fix Pauli.apply_layout

qiskit/quantum_info/operators/symplectic/pauli.py

@@ -741,6 +741,9 @@ def apply_layout(
                 raise QiskitError("Provided layout contains indices outside the number of qubits.")
             if len(set(layout)) != len(layout):
                 raise QiskitError("Provided layout contains duplicate indices.")
+        if self.num_qubits == 0:
+            phases = ["", "-i", "-", "i"]
+            return type(self)(phases[self.phase] + "I" * n_qubits)
         new_op = type(self)("I" * n_qubits)
         return new_op.compose(self, qargs=layout)
 

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

@@ -1,3 +1,4 @@
 fixes:
   - |
-    Fixed :meth:`~.SparsePauliOp.apply_layout` to work correctly with zero-qubit operators.
+    Fixed :meth:`.SparsePauliOp.apply_layout` and :meth:`.Pauli.apply_layout`
+    to work correctly with zero-qubit operators.

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

@@ -618,6 +618,29 @@ def test_apply_layout_duplicate_indices(self):
         with self.assertRaises(QiskitError):
             op.apply_layout(layout=[0, 0], num_qubits=3)
 
+    def test_apply_layout_zero_qubit(self):
+        """Test apply_layout with a zero-qubit operator"""
+        with self.subTest("default"):
+            op = Pauli("")
+            res = op.apply_layout(layout=None, num_qubits=5)
+            self.assertEqual(Pauli("IIIII"), res)
+        with self.subTest("phase -1j"):
+            op = Pauli("-i")
+            res = op.apply_layout(layout=None, num_qubits=5)
+            self.assertEqual(Pauli("-iIIIII"), res)
+        with self.subTest("phase -1"):
+            op = Pauli("-")
+            res = op.apply_layout(layout=None, num_qubits=5)
+            self.assertEqual(Pauli("-IIIII"), res)
+        with self.subTest("phase 1j"):
+            op = Pauli("i")
+            res = op.apply_layout(layout=None, num_qubits=5)
+            self.assertEqual(Pauli("iIIIII"), res)
+        with self.subTest("layout"):
+            op = Pauli("")
+            res = op.apply_layout(layout=[], num_qubits=5)
+            self.assertEqual(Pauli("IIIII"), res)
+
 
 if __name__ == "__main__":
     unittest.main()