  We use quantum detector tomography to characterize the qubit readout in terms
of measurement POVMs on IBM Quantum Computers IBM Q 5 Tenerife and IBM Q 5
Yorktown. Our results suggest that the characterized detector model deviates
from the ideal projectors by a few percent. Further improvement on this
characterization can be made by adopting two- or more-qubit detector models
instead of independent single-qubit detectors for all the qubits in one device.
An unexpected behavior was seen in the physical qubit labelled as qubit 3 of
IBM Q 5 Tenerife, which can be a consequence of detector crosstalk or qubit
operations influencing each other and requires further investigation. This
peculiar behavior is consistent with characterization from the more
sophisticated approach of the gate set tomography. We also discuss how the
characterized detectors' POVM, despite deviation from the ideal projectors, can
be used to estimate the ideal detection distribution.
