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I have a 2 qubits system, maximally entangled, and I performed measurement on one of the qubit.

q = QuantumRegister(2)
c = ClassicalRegister(1)
qc = QuantumCircuit(q, c)
###create Bell State Qubit########
qc.h(q[1])
qc.cx(q[1],q[0])
### Perform Measurement ##########
qc.measure(q[0],c)
job = execute(qc,simulator,shots = 1000)
result = job.result()
counts = result.get_counts(qc)
print("\nTotal count for 0 and 1 are:",counts)

Before measurement, the state of the 2 qubit is $|00\rangle+|11\rangle/\sqrt{2}$. After obtaining the measurement result, I measured another qubit. I expected the 2nd qubit to collapse to either 1 of the 2 states($|0\rangle$ or $|1\rangle$) after measurement, but the result shows it is still in the superposition state. How do I renormalise the qubit state after measurement?

qc.measure(q[1],c)
job = execute(qc,simulator,shots=1000)
result = job.result()
counts = result.get_counts(qc)
print("\n Total count for 0 and 1 are:,counts")
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    $\begingroup$ It's not clear what you think is the problem here. You measure Qubit 0 and you should see 50% 0s and 50% 1s. You rerun the simulator and measure Qubit 1 instead and are still seeing the same results. These are new runs of the simulator. They're not remembering what happened the last time you ran it. $\endgroup$
    – Frank Yellin
    Nov 13, 2022 at 20:38
  • $\begingroup$ Also note that if you measure both bits in the same run after entangling them. You are guaranteed to get 00 or 11 in the classical registers. The value of a qubit after reading it is undefined. $\endgroup$
    – Frank Yellin
    Nov 13, 2022 at 20:54

1 Answer 1

Reset to default
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It looks like you are using the simulator, so adding a reset on the qubit you want to renormalize would work. The reset operation sets a qubit back to the ground state, $|0\rangle$. You would call this like any other operation: circuit.reset(<qubits>). Note: This will not work on the real devices.

If you think it is still in superposition after the measurement, applying another H-Gate would fix that as well.

However, if you plan to continue using the simulator, applying the reset might be the better option as you will always be sure, no matter what, that that qubit is back in the $|0\rangle$ state.

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