Add zero-qubit tests of Pauli.apply_layout

Qiskit · May 13, 2024 · 5cdaf81 · 5cdaf81
1 parent 49e96fa
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diff --git a/releasenotes/notes/fix-sparse-pauli-op-apply-layout-zero-43b9e70f0d1536a6.yaml b/releasenotes/notes/fix-sparse-pauli-op-apply-layout-zero-43b9e70f0d1536a6.yaml
@@ -1,3 +1,3 @@
 fixes:
   - |
-    Fixed :meth:`~.SparsePauliOp.apply_layout` to work correctly with zero-qubit operators.
+    Fixed :meth:`.SparsePauliOp.apply_layout` to work correctly with zero-qubit operators.
diff --git a/test/python/quantum_info/operators/symplectic/test_pauli.py b/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()