barriers are ignored in QuantumCircuit.depth()

The depth method on QuantumCircuit ignores barriers:

from qiskit import QuantumCircuit
circ_serial = QuantumCircuit(4)
circ_serial.h(0)
circ_serial.cx(0, 1)
circ_serial.barrier()
circ_serial.h(2)
circ_serial.cx(2, 3)
print(circ_serial)
print('depth: ', circ_serial.depth())

circ_parallel = QuantumCircuit(4)
circ_parallel.h(0)
circ_parallel.cx(0, 1)
circ_parallel.h(2)
circ_parallel.cx(2, 3)
print(circ_parallel)
print('depth: ', circ_parallel.depth())

        ┌───┐      ░           
q_0: |0>┤ H ├──■───░───────────
        └───┘┌─┴─┐ ░           
q_1: |0>─────┤ X ├─░───────────
             └───┘ ░ ┌───┐     
q_2: |0>───────────░─┤ H ├──■──
                   ░ └───┘┌─┴─┐
q_3: |0>───────────░──────┤ X ├
                   ░      └───┘
depth:  2
        ┌───┐     
q_0: |0>┤ H ├──■──
        └───┘┌─┴─┐
q_1: |0>─────┤ X ├
        ┌───┐└───┘
q_2: |0>┤ H ├──■──
        └───┘┌─┴─┐
q_3: |0>─────┤ X ├
             └───┘
depth:  2

The depth method on DAGCircuit is correct however:

from qiskit.converters import circuit_to_dag
circuit_to_dag(circ_serial).depth()

5

Note: snapshot should also behave similarly to barrier and extend the depth.

[nonhermitian]

This was by design. Should a snapshot actually change the depth? I think not as it is not a physical operation. As for barrier. Otherwise depth losses it property of being a measure of circuit length and gauge of whether a circuit can actually run.

[taalexander]

| I consider scheduling part of compilation.

You’re right, corrected to transpiler directive.