I have the following circuit in qiskit. It is a simple CNOT operation with the control qubit set to 1, by running the control qubit through an X-gate.
However, when I measure the 2nd qubit, I get the state $|00\rangle$, not $|11\rangle$.
If I add a measurement of the control qubit, I do see $|11\rangle$ as the final result.
I fully expected $|11\rangle$ as the outcome regardless of whether I measure a single or both qubits
from qiskit import ClassicalRegister, QuantumRegister, QuantumCircuit from qiskit import execute from qiskit import BasicAer from math import * q0 = QuantumRegister(2, "q0") # one ancilla qubit c0 = ClassicalRegister(2, "c0") # n classical bits for output qc = QuantumCircuit(q0,c0) qc.x(q0) qc.cx(q0, q0) #qc.measure(q0,c0) <= when commented out I measure |00> qc.measure(q0,c0) qc.draw() job = execute(qc,backend=BasicAer.get_backend('qasm_simulator')) job.result().get_counts()