  Quantum technologies rely heavily on accurate control and reliable readout of
quantum systems. Current experiments are limited by numerous sources of noise
that can only be partially captured by simple analytical models and additional
characterization of the noise sources is required. We test the ability of
readout error mitigation to correct realistic noise found in systems composed
of quantum two-level objects (qubits). To probe the limit of such methods, we
designed a beyond-classical readout error mitigation protocol based on quantum
state tomography (QST), which estimates the density matrix of a quantum system,
and quantum detector tomography (QDT), which characterizes the measurement
procedure. By treating readout error mitigation in the context of state
tomography the method becomes largely readout mode-, architecture-, noise
source-, and quantum state-independent. We implement this method on a
superconducting qubit and evaluate the increase in reconstruction fidelity for
QST. We characterize the performance of the method by varying important noise
sources, such as suboptimal readout signal amplification, insufficient
resonator photon population, off-resonant qubit drive, and effectively
shortened $T_1$ and $T_2$ decay times. As a result, we identified noise sources
for which readout error mitigation worked well, and observed decreases in
readout infidelity by a factor of up to 30.
