BUG: `measure_all` adds extra register in `ghz` test

[MattePalte]

Environment

• qiskit.version: 0.22.0
• Python version: 3.8.0
• Operating system: Ubuntu 18.04

What is happening?

The measure_all in the file ghz.py of the test suite adds (perhaps involuntarily) another classical register instead of using the existing one. This makes the output double as big as it should be, with an empty register.

How can we reproduce the issue?

Run the following section of the circuit:

from qiskit import QuantumCircuit
from qiskit import execute, Aer

n_qubits = 10

def cx_chain(qc,n):
    for i in range(0,n-1):
        qc.cx(i,i+1)

qc = QuantumCircuit(n_qubits, n_qubits)
qc.h(0)
cx_chain(qc,n_qubits)
qc.measure_all()

simulator = Aer.get_backend('statevector_simulator')
job1 = execute(qc,simulator,shots=1000)
result1 = job1.result()
counts1 = result1.get_counts(qc)
print (counts1)

Output:

{'1111111111 0000000000': 498, '0000000000 0000000000': 502}

What should happen?

The existing register should be used instead of adding a new one.

Any suggestions?

Following the documentation of the measure_all method, it is possible to measure the qubits directly, without adding an extra register. The code should be changed to:

qc.measure_all(add_bits=False)

This produces the following:

{'1111111111': 531, '0000000000': 469}