construction yields finite-ring saturation, a square-rootreceiver-width enhancement in a specified diffusive regime, and threshold-dependent first-crossing laws without a Gaussian closure.A complementary coherent benchmark comes from an interacting, number-conservingspin chain in which adjacent excitations propagate while isolated excitations remain sta-tionary. Here the optimal receiver advantage is an exact sum of local pair probabilities,and a distance-scaled threshold produces a ballistic first-crossing time with a calculableAiry correction. These contrasting examples show why locality alone does not determinean operational transport exponent. For generic closed Hamiltonian systems, we identify thememory-kernel assumptions needed for a hydrodynamic reduction and prove an obstructionto imposing a dissipative quadratic-form gap on the exact unitary Mori block. Finite-sizeXXZ and TEBD/MPS diagnostics are reported without inferring thermodynamic scaling.