CTMP mitigator works only for circuits with even number of qubits

[HwajungKang]

Informations

• Qiskit Ignis version: latest
• Python version:
• Operating system:

What is the current behavior?

Applying CTMP mitigator on circuits with odd number of qubits makes expectation value worse rather than making it closer to the ideal value. works opposite to error mitigation. It behaves right for the circuits with even number of qubits, though. You can check with the code below by changing the variable n_q for the number of qubits.

Steps to reproduce the problem

import numpy as np
from qiskit import *
from qiskit.providers.aer.noise import NoiseModel
from qiskit.ignis.mitigation import (expval_meas_mitigator_circuits,
                                     ExpvalMeasMitigatorFitter,
                                     expectation_value)
from qiskit.test.mock import FakeMelbourne


###################

def circ(n_q):
    
    qc = QuantumCircuit(n_q)
    qc.h(0)
    qc.cx(range(n_q-1), range(1, n_q))
    qc.measure_all()
    
    return qc

####################

sim = Aer.get_backend('qasm_simulator')
backend = FakeMelbourne()


n_q = 7 # number of qubits

qc = circ(n_q)
counts_raw = execute(qc, backend, shots=8192).result().get_counts()

circ_ctmp, data_ctmp = expval_meas_mitigator_circuits(n_q, method='CTMP')
result_ctmp = execute(circ_ctmp, backend, shots=8192).result()
ctmp_mitigator = ExpvalMeasMitigatorFitter(result_ctmp, data_ctmp).fit()

counts_ideal = execute(qc, sim, shots=8192).result().get_counts()

ideal = i1nt(expectation_value(counts_ideal)[0])
raw = expectation_value(counts_raw)[0]
mitigated = expectation_value(counts_raw, meas_mitigator=ctmp_mitigator)[0]

print('Ideal expectation value: {}'.format(ideal))
print('Raw expectation value: {}'.format(raw))
print('CTMP mitigated expectation value: {}'.format(mitigated))

What is the expected behavior?

Suggested solutions

[chriseclectic]

@georgesbarron Do you have any ideas for this?