Add option to skip deepcopy on circuit_to_dag (Qiskit#9848)

Just as was done for Qiskit#9825 for dag_to_circuit(), this commit adds a new
option, copy_operations, which allows to disable the deepcopy on
circuit_to_dag(). Previously, the circuit to dag converter would always
deep copy every operation, however we don't need this extra overhead
in every case; where the input QuantumCircuit is discarded after
conversion. This new flag lets us avoid the copy overhead in these
situations, however in general the converter still needs to default to
copying.

qiskit/circuit/quantumcircuit.py

@@ -496,7 +496,9 @@ def __eq__(self, other) -> bool:
         # TODO: remove the DAG from this function
         from qiskit.converters import circuit_to_dag
 
-        return circuit_to_dag(self) == circuit_to_dag(other)
+        return circuit_to_dag(self, copy_operations=False) == circuit_to_dag(
+            other, copy_operations=False
+        )
 
     @classmethod
     def _increment_instances(cls):

qiskit/converters/circuit_to_dag.py

@@ -16,11 +16,18 @@
 from qiskit.dagcircuit.dagcircuit import DAGCircuit
 
 
-def circuit_to_dag(circuit):
+def circuit_to_dag(circuit, copy_operations=True):
     """Build a ``DAGCircuit`` object from a ``QuantumCircuit``.
 
     Args:
         circuit (QuantumCircuit): the input circuit.
+        copy_operations (bool): Deep copy the operation objects
+            in the :class:`~.QuantumCircuit` for the output :class:`~.DAGCircuit`.
+            This should only be set to ``False`` if the input :class:`~.QuantumCircuit`
+            will not be used anymore as the operations in the output
+            :class:`~.DAGCircuit` will be shared instances and modifications to
+            operations in the :class:`~.DAGCircuit` will be reflected in the
+            :class:`~.QuantumCircuit` (and vice versa).
 
     Return:
         DAGCircuit: the DAG representing the input circuit.
@@ -57,9 +64,10 @@ def circuit_to_dag(circuit):
         dagcircuit.add_creg(register)
 
     for instruction in circuit.data:
-        dagcircuit.apply_operation_back(
-            copy.deepcopy(instruction.operation), instruction.qubits, instruction.clbits
-        )
+        op = instruction.operation
+        if copy_operations:
+            op = copy.deepcopy(op)
+        dagcircuit.apply_operation_back(op, instruction.qubits, instruction.clbits)
 
     dagcircuit.duration = circuit.duration
     dagcircuit.unit = circuit.unit

releasenotes/notes/deepcopy-option-circuit_to_dag-1d494b7f9824ec93.yaml

@@ -0,0 +1,10 @@
+---
+features:
+  - |
+    Added a new option, ``copy_operations``, to :func:`~.circuit_to_dag` to
+    enable optionally disabling deep copying the operations from the input
+    :class`~.QuantumCircuit` to the output :class:`~.QuantumCircuit`. In cases
+    where the input :class`~.QuantumCircuit` is not used anymore after
+    conversion this deep copying is unnecessary overhead as any shared
+    references wouldn't have any potential unwanted side effects if the input
+    :class`~.QuantumCircuit` is discarded.