Removing Ambiguity by Adjusting `c_if` Gates in Quantum Circuit

[MattePalte]

Environment

• qiskit.version: 0.45.2
• Python version: 3.10.12
• Operating system: Ubuntu 20.04

What is happening?

In the Python file example_qiskit_conditional.py, the c_if gates are used without preceding measurements. This could lead to ambiguities as the c_if gates are conditional and depend on the state of the classical registers.

How can we reproduce the issue?

Run the following code in the Python file:

q = QuantumRegister(3, "q")
c0 = ClassicalRegister(1, "c0")
c1 = ClassicalRegister(1, "c1")
c2 = ClassicalRegister(1, "c2")
qc = QuantumCircuit(q, c0, c1, c2, name="conditional")

qc.h(q[0])
qc.h(q[1]).c_if(c0, 0)  # h-gate on q[1] is executed
qc.h(q[2]).c_if(c1, 1)  # h-gate on q[2] is not executed

qc.measure(q[0], c0)
qc.measure(q[1], c1)
qc.measure(q[2], c2)

What should happen?

I would have expected either to have preceding measurements, such as:

q = QuantumRegister(3, "q")
c0 = ClassicalRegister(1, "c0")
c1 = ClassicalRegister(1, "c1")
c2 = ClassicalRegister(1, "c2")
qc = QuantumCircuit(q, c0, c1, c2, name="conditional")

qc.h(q[0])
qc.measure(q[0], c0)
qc.h(q[1]).c_if(c0, 0)  # h-gate on q[1] is executed
qc.measure(q[1], c1)
qc.h(q[2]).c_if(c1, 1)  # h-gate on q[2] is not executed

qc.measure(q[0], c0)
qc.measure(q[1], c1)
qc.measure(q[2], c2)

Or to have the c_if gates removed, such as:

q = QuantumRegister(3, "q")
c0 = ClassicalRegister(1, "c0")
c1 = ClassicalRegister(1, "c1")
c2 = ClassicalRegister(1, "c2")
qc = QuantumCircuit(q, c0, c1, c2, name="conditional")

qc.h(q[0])
qc.h(q[1])
# qc.h(q[2]) <- removed

qc.measure(q[0], c0)
qc.measure(q[1], c1)
qc.measure(q[2], c2)

To avoid ambiguity.
Thanks in advance, I wish you a happy and productive day.