Surviving readout errors and incomplete postselection in monitored quantum circuits
We study how readout errors affect dynamics in monitored quantum circuits. Such errors make postselection effectively incomplete, conditioning trajectories on imperfect rather than true records. Focusing on U(1)-symmetric circuits, we find that the charge-sharpening transition survives at a finite error rate, with the sharp phase's vanishing fluctuations replaced by a finite steady-state plateau. The charge fluctuations show a shallow minimum near the ideal transition point as a function of measurement rate, signaling the competition between measurement-induced purification and noise-induced mixing. By decomposing the fluctuations into quantum and classical contributions, we show the minimum marks where incomplete postselection still retains enough trajectory memory to attenuate noise-induced mixing, leaving a detectable signature of the transition even in a noisy setting. Mid-circuit measurements are far harder to mitigate against readout errors than standard end-of-circuit ones, because the errors corrupt the postselection itself; the decomposition therefore offers a minimal mitigation framework and a route to locate the transition directly from experimental data. Our results show that charge sharpening survives readout errors in a quantitative and experimentally accessible way.