Repository: Qiskit/qiskit

Issue:
SparsePauliOp.apply_layout should do nothing if given None
### What should we add?

Simulators give `None` for the layout when a circuit goes through the transpiler.  Currently this raises in `SparsePauliOp.apply_layout`.  Rather, it would be nice if `None` is a null op in `apply_layout` so that I can run the same code for a sim and a real backend.


Discussion:
From my understanding, the only change will be in the apply_layout function by adding the case for a null layout (in which case the function should do nothing). If this is good for a first issue I would like to help with this one :) 
It is a good first issue, at least in my mind.
Great! Then I would like to work on it :)
I've assigned this issue to you @SoranaAurelia 

Resolving patch:
diff --git a/qiskit/quantum_info/operators/symplectic/sparse_pauli_op.py b/qiskit/quantum_info/operators/symplectic/sparse_pauli_op.py
--- a/qiskit/quantum_info/operators/symplectic/sparse_pauli_op.py
+++ b/qiskit/quantum_info/operators/symplectic/sparse_pauli_op.py
@@ -1109,17 +1109,20 @@ def assign_parameters(
         return None if inplace else bound
 
     def apply_layout(
-        self, layout: TranspileLayout | List[int], num_qubits: int | None = None
+        self, layout: TranspileLayout | List[int] | None, num_qubits: int | None = None
     ) -> SparsePauliOp:
         """Apply a transpiler layout to this :class:`~.SparsePauliOp`
 
         Args:
-            layout: Either a :class:`~.TranspileLayout` or a list of integers.
+            layout: Either a :class:`~.TranspileLayout`, a list of integers or None.
+                    If both layout and num_qubits are none, a copy of the operator is
+                    returned.
             num_qubits: The number of qubits to expand the operator to. If not
                 provided then if ``layout`` is a :class:`~.TranspileLayout` the
                 number of the transpiler output circuit qubits will be used by
                 default. If ``layout`` is a list of integers the permutation
-                specified will be applied without any expansion.
+                specified will be applied without any expansion. If layout is
+                None, the operator will be expanded to the given number of qubits.
 
 
         Returns:
@@ -1127,6 +1130,9 @@ def apply_layout(
         """
         from qiskit.transpiler.layout import TranspileLayout
 
+        if layout is None and num_qubits is None:
+            return self.copy()
+
         n_qubits = self.num_qubits
         if isinstance(layout, TranspileLayout):
             n_qubits = len(layout._output_qubit_list)
@@ -1138,8 +1144,10 @@ def apply_layout(
                     f"applied to a {n_qubits} qubit operator"
                 )
             n_qubits = num_qubits
-        if any(x >= n_qubits for x in layout):
+        if layout is not None and any(x >= n_qubits for x in layout):
             raise QiskitError("Provided layout contains indicies outside the number of qubits.")
+        if layout is None:
+            layout = list(range(self.num_qubits))
         new_op = type(self)("I" * n_qubits)
         return new_op.compose(self, qargs=layout)
 


Test patch:
diff --git a/test/python/quantum_info/operators/symplectic/test_sparse_pauli_op.py b/test/python/quantum_info/operators/symplectic/test_sparse_pauli_op.py
--- a/test/python/quantum_info/operators/symplectic/test_sparse_pauli_op.py
+++ b/test/python/quantum_info/operators/symplectic/test_sparse_pauli_op.py
@@ -1088,6 +1088,25 @@ def test_apply_layout_layout_list_and_num_qubits(self):
         res = op.apply_layout([4, 0], 5)
         self.assertEqual(SparsePauliOp.from_list([("IIIIY", 2), ("IIIIX", 1)]), res)
 
+    def test_apply_layout_null_layout_no_num_qubits(self):
+        """Test apply_layout with a null layout"""
+        op = SparsePauliOp.from_list([("II", 1), ("IZ", 2), ("XI", 3)])
+        res = op.apply_layout(layout=None)
+        self.assertEqual(op, res)
+
+    def test_apply_layout_null_layout_and_num_qubits(self):
+        """Test apply_layout with a null layout a num_qubits provided"""
+        op = SparsePauliOp.from_list([("II", 1), ("IZ", 2), ("XI", 3)])
+        res = op.apply_layout(layout=None, num_qubits=5)
+        # this should expand the operator
+        self.assertEqual(SparsePauliOp.from_list([("IIIII", 1), ("IIIIZ", 2), ("IIIXI", 3)]), res)
+
+    def test_apply_layout_null_layout_invalid_num_qubits(self):
+        """Test apply_layout with a null layout and num_qubits smaller than capable"""
+        op = SparsePauliOp.from_list([("II", 1), ("IZ", 2), ("XI", 3)])
+        with self.assertRaises(QiskitError):
+            op.apply_layout(layout=None, num_qubits=1)
+
 
 if __name__ == "__main__":
     unittest.main()