Fix missing layout in Commuting2qGateRouter

qiskit/transpiler/passes/routing/commuting_2q_gate_routing/commuting_2q_gate_router.py

@@ -160,8 +160,13 @@ def run(self, dag: DAGCircuit) -> DAGCircuit:
         if len(dag.qubits) != next(iter(dag.qregs.values())).size:
             raise TranspilerError("Circuit has qubits not contained in the qubit register.")
 
-        new_dag = dag.copy_empty_like()
+        # Fix output permutation -- copied from ElidePermutations
+        input_qubit_mapping = {qubit: index for index, qubit in enumerate(dag.qubits)}
+        self.property_set["original_layout"] = Layout(input_qubit_mapping)
+        if self.property_set["original_qubit_indices"] is None:
+            self.property_set["original_qubit_indices"] = input_qubit_mapping
 
+        new_dag = dag.copy_empty_like()
         current_layout = Layout.generate_trivial_layout(*dag.qregs.values())
 
         # Used to keep track of nodes that do not decompose using swap strategies.
@@ -183,6 +188,8 @@ def run(self, dag: DAGCircuit) -> DAGCircuit:
 
         self._compose_non_swap_nodes(accumulator, current_layout, new_dag)
 
+        self.property_set["virtual_permutation_layout"] = current_layout
+
         return new_dag
 
     def _compose_non_swap_nodes(

releasenotes/notes/fix-swap-router-layout-f28cf0a2de7976a8.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    Fixed an oversight in the :class:`.Commuting2qGateRouter` transpiler pass where the qreg permutations
+    were not added to the pass property set, so they would have to be tracked manually by the user. Now it's 
+    possible to access the permutation through the output circuit's ``layout`` property and plug the pass
+    into any transpilation pipeline without loss of information.

test/python/transpiler/test_swap_strategy_router.py

@@ -1,6 +1,6 @@
 # This code is part of Qiskit.
 #
-# (C) Copyright IBM 2022, 2023.
+# (C) Copyright IBM 2022, 2024.
 #
 # This code is licensed under the Apache License, Version 2.0. You may
 # obtain a copy of this license in the LICENSE.txt file in the root directory
@@ -15,12 +15,14 @@
 from ddt import ddt, data
 
 from qiskit.circuit import QuantumCircuit, Qubit, QuantumRegister
+from qiskit.providers.fake_provider import GenericBackendV2
 from qiskit.transpiler import PassManager, CouplingMap, Layout, TranspilerError
 from qiskit.circuit.library import PauliEvolutionGate, CXGate
 from qiskit.circuit.library.n_local import QAOAAnsatz
 from qiskit.converters import circuit_to_dag
 from qiskit.exceptions import QiskitError
 from qiskit.quantum_info import Pauli, SparsePauliOp
+from qiskit.transpiler.preset_passmanagers import generate_preset_pass_manager
 from qiskit.transpiler.passes import FullAncillaAllocation
 from qiskit.transpiler.passes import EnlargeWithAncilla
 from qiskit.transpiler.passes import ApplyLayout
@@ -562,9 +564,47 @@ def test_edge_coloring(self, edge_coloring):
 
         self.assertEqual(pm_.run(circ), expected)
 
+    def test_permutation_tracking(self):
+        """Test that circuit layout permutations are properly tracked in the pass property
+        set and returned with the output circuit."""
+
+        # We use the same scenario as the QAOA test above
+        mixer = QuantumCircuit(4)
+        for idx in range(4):
+            mixer.ry(-idx, idx)
+
+        op = SparsePauliOp.from_list([("IZZI", 1), ("ZIIZ", 2), ("ZIZI", 3)])
+        circ = QAOAAnsatz(op, reps=2, mixer_operator=mixer)
+
+        expected_swap_permutation = [3, 1, 2, 0]
+        expected_full_permutation = [1, 3, 2, 0]
+
+        cmap = CouplingMap(couplinglist=[(0, 1), (1, 2), (2, 3)])
+        swap_strat = SwapStrategy(cmap, swap_layers=[[(0, 1), (2, 3)], [(1, 2)]])
+
+        # test standalone
+        swap_pm = PassManager(
+            [
+                FindCommutingPauliEvolutions(),
+                Commuting2qGateRouter(swap_strat),
+            ]
+        )
+        swapped = swap_pm.run(circ.decompose())
+
+        # test as pre-routing step
+        backend = GenericBackendV2(num_qubits=4, coupling_map=[[0, 1], [0, 2], [0, 3]], seed=42)
+        pm = generate_preset_pass_manager(
+            optimization_level=3, target=backend.target, seed_transpiler=40
+        )
+        pm.pre_routing = swap_pm
+        full = pm.run(circ.decompose())
+
+        self.assertEqual(swapped.layout.routing_permutation(), expected_swap_permutation)
+        self.assertEqual(full.layout.routing_permutation(), expected_full_permutation)
+
 
 class TestSwapRouterExceptions(QiskitTestCase):
-    """Test that exceptions are properly raises."""
+    """Test that exceptions are properly raised."""
 
     def setUp(self):
         """Setup useful variables."""