Circuit Introspection Cheat Sheet

qc/qiskit

A collection of QuantumCircuit methods/properties for pulling structural facts out of a built circuit — useful for benchmarking, comparing transpilation results across backends, or just debugging.

What you want	How to get it
Total qubit count	circuit.num_qubits (property)
Ancilla qubit count	circuit.num_ancillas
Circuit depth (longest dependent-gate chain)	circuit.depth()
Depth counting only 2-qubit gates	circuit.depth(lambda instr: instr.operation.num_qubits >= 2)
Gate name → count mapping	dict(circuit.count_ops())
Count of multi-qubit gate operations	circuit.num_nonlocal_gates()
Iterate every instruction in order	for instr in circuit.data: instr.operation.name

Depth vs size, the important distinction: .size() counts total gates; .depth() counts the longest chain assuming independent gates run in parallel. Depth is the better proxy for actual circuit runtime on hardware, since gates on disjoint qubits can execute simultaneously.

Why 2-qubit depth specifically matters: two-qubit gates are typically the noisiest, slowest operations on real hardware (see Backend Properties), so isolating how much of the total depth comes from them specifically is a much sharper signal than raw depth when comparing e.g. two different topologies (Coupling Map and Topology) or layouts (Physical vs Logical Qubits and Layout).

def quantum_circuit_params(circuit: QuantumCircuit) -> dict:
    return {
        "Number of qubits": circuit.num_qubits,
        "Depth": circuit.depth(),
        "2-qubit depth": circuit.depth(lambda instr: instr.operation.num_qubits >= 2),
        "Gates": dict(circuit.count_ops()),
        "Multi-qubit gates": circuit.num_nonlocal_gates(),
        "Number of ancillas": circuit.num_ancillas,
    }

Related

• SWAP Overhead and Routing
• Backend Properties
• Coupling Map and Topology
• Qiskit API Gotchas

Self-Check

• What’s the difference between circuit.size() and circuit.depth(), and which better predicts real runtime?
• Why would you want 2-qubit-only depth instead of total depth when comparing topologies or layouts?
• Which method would you reach for to count how many cx gates are in a circuit?