Add __iadd__ method to PauliSentence

doc/releases/changelog-dev.md

@@ -9,6 +9,9 @@
 * Updates to some relevant Pytests to enable its use as a suite of benchmarks.
   [(#4703)](https://github.com/PennyLaneAI/pennylane/pull/4703)
 
+* Added `__iadd__` method to PauliSentence, which enables inplace-addition using `+=`, we no longer need to perform a copy, leading to performance improvements.
+[(#4662)](https://github.com/PennyLaneAI/pennylane/pull/4662) 
+
 * `qml.ArbitraryUnitary` now supports batching.
   [(#4745)](https://github.com/PennyLaneAI/pennylane/pull/4745)
 
@@ -28,5 +31,6 @@
 This release contains contributions from (in alphabetical order):
 
 Amintor Dusko,
+Anurav Modak,
 David Wierichs,
 Justin Woodring

pennylane/pauli/pauli_arithmetic.py

@@ -343,6 +343,15 @@ def __add__(self, other):
 
         return larger_ps
 
+    def __iadd__(self, other):
+        """Inplace addition of two Pauli sentence together by adding terms of other to self"""
+        for key in other:
+            if key in self:
+                self[key] += other[key]
+            else:
+                self[key] = other[key]
+        return self
+
     def __copy__(self):
         """Copy the PauliSentence instance."""
         copied_ps = {}

tests/pauli/test_pauli_arithmetic.py

@@ -409,6 +409,17 @@ def test_add(self, string1, string2, result):
         (ps5, "Can't get the matrix of an empty PauliSentence."),
     )
 
+    @pytest.mark.parametrize("string1, string2, result", tup_ps_add)
+    def test_iadd(self, string1, string2, result):
+        """Test that the correct result of inplace addition is produced and other object is not changed."""
+        copied_string1 = copy(string1)
+        copied_string2 = copy(string2)
+        copied_string1 += copied_string2
+        copied_string1.simplify()
+
+        assert copied_string1 == result  # Check if the modified object matches the expected result
+        assert copied_string2 == string2  # Ensure the original object is not modified
+
     @pytest.mark.parametrize("ps, match", ps_match)
     def test_to_mat_error_empty(self, ps, match):
         """Test that an appropriate error is raised when an empty