fix a corner case of SparsePauliOp.apply_layout

qiskit/quantum_info/operators/symplectic/sparse_pauli_op.py

@@ -1164,6 +1164,9 @@ def apply_layout(
         if layout is None:
             layout = list(range(self.num_qubits))
         new_op = type(self)("I" * n_qubits)
+        if self.num_qubits == 0:
+            coeff = self.simplify(atol=0).coeffs[0]
+            return new_op * coeff
         return new_op.compose(self, qargs=layout)
 
 

releasenotes/notes/fix-sparse-pauli-op-apply-layout-zero-43b9e70f0d1536a6.yaml

@@ -0,0 +1,3 @@
+fixes:
+  - |
+    Fixed :meth:`~.SparsePauliOp.apply_layout` to work correctly with zero-qubit operators.

test/python/quantum_info/operators/symplectic/test_sparse_pauli_op.py

@@ -1179,6 +1179,25 @@ def test_apply_layout_null_layout_invalid_num_qubits(self):
         with self.assertRaises(QiskitError):
             op.apply_layout(layout=None, num_qubits=1)
 
+    def test_apply_layout_zero_qubit(self):
+        """Test apply_layout with a zero-qubit operator"""
+        with self.subTest("default"):
+            op = SparsePauliOp("")
+            res = op.apply_layout(layout=None, num_qubits=5)
+            self.assertEqual(SparsePauliOp("IIIII"), res)
+        with self.subTest("coeff"):
+            op = SparsePauliOp("", 2)
+            res = op.apply_layout(layout=None, num_qubits=5)
+            self.assertEqual(SparsePauliOp("IIIII", 2), res)
+        with self.subTest("layout"):
+            op = SparsePauliOp("")
+            res = op.apply_layout(layout=[], num_qubits=5)
+            self.assertEqual(SparsePauliOp("IIIII"), res)
+        with self.subTest("multiple ops"):
+            op = SparsePauliOp.from_list([("", 1), ("", 2)])
+            res = op.apply_layout(layout=None, num_qubits=5)
+            self.assertEqual(SparsePauliOp("IIIII", 3), res)
+
 
 if __name__ == "__main__":
     unittest.main()