Add QuantumCircuit.ensure_physical
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This is similar but stronger to `DAGCircuit::make_physical`. Since `QuantumCircuit` also has layout information available, this method safely handles this, and the case that the circuit has already been made physical. The new documentation in the circuit module and the `QuantumCircuit` class is intended to help, since this is a common point of confusion among both users and developers; the documentation isn't supposed to be a strong personal endorsement of the current situation, just an explicit statement of what the current system _is_. This commit was motivated by ad-hoc attempts to do this, such as in `QuantumCircuit._from_circuit_data(add_regs=True)` (which can be updated to use this in a follow-up).
jakelishman
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ihincks
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Jul 22, 2025
| /// Args: | ||
| /// num_qubits: if given, the total number of physical qubits in the output; it must be at | ||
| /// least as large as the number of qubits in the circuit. If not given, the number of | ||
| /// qubits is unchanged. |
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You know qiskit style better than me, but I'll mention in case it is an oversight rather than intentional: do you want a Raises: section here?
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we're pretty lax with "Raises" sections - I don't usually add them when the only exceptions are things like ValueError for things that are fairly obviously incorrect inputs. If you go down that route, basically everything in Python can raise runtime exceptions (TypeError/IndexError/ValueError and all sorts), so I tend to only put them for non-obvious Qiskit-specific things.
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This is similar but stronger to
DAGCircuit::make_physical. SinceQuantumCircuitalso has layout information available, this method safely handles this, and the case that the circuit has already been made physical.The new documentation in the circuit module and the
QuantumCircuitclass is intended to help, since this is a common point of confusion among both users and developers; the documentation isn't supposed to be a strong personal endorsement of the current situation, just an explicit statement of what the current system is.This commit was motivated by ad-hoc attempts to do this, such as in
QuantumCircuit._from_circuit_data(add_regs=True)(which can be updated to use this in a follow-up).Summary
Details and comments
The reason that the
QuantumCircuitmethod is calledensure_physicaland theDAGCircuitone is calledmake_physicalis because the DAG one doesn't have the same information available to it about what (if anything) the current layout is (in the current Qiskit data flow, the layout, if any, is in thePropertySetwhile transpilation is ongoing), so it can't really do a proper verification; it's always an active modification. TheQuantumCircuitvariant can tell if a circuit is already laid out.