I have a circuit composed by n qubits, plus a single one which is an ancilla. I'm making multiple measurements on the ancilla at different stages of the circuit, while working on the n qubits. These measurements are not really needed at all: they are just a way to collapse the state of the qubit at some point during the computation and then reuse the same qubit in a different way.

At the end of the circuit, when I'm measuring the outcome of the n qubits, I don't want the result of this ancilla to be shown in the output of the get_counts() function; what I want is only the output of the n qubits. Is there any way to achieve this result?


1 Answer 1


If you keep measuring to the same bit, the value should get overridden every time. So you won't receive the intermediary values.

For example, the following will output a single 1 from the second measurement, with no trace of the first.

from qiskit import ClassicalRegister, QuantumRegister, QuantumCircuit
from qiskit import execute
from qiskit import BasicAer

q = QuantumRegister(1)
c = ClassicalRegister(1)
qc = QuantumCircuit(q,c)


job = execute(qc,backend=BasicAer.get_backend('qasm_simulator'))

I guess what you want is something more like

q = QuantumRegister(n) # n qubits
a = QuantumRegister(1) # one ancilla qubit
c = ClassicalRegister(n) # n classical bits for output

qc = QuantumCircuit(q,a,c)

qc.measure(a[0],c[0]) # measure the ancilla to one of the classical bits

qc.measure(q,c) # measure the n qubits to the n bits (overwriting the output from the previous measurement
  • $\begingroup$ I've updated my question to better clarify the issue. $\endgroup$
    – tigerjack
    Dec 15, 2018 at 17:48
  • $\begingroup$ I'm not sure I understand. It is only the classical register that is shown in the output. Results from qubits only appear if measured. In that case, they appear on the classical bit you chose when measuring. $\endgroup$ Dec 15, 2018 at 17:58
  • $\begingroup$ Sure, but when I measure the n qubits, the result of the ancilla is still shown in the output. Instead, I only want the results from the n qubits, i.e. the one that I'm interested to. $\endgroup$
    – tigerjack
    Dec 15, 2018 at 18:11
  • $\begingroup$ Does the new snippet in my edit do the kind of thing you want? $\endgroup$ Dec 15, 2018 at 18:23
  • $\begingroup$ Exactly this... $\endgroup$
    – tigerjack
    Dec 15, 2018 at 18:25

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