Measure and Forget Dynamics in Random Circuits

Abstract: "Forgetful" measurements-physically similar to dephasing-are of interest both for applications to fault-tolerant quantum computing and fundamentally, in studying how entanglement and entropy spread. This paper investigates measurement-induced phase transitions (MIPT) in random Clifford circuits when measurement outcomes are partially forgotten. Our findings reveal a local thermalization rate that remains constant regardless of system size. We also numerically calculate the decay behavior at the turning points in the entropy diagram. We observe a counterintuitive phenomenon where the entropy reaches a threshold and stops evolving, even as the system size increases. This challenges an intuition, drawn from previous studies of noisy random circuits, that noise will cause the thermalization of the whole system. Additionally, we identify the disappearance of the purification transition and discuss the implications of these entanglement dynamics for quantum error-correction codes.