Fix/commuting2q block parameter preservation

qiskit/transpiler/passes/routing/commuting_2q_gate_routing/commuting_2q_block.py

@@ -16,7 +16,7 @@
 from collections.abc import Iterable
 
 from qiskit.exceptions import QiskitError
-from qiskit.circuit import Gate, Qubit, Clbit
+from qiskit.circuit import Clbit, Gate, ParameterExpression, Qubit
 from qiskit.dagcircuit import DAGOpNode
 
 
@@ -37,20 +37,30 @@ def __init__(self, node_block: Iterable[DAGOpNode]) -> None:
         """
         qubits: set[Qubit] = set()
         cbits: set[Clbit] = set()
+        params_set: set[ParameterExpression] = set()
+
         for node in node_block:
             if len(node.qargs) != 2:
                 raise QiskitError(f"Node {node.name} does not apply to two-qubits.")
 
             qubits.update(node.qargs)
             cbits.update(node.cargs)
 
+            for param in node.op.params:
+                if isinstance(param, ParameterExpression):
+                    params_set.update(param.parameters)
+        params_list = sorted(params_set, key=lambda p: p.name)
+
         if cbits:
             raise QiskitError(
                 f"{self.__class__.__name__} does not accept nodes with classical bits."
             )
 
         super().__init__(
-            "commuting_2q_block", num_qubits=len(qubits), params=[], label="Commuting 2q gates"
+            "commuting_2q_block",
+            num_qubits=len(qubits),
+            params=params_list,
+            label="Commuting 2q gates",
         )
         self.node_block = node_block
         self.qubits = qubits

releasenotes/notes/fix-commuting2qblock-parameters-bc0ad1c462ad72d6.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - Fixed parameter loss in :class:`~.Commuting2qBlock` where the ``params``
+    attribute was hardcoded to an empty list, causing all parameters from
+    input parametric gates to be lost. 
+    The fix properly extracts and preserves parameters from gate operations,
+    including those in :class:`~.ParameterExpression` objects.

test/python/transpiler/test_swap_strategy_router.py

@@ -14,7 +14,7 @@
 
 from ddt import ddt, data
 
-from qiskit.circuit import QuantumCircuit, Qubit, QuantumRegister
+from qiskit.circuit import QuantumCircuit, Qubit, QuantumRegister, Parameter
 from qiskit.providers.fake_provider import GenericBackendV2
 from qiskit.transpiler import PassManager, CouplingMap, Layout, TranspilerError
 from qiskit.circuit.library import PauliEvolutionGate, CXGate
@@ -676,3 +676,23 @@ def test_commuting2qblocks_errors(self):
 
         with self.assertRaises(QiskitError):
             Commuting2qBlock(circuit_to_dag(circ).op_nodes())
+
+    def test_commuting2qblock_preserves_parameters(self):
+        """Test that Commuting2qBlock preserves parameters from parametric gates."""
+
+        c_0 = Parameter("c_0")
+        c_1 = Parameter("c_1")
+        gamma = Parameter("gamma")
+
+        qc = QuantumCircuit(3)
+        qc.rzz(c_0 * gamma, 0, 1)
+        qc.rzz(c_1, 1, 2)
+
+        dag = circuit_to_dag(qc)
+        nodes = list(dag.topological_op_nodes())
+        block = Commuting2qBlock(nodes)
+
+        # Check that both parameters are preserved
+        self.assertEqual(len(block.params), 3)
+        param_names = {p.name for p in block.params}
+        self.assertEqual(param_names, {"c_0", "gamma", "c_1"})