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I'm trying to write a 1-bit teleportation error correction code and there is one part in it where I need to add a gate that's dependent on a measurement from earlier in the circuit. The measurement is stored on a single classical bit, and I'm trying to use c_if to apply the gate based on the measurement result of that bit.

Intuitively I would think this could be written as: qc.h(q[0]).c_if(c[3], 1) but that only returns an error while qc.h(q[0]).c_if(c, 1) gives no error (though it doesn't give the result I'm looking for). Is there a way to link c_if to a single classical bit instead of the classical register?

This question is similar and the answer seems to imply that it isn't possible to use c_if this way.

If c_if can't do this, would something like what they mentioned in their question: if(c[3]==1) qc.h(q[0]) work in qiskit? For my purposes the operation needs to be integrated into the circuit, so if it does work would the gate be applied automatically?

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c_if must be used on an entire ClassicalRegister. However, it is still possible to use it on a single classical bit. You would need to create a ClassicalRegister of size 1, and attach that to your circuit. This would be the register that you input into the c_if call.

from qiskit import QuantumCircuit, ClassicalRegister, QuantumRegister

c1 = ClassicalRegister(1)
c3 = ClassicalRegister(3)
q4 = QuantumRegister(4)
qc = QuantumCircuit(q4, c3)  # Instantiate with one of the classical registers 
qc.add_register(c1) # Add the other classical register

qc.x(0)
qc.measure(range(4), range(4)
qc.x(0).c_if(c1, 1)  # Add an X-Gate if the classical register c1 outputs 1

Do note though, using c_if is not supported on the real hardware, so you would only be able to run this circuit on the simulator.

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