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I want to apply the projector $|0\rangle\langle 0|$ onto a previously prepared state $|\psi_1\rangle$ in qiskit. I have already initialized a quantum circuit (with two registers) into $|\psi_1\rangle$, but I don't know how to apply a gate that projects one of the registers into $|0\rangle$.

How can I create a projector operator to obtain this? I want to run this program in QASM Simulator (and later in a real device).

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Projecting a state to $|0\rangle\langle 0|$ with probability 1 is equivalent to initialization (indeed, this is why it’s called “projective measurement”), so you can simply call reset (see here: https://qiskit.org/documentation/stubs/qiskit.circuit.QuantumCircuit.reset.html )

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