How to split a Quantum Circuit on a barrier in Qiskit?

Question

Let's say I have a QuantumCircuit with multiple barriers as shown in the visual below:

How would I split up the QuantumCircuit on the barriers into a list of 3 QuantumCircuits in Qiskit?

NB: Not directly relevant to the question, but the circuit is a random Unitary circuit, as I'm doing randomised-benchmarking. The circuit is generated as shown in the Qiskit Textbook: 5.3 Randomized Benchmarking.

Answer 1

For such a simple circuit, probably the easier way is to dump it as QASM and read it back.

def split_circuit_by_barrier(circuit):
    qasm = circuit.qasm()
    prelude = []
    circuits = [[]]
    for line in qasm.splitlines():
        if any([line.startswith(t) for t in ['OPENQASM', 'include', 'qreg', 'creg']]):
            prelude.append(line)
        elif line.startswith('barrier'):
            circuits.append([])
        else:
            circuits[-1].append(line)
    circuits_with_prelude = [prelude+circuit for circuit in circuits]
    for circuit_with_prelude in circuits_with_prelude:
        yield QuantumCircuit.from_qasm_str('\n'.join(circuit_with_prelude))

Taking your example:

for subcircuit in (split_circuit_by_barrier(circuit)):
    print(subcircuit)

                             ┌───┐ ┌───┐
   q_0: ─────────────────■───┤ H ├─┤ S ├─────
        ┌───┐┌───┐┌───┐┌─┴─┐┌┴───┴┐├───┤┌───┐
   q_1: ┤ H ├┤ H ├┤ S ├┤ X ├┤ SDG ├┤ H ├┤ X ├
        └───┘└───┘└───┘└───┘└─────┘└───┘└───┘
meas: 2/═════════════════════════════════════

        ┌─────┐┌───┐
   q_0: ┤ SDG ├┤ H ├───────■───────────────────
        └┬───┬┘├───┤┌───┐┌─┴─┐┌─────┐┌───┐┌───┐
   q_1: ─┤ X ├─┤ H ├┤ S ├┤ X ├┤ SDG ├┤ H ├┤ H ├
         └───┘ └───┘└───┘└───┘└─────┘└───┘└───┘
meas: 2/═══════════════════════════════════════

        ┌─┐
   q_0: ┤M├───
        └╥┘┌─┐
   q_1: ─╫─┤M├
         ║ └╥┘
meas: 2/═╩══╩═
         0  1