Fix VF2PostLayout with uncoupled qubits in strict_direction=True (Qiskit#14998) (Qiskit#15001)

When `VF2PostLayout` is set in strict mode, we have to include even
active but uncoupled qubits in the interaction graph, because of the
semantic constraints.  We need to limit the number of these uncoupled
qubits we'll consider, though, because there's a factorial overhead to
handling them.

A previous commit[^1] added this skip to `strict_direction=False`, but
didn't correctly handle the continuation case where we're within the
limit.  This commit correctly clears out the uncoupled-qubit metadata
that was causing problems in later layout reconstruction.

[^1]: b4094dd: Fix calls to `VF2PostLayout` after optimization loop at O3

(cherry picked from commit 2d73e5d)

Co-authored-by: Jake Lishman <[email protected]>

Author: mergify[bot]

qiskit/transpiler/passes/layout/vf2_post_layout.py

@@ -153,11 +153,19 @@ def run(self, dag):
             self.property_set["VF2PostLayout_stop_reason"] = VF2PostLayoutStopReason.MORE_THAN_2Q
             return
         im_graph, im_graph_node_map, reverse_im_graph_node_map, free_nodes = result
-        if len(free_nodes) > 1 and self.strict_direction:
-            self.property_set["VF2PostLayout_stop_reason"] = (
-                VF2PostLayoutStopReason.NO_BETTER_SOLUTION_FOUND
-            )
-            return
+        if self.strict_direction and free_nodes:
+            # If there are uncoupled qubits, in non-strict modes we just allocate them to the
+            # lowest-error states at the end.  However, in strict mode, we have to consider them at
+            # the same time to handle heterogeneous targets correctly.  This risks a factorial
+            # combinatoric explosion in complexity, though, so we put a limit on how many we'll
+            # handle.  The builder still builds the graph entirely, it just returns the free nodes
+            # for us to check on, so we clear that out after we've checked it.
+            if len(free_nodes) > 1:
+                self.property_set["VF2PostLayout_stop_reason"] = (
+                    VF2PostLayoutStopReason.NO_BETTER_SOLUTION_FOUND
+                )
+                return
+            free_nodes.clear()
         scoring_bit_list = vf2_utils.build_bit_list(im_graph, im_graph_node_map)
         scoring_edge_list = vf2_utils.build_edge_list(im_graph)
 

test/python/transpiler/test_vf2_post_layout.py

@@ -18,7 +18,7 @@
 from qiskit import QuantumRegister, QuantumCircuit
 from qiskit.circuit import ControlFlowOp
 from qiskit.circuit.library import CXGate, XGate
-from qiskit.transpiler import Layout, TranspilerError
+from qiskit.transpiler import Layout, TranspilerError, PassManager, passes
 from qiskit.transpiler.passes.layout.vf2_post_layout import VF2PostLayout, VF2PostLayoutStopReason
 from qiskit.converters import circuit_to_dag
 from qiskit.providers.fake_provider import GenericBackendV2
@@ -352,6 +352,33 @@ def test_complete_layout_with_idle_qubits(self, seed, strict_direction):
         }
         self.assertEqual(unallocated, set())
 
+    @ddt.data(-1, 12)
+    def test_complete_layout_with_one_uncoupled_qubit(self, seed):
+        """Test that a single uncoupled qubit is handled correctly.
+
+        Regression test of https://github.com/Qiskit/qiskit/issues/14997."""
+        qc = QuantumCircuit(3)
+        qc.x(0)
+        qc.cx(1, 2)
+
+        bad = InstructionProperties(error=1e-1)
+        good = InstructionProperties(error=1e-5)
+
+        target = Target()
+        target.add_instruction(XGate(), {(0,): bad, (1,): good, (2,): good})
+        target.add_instruction(CXGate(), {(0, 1): good, (1, 2): bad})
+
+        pm = PassManager(
+            [
+                passes.TrivialLayout(target),
+                passes.ApplyLayout(),
+                passes.VF2PostLayout(target, seed=seed, strict_direction=True),
+                passes.ApplyLayout(),
+            ]
+        )
+        out = pm.run(qc)
+        self.assertEqual(out.layout.final_index_layout(), [2, 0, 1])
+
 
 class TestVF2PostLayoutScoring(QiskitTestCase):
     """Test scoring heuristic function for VF2PostLayout."""