How to collapse the subcircuits in a QuantumCircuit?

Question

Suppose I have a QuantumCircuit object named FC, which explicitly contains the identity gate, and convert it into a gate using .to_instruction():

gate = FC.to_instruction()

and I use this to create a bigger QuantumCircuit object named circuit, which looks like this:

Now, when I iterate over the elements of the circuit, using this code:

for instruction, qarg, carg in circuit:
    print(instruction)

I get this:

Instruction(name='gate', num_qubits=3, num_clbits=0, params=[])
Instruction(name='gate', num_qubits=3, num_clbits=0, params=[])

However, gate itself is made up of its constituent elements, which don't show up in the above list. How do I "collapse" the circuit, so that I can iterate over all the components present in circuit, rather than just gate? One way you can go about doing this is to transpile the circuit like this:

from qiskit import Aer, transpile

simulator = Aer.get_backend('aer_simulator')
circ = transpile(circ, simulator)

and iterate over circ, but when I do that, the identity gate does not show up.

Answer 1

You can achieve that by calling QuantumCircuit's decompose() method as follows:

for instruction, qarg, carg in circuit.decompose():
    print(instruction)

Another option is to add FC to your circuit using compose method intead of append:

circuit = circuit.compose(FC, [0, 1, 2])

Note that you should use FC directly without converting it into a gate.