A6.2: Programmable Global Which‑Path Tagging and Conditional Interference Restoration on Superconducting Quantum Hardware

Decoherence is often described as “loss of quantum coherence,” but in standard quantum theory it is more precisely a redistribution of coherence from a system into correlations with other degrees of freedom (environment, measurement record, or marker). A6.2 implements a programmable, global which‑path “MARK” interaction between a multi‑qubit register and a marker ancilla, and demonstrates on superconducting quantum hardware that (i) unconditional interference in the register is suppressed as the marker strength is increased, while (ii) interference is restored when the data are conditioned on the marker outcome measured in an eraser basis. This is a hardware‑native, circuit‑level realization of the quantum‑eraser mechanism in a multi‑qubit setting, providing an operational demonstration that coherence can be exported into system–marker correlations and recovered by conditional inference rather than being irreversibly destroyed.