Can I only use a 'subset' of calibration matrix to reduce measurement errors?

Question

Suppose I have the following calibration matrix generated using 4 qubits:

from qiskit.ignis.mitigation.measurement import (complete_meas_cal, CompleteMeasFitter)
cal_circuits, state_labels = complete_meas_cal(qr = qc.qregs[0],circlabel = 'measerrormitigationcal')
cal_job = execute(cal_circuits, 
                 backend = device,
                 shots = 1024,
                  optimization_level = 0
                 )
print(cal_job.job_id())
job_monitor(cal_job)
cal_results = cal_job.result()
meas_fitter = CompleteMeasFitter(cal_results, state_labels)
import matplotlib.pyplot as plt
fig, ax = plt.subplots(figsize=(10, 10))
meas_fitter.plot_calibration(ax)

Is there a way I can use this filter to reduce another quantum circuit with 3 qubits? (i.e. A subset with 8 elements) (I was wondering if I could add a '0' after the keys but it seems like this won't work since the additional qubit might 'flip'. )

Thanks!!

Answer 1

Yes, I think this can be done through redefine a new meas_fitter that is a subset of the original one.

fig, ax = plt.subplots(figsize=(10, 10))
subset_meas_fitter = meas_fitter.subset_fitter([0,1,2]) #defining new meas_fitter for a set of qubits
subset_meas_fitter.plot_calibration(ax)
subset_meas_fitter.plot_calibration(ax)

Which should give you something like: