Swap removal fix (#38)

Co-authored-by: Elena Peña Tapia <[email protected]>

qopt_best_practices/transpilation/swap_cancellation_pass.py

@@ -18,8 +18,6 @@ def run(self, dag: DAGCircuit):
 
         qmap = self.property_set["virtual_permutation_layout"]
 
-        qreg = dag.qregs[next(iter(dag.qregs))]
-
         # This will remove SWAP gates that are applied before anything else
         # This remove is executed multiple times until there are no more SWAP
         # gates left to remove. Note: a more inteligent DAG traversal could
@@ -34,8 +32,7 @@ def run(self, dag: DAGCircuit):
                     successors = list(dag.successors(node))
                     if len(successors) == 2:
                         if all(isinstance(successors[idx], DAGOutNode) for idx in [0, 1]):
-                            bits = [qreg.index(qubit) for qubit in node.qargs]
-                            qmap[bits[0]], qmap[bits[1]] = qmap[bits[1]], qmap[bits[0]]
+                            qmap.swap(node.qargs[0], node.qargs[1])
                             dag.remove_op_node(node)
                             permuted = True
 

test/test_qubit_selection.py

@@ -4,7 +4,7 @@
 import os
 from unittest import TestCase
 
-from qiskit_ibm_runtime.fake_provider import FakeSherbrooke, FakeHanoi, FakeHanoiV2
+from qiskit_ibm_runtime.fake_provider import FakeSherbrooke, FakeHanoiV2
 from qiskit.providers.fake_provider import GenericBackendV2
 
 
@@ -52,12 +52,9 @@ def test_qubit_selection(self):
         expected_path = [33, 39, 40, 72, 41, 81, 53, 60, 61, 62]
         self.assertEqual(set(path), set(expected_path))
 
-    def test_qubit_selection_v1_v2(self):
+    def test_qubit_selection_v2(self):
         """Test backend evaluation for 10 qubit line"""
-        backends = [FakeHanoi(), FakeHanoiV2()]
-
-        for backend in backends:
-            path_finder = BackendEvaluator(backend)
-            path, _, _ = path_finder.evaluate(len(self.mapped_graph))
-            expected_path = [8, 9, 11, 12, 13, 14, 15, 18, 21, 23]
-            self.assertEqual(set(path), set(expected_path))
+        path_finder = BackendEvaluator(FakeHanoiV2())
+        path, _, _ = path_finder.evaluate(len(self.mapped_graph))
+        expected_path = [8, 9, 11, 12, 13, 14, 15, 18, 21, 23]
+        self.assertEqual(set(path), set(expected_path))

test/test_swap_cancellation.py

@@ -0,0 +1,45 @@
+"""The the cancellation of unneeded SWAP gates."""
+
+
+from unittest import TestCase
+
+from qiskit import QuantumCircuit
+from qiskit.converters import circuit_to_dag
+from qiskit.transpiler import Layout
+
+from qopt_best_practices.transpilation.swap_cancellation_pass import SwapToFinalMapping
+
+
+class TestSwapCancellation(TestCase):
+    """Test the swap cancellation pass."""
+
+    def test_simple(self):
+        """Simple test."""
+
+        qc = QuantumCircuit(4, 4)
+        qc.swap(3, 2)
+        qc.rzz(1.234, 1, 2)
+        qc.swap(1, 2)
+
+        qreg = next(iter(qc.qregs))
+
+        swap_pass = SwapToFinalMapping()
+        layout = Layout(
+            {
+                0: qreg[0],
+                1: qreg[2],
+                2: qreg[3],
+                3: qreg[1],
+            }
+        )
+        swap_pass.property_set["virtual_permutation_layout"] = layout
+
+        dag = circuit_to_dag(qc)
+        _ = swap_pass.run(dag)
+
+        new_layout = swap_pass.property_set["virtual_permutation_layout"]
+
+        self.assertTrue(new_layout[0] == qreg[0])
+        self.assertTrue(new_layout[1] == qreg[1])
+        self.assertTrue(new_layout[2] == qreg[3])
+        self.assertTrue(new_layout[3] == qreg[2])