Information

• Qiskit Terra version: 0.18.0
• Python version: 3.9.5 (default, May 18 2021, 12:31:01) [Clang 10.0.0 ]
• Operating system: Darwin

What is the current behavior?

I am currently seeing

QiskitError: 'Unknown fitter <qiskit.ignis.mitigation.measurement.fitters.CompleteMeasFitter object at 0x7fcbcace6c40>'

error, when I pass an qiskit.ignis.mitigation.measurement.fitters.CompleteMeasFitter object as a value to the parameter measurement_error_mitigation_cls in the constructor of QuantumInstance and use the QuantumInstance object with QuantumKernel.evaluate() .

Steps to reproduce the problem

1. Generate the calibration circuits using complete_meas_cal().
2. Execute the calibration circuits on target hardware (for this week ibm_toronto).
3. Calculate the calibration matrix using CompleteMeasFitter().
4. Pass the calibration matrix calculated in step # 3 as a value for the parameter measurement_error_mitigation_clswhile creating an instance of QuantumInstance . For simplicity, I am not overriding the default values for cals_matrix_refresh_period and measurement_error_mitigation_shots.
5. Use the QuantumInstance object in the standard QSVM pipeline.

What is the expected behavior?

It should not throw any runtime error.

Suggested solutions

CompleteMeasFitter incorrectly imported in measurement_error_mitigation.py. At line # 141 of measurement_error_mitigation.py, CompleteMeasFitter should be imported from qiskit.ignis.mitigation.measurement.fitters instead of qiskit.ignis.mitigation.measurement.