FullAncillaAllocation for backends witout a coupling map

qiskit/transpiler/passes/layout/full_ancilla_allocation.py

@@ -19,11 +19,11 @@
 
 
 class FullAncillaAllocation(AnalysisPass):
-    """Allocate all idle nodes from the coupling map as ancilla on the layout.
+    """Allocate all idle nodes from the coupling map or target as ancilla on the layout.
 
     A pass for allocating all idle physical qubits (those that exist in coupling
-    map but not the dag circuit) as ancilla. It will also choose new virtual
-    qubits to correspond to those physical ancilla.
+    map or target but not the dag circuit) as ancilla. It will also choose new
+    virtual qubits to correspond to those physical ancilla.
 
     Note:
         This is an analysis pass, and only responsible for choosing physical
@@ -81,7 +81,11 @@ def run(self, dag):
 
         idle_physical_qubits = [q for q in layout_physical_qubits if q not in physical_bits]
 
-        if self.coupling_map:
+        if self.target:
+            idle_physical_qubits = [
+                q for q in range(self.target.num_qubits) if q not in physical_bits
+            ]
+        elif self.coupling_map:
             idle_physical_qubits = [
                 q for q in self.coupling_map.physical_qubits if q not in physical_bits
             ]

releasenotes/notes/ancilla_allocation_no_cmap-ac3ff65b3639988e.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    Fixed an issue with :class:`~.FullAncillaAllocation` so it can now function with :class:`~.Target` objects that do not have a
+    coupling map (typically because there are no 2 qubit gates in the :class:`~.Target`). In this case :class:`~.FullAncillaAllocation` will add
+    ancilla qubits so that the number of qubits in the :class:`~.DAGCircuit` matches the number
+    :attr:`.Target.num_qubits`.

test/python/transpiler/test_full_ancilla_allocation.py

@@ -214,6 +214,28 @@ def test_bad_layout(self):
             cm.exception.message,
         )
 
+    def test_target_without_cmap(self):
+        """Test that FullAncillaAllocation works when the target does not have a coupling map.
+
+        This situation occurs at the early stages of backend bring-up.
+        """
+        target_data = {"basis_gates": ["h"], "num_qubits": 3}
+        target = Target.from_configuration(**target_data)
+
+        circ = QuantumCircuit(1)
+        circ.h(0)
+
+        pass_ = FullAncillaAllocation(target)
+        pass_.property_set["layout"] = Layout.from_intlist([0], *circ.qregs)
+
+        # Pre pass check
+        self.assertEqual(len(pass_.property_set["layout"]), 1)
+
+        pass_.run(circuit_to_dag(circ))
+
+        # Post pass check
+        self.assertEqual(len(pass_.property_set["layout"]), target.num_qubits)
+
 
 if __name__ == "__main__":
     unittest.main()