Qiskit - How do we measure the same single qubit system multiple times while also getting the counts/results after each measurement?

Question

I am able to apply multiple measurements (2) to the same qubit system. However, I would like to get the counts right after each measurement is executed. I realize that the counts should be the same after every measurement of the same system. I would just like to see empirical proof of the so-called "repeated measurement principle" without any evolution of the system. I attempted to write a program which would achieve this, but had no success:

import time 
from qiskit import QuantumCircuit, ClassicalRegister, QuantumRegister 
from qiskit import execute, IBMQ, Aer 
from qiskit.backends.ibmq import least_busy 
from qiskit import compile as q_compile

counter = 0

def generate_circuits(num_q):
    # Create a Quantum Register with 1 qubits.
    q = QuantumRegister(num_q, 'q')
    # Create a Classical Register with 1 bits.
    c = ClassicalRegister(num_q, 'c')
    # Create a Quantum Circuit
    qc = QuantumCircuit(q, c)

    if counter == 0:
        # Add a H gate on qubit 0, putting this qubit in superposition.
        qc.h(q[0])
        # Add a Measure gate to see the state.
        qc.measure(q, c)
        qc.barrier(q)
        return qc
    else:
        qc.measure(q, c)
        return qc

backend = Aer.get_backend('qasm_simulator')

qc_gen = generate_circuits(1)

qobj = q_compile(qc_gen, backend, shots=1000)

job = backend.run(qobj)

result = job.result()

data = result.get_counts(qc_gen)

print(data)

counter += 1

qobj = q_compile(qc_gen, backend, shots=1000)

job = backend.run(qobj)

result = job.result()

data = result.get_counts(qc_gen)

print(data)

which returned:

{'0': 489, '1': 511} 
{'0': 491, '1': 509}

Any ideas?

Answer 1

The two quantum circuits generated and submitted to the IBMQ system by your code are the same. So ideally, the result should be both {'0':500,'1':500}, but since those circuits are run on an experimental platform, a little fluctuation is tolerable.