Fix DenseLayout when loose bits are present in input circuit

qiskit/transpiler/passes/layout/dense_layout.py

@@ -98,11 +98,9 @@ def run(self, dag):
 
         best_sub = self._best_subset(num_dag_qubits, num_meas, num_cx)
         layout = Layout()
-        map_iter = 0
+        for i, qubit in enumerate(dag.qubits):
+            layout.add(qubit, int(best_sub[i]))
         for qreg in dag.qregs.values():
-            for i in range(qreg.size):
-                layout[qreg[i]] = int(best_sub[map_iter])
-                map_iter += 1
             layout.add_register(qreg)
         self.property_set["layout"] = layout
 

releasenotes/notes/denselayout-loose-bits-3e66011432bc6232.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    Fixed an issue in the :class:`~.DenseLayout` transpiler pass where any
+    loose :class:`~.Qubit` objects (i.e. not part of a :class:`~.QuantumRegister`)
+    that were part of a :class:`~.QuantumCircuit` would not be included in the
+    output :class:`~.Layout` that was generated by the pass.

test/python/transpiler/test_dense_layout.py

@@ -17,7 +17,7 @@
 import numpy as np
 
 from qiskit import QuantumRegister, QuantumCircuit, ClassicalRegister
-from qiskit.circuit import Parameter
+from qiskit.circuit import Parameter, Qubit
 from qiskit.circuit.library import CXGate, UGate, ECRGate, RZGate
 from qiskit.transpiler import CouplingMap, Target, InstructionProperties, TranspilerError
 from qiskit.transpiler.passes import DenseLayout
@@ -218,6 +218,26 @@ def test_5q_circuit_20q_with_if_else(self):
         self.assertEqual(layout[qr[3]], 5)
         self.assertEqual(layout[qr[4]], 0)
 
+    def test_loose_bit_circuit(self):
+        """Test dense layout works with loose bits outside a register."""
+        bits = [Qubit() for _ in range(5)]
+        circuit = QuantumCircuit()
+        circuit.add_bits(bits)
+        circuit.h(3)
+        circuit.cx(3, 4)
+        circuit.cx(3, 2)
+        circuit.cx(3, 0)
+        circuit.cx(3, 1)
+        dag = circuit_to_dag(circuit)
+        pass_ = DenseLayout(CouplingMap(self.cmap20))
+        pass_.run(dag)
+        layout = pass_.property_set["layout"]
+        self.assertEqual(layout[bits[0]], 11)
+        self.assertEqual(layout[bits[1]], 10)
+        self.assertEqual(layout[bits[2]], 6)
+        self.assertEqual(layout[bits[3]], 5)
+        self.assertEqual(layout[bits[4]], 0)
+
 
 if __name__ == "__main__":
     unittest.main()