Remove string support for control_values of MutliControlledX (#6835)

**Context:**
In PL0.36 we deprecated the string type support for argument
`control_values` of the operator `MultiControlledX` initialization, e.g.
`control_values='01'` shall not be allowed anymore. In this PR we would
like to remove this entirely with a strict type check.

**Description of the Change:**
- Added a type check with `isinstance` to raise `ValueError` if the
`control_values` does not follow the designed pattern.
- Added a type hint at correpsonding position within the init func
signature
 - Removed a test that presumes the validity of str input
 - Alter the deprecation check into error check

**Benefits:**
Less redundancy

**Possible Drawbacks:**
Potential existence of uncovered indirect usage of string input in
`MultiControlledX`

**Related GitHub Issues:**
[sc-81527]

doc/development/deprecations.rst

@@ -25,11 +25,6 @@ Pending deprecations
 
   - Deprecated in v0.36
 
-* ``MultiControlledX`` is the only controlled operation that still supports specifying control
-  values with a bit string. In the future, it will no longer accepts strings as control values.
-
-  - Deprecated in v0.36
-
 Completed removal of legacy operator arithmetic
 -----------------------------------------------
 
@@ -58,6 +53,11 @@ for details on how to port your legacy code to the new system. The following fun
 Completed deprecation cycles
 ----------------------------
 
+* ``MultiControlledX`` no longer accepts strings as control values.
+
+  - Deprecated in v0.36
+  - Removed in v0.41
+
 * The input argument ``control_wires`` of ``MultiControlledX`` has been removed.
 
   - Deprecated in v0.22

doc/releases/changelog-dev.md

@@ -46,6 +46,9 @@
 
 <h3>Breaking changes 💔</h3>
 
+* `MultiControlledX` no longer accepts strings as control values.
+  [(#6835)](https://github.com/PennyLaneAI/pennylane/pull/6835)
+
 * The input argument `control_wires` of `MultiControlledX` has been removed.
   [(#6832)](https://github.com/PennyLaneAI/pennylane/pull/6832)
 

pennylane/ops/op_math/controlled_ops.py

@@ -18,6 +18,7 @@
 import warnings
 from collections.abc import Iterable
 from functools import lru_cache
+from typing import List, Union
 
 import numpy as np
 from scipy.linalg import block_diag
@@ -1154,23 +1155,31 @@ def _primitive_bind_call(cls, wires, control_values=None, work_wires=None, id=No
             *wires, n_wires=len(wires), control_values=control_values, work_wires=work_wires
         )
 
+    @staticmethod
+    def _validate_control_values(control_values):
+        if control_values is not None:
+            if not (
+                isinstance(control_values, (bool, int))
+                or (
+                    (
+                        isinstance(control_values, (list, tuple))
+                        and all(isinstance(val, (bool, int)) for val in control_values)
+                    )
+                )
+            ):
+                raise ValueError(f"control_values must be boolean or int. Got: {control_values}")
+
     # pylint: disable=too-many-arguments
     def __init__(
         self,
         wires: WiresLike = (),
-        control_values=None,
+        control_values: Union[bool, List[bool], int, List[int]] = None,
         work_wires: WiresLike = (),
     ):
         wires = Wires(() if wires is None else wires)
         work_wires = Wires(() if work_wires is None else work_wires)
 
-        # First raise deprecation warnings regardless of the validity of other arguments
-        if isinstance(control_values, str):
-            warnings.warn(
-                "Specifying control values using a bitstring is deprecated, and will not be "
-                "supported in future releases, Use a list of booleans or integers instead.",
-                qml.PennyLaneDeprecationWarning,
-            )
+        self._validate_control_values(control_values)
 
         if len(wires) == 0:
             raise ValueError("Must specify the wires where the operation acts on")

tests/ops/qubit/test_non_parametric_ops.py

@@ -606,11 +606,11 @@ class TestMultiControlledX:
 
     X = np.array([[0, 1], [1, 0]])
 
-    def test_str_control_values_deprecation(self):
+    def test_str_control_values_error(self):
         """Tests that control_values specified with a bit string is deprecated."""
-        with pytest.warns(
-            qml.PennyLaneDeprecationWarning,
-            match="Specifying control values using a bitstring is deprecated",
+        with pytest.raises(
+            ValueError,
+            match="control_values must be boolean or int",
         ):
             _ = qml.MultiControlledX(wires=[0, 1, 2], control_values="01")
 
@@ -631,26 +631,6 @@ def test_invalid_arguments_to_init(self, wires, control_values, error_message):
         with pytest.raises(ValueError, match=error_message):
             _ = qml.MultiControlledX(wires=wires, control_values=control_values)
 
-    @pytest.mark.parametrize(
-        "wires, control_values, error_message",
-        [
-            ([0, 1, 2], "ab", "String of control values can contain only '0' or '1'."),
-            (
-                [0, 1, 2],
-                "011",
-                "Length of control values must equal number of control wires.",
-            ),
-        ],
-    )
-    def test_invalid_str_control_values(self, wires, control_values, error_message):
-        """Tests control_values specified with invalid strings"""
-        with pytest.warns(
-            qml.PennyLaneDeprecationWarning,
-            match="Specifying control values using a bitstring is deprecated",
-        ):
-            with pytest.raises(ValueError, match=error_message):
-                _ = qml.MultiControlledX(wires=wires, control_values=control_values)
-
     def test_decomposition_not_enough_wires(self):
         """Test that the decomposition raises an error if the number of wires is lower than two"""
         with pytest.raises(ValueError, match="Wrong number of wires"):