Add __iadd__ method to PauliSentence

Signed-off-by: Anurav Modak <[email protected]>

pennylane/pauli/pauli_arithmetic.py

@@ -342,6 +342,18 @@ def __add__(self, other):
             larger_ps[key] += smaller_ps[key]
 
         return larger_ps
+
+    def __iadd__(self, other):
+        """Adding two Pauli sentence together in-place by iterating over the smaller
+        one and adding its terms to the larger one, modifying self in place"""
+        smaller_ps, larger_ps = (self, other) if len(self) < len(other) else (other, self)
+        for key in smaller_ps:
+            if key in self:
+                self[key]+=smaller_ps[key]
+            else:
+                self[key]=smaller_ps[key]
+
+        return self  # Return self to indicate in-place modification
 
     def __copy__(self):
         """Copy the PauliSentence instance."""

tests/pauli/test_pauli_arithmetic.py

@@ -409,6 +409,16 @@ 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."""
+        original_string2 = copy(string2)
+        string1 += string2  # Perform in-place addition using +=
+        string1.simplify()
+
+        assert string1 == result  # Check if the modified object matches the expected result
+        assert original_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