Remove simplify argument from Hamiltonian and LinearCombination (…

…#6279) As name says. [sc-72003]
PennyLaneAI · Sep 18, 2024 · 68fd307 · 68fd307
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diff --git a/doc/development/deprecations.rst b/doc/development/deprecations.rst
@@ -45,12 +45,6 @@ Pending deprecations
   - Deprecated in v0.39
   - Will be removed in v0.40
 
-* The ``simplify`` argument in ``qml.Hamiltonian`` and ``qml.ops.LinearCombination`` is deprecated. 
-  Instead, ``qml.simplify()`` can be called on the constructed operator.
-
-  - Deprecated in v0.37
-  - Will be removed in v0.39
-
 * The ``QubitStateVector`` template is deprecated.
   Instead, use ``StatePrep``.
 
@@ -101,6 +95,12 @@ Other deprecations
 Completed deprecation cycles
 ----------------------------
 
+* The ``simplify`` argument in ``qml.Hamiltonian`` and ``qml.ops.LinearCombination`` has been removed.
+  Instead, ``qml.simplify()`` can be called on the constructed operator.
+
+  - Deprecated in v0.37
+  - Removed in v0.39
+
 * The ``decomp_depth`` argument in ``qml.device`` is removed.
 
   - Deprecated in v0.38

diff --git a/doc/news/new_opmath.rst b/doc/news/new_opmath.rst
@@ -245,10 +245,10 @@ To help identify a fix, select the option below that describes your situation.
     >>> qml.Hamiltonian([0.5], [X(0) @ X(1)])
     0.5 * (X(0) @ X(1))
 
-    The API of :class:`~.ops.op_math.LinearCombination` is identical to that of :class:`~.Hamiltonian`. We can group observables or simplify upon initialization.
+    The API of :class:`~.ops.op_math.LinearCombination` is identical to that of :class:`~.Hamiltonian`. We can group observables upon initialization.
 
-    >>> H1 = qml.Hamiltonian([0.5, 0.5, 0.5], [X(0) @ X(1), X(0), Y(0)], grouping_type="qwc", simplify=True)
-    >>> H2 = qml.ops.LinearCombination([0.5, 0.5, 0.5], [X(0) @ X(1), X(0), Y(0)], grouping_type="qwc", simplify=True)
+    >>> H1 = qml.Hamiltonian([0.5, 0.5, 0.5], [X(0) @ X(1), X(0), Y(0)], grouping_type="qwc")
+    >>> H2 = qml.ops.LinearCombination([0.5, 0.5, 0.5], [X(0) @ X(1), X(0), Y(0)], grouping_type="qwc")
     >>> H1 == H2
     True
 

diff --git a/doc/releases/changelog-dev.md b/doc/releases/changelog-dev.md
@@ -63,6 +63,10 @@
 
 <h3>Breaking changes 💔</h3>
 
+* The `simplify` argument in `qml.Hamiltonian` and `qml.ops.LinearCombination` has been removed.
+  Instead, `qml.simplify()` can be called on the constructed operator.
+  [(#6279)](https://github.com/PennyLaneAI/pennylane/pull/6279)
+
 * The functions `qml.qinfo.classical_fisher` and `qml.qinfo.quantum_fisher` have been removed and migrated to the `qml.gradients`
   module. Therefore, `qml.gradients.classical_fisher` and `qml.gradients.quantum_fisher` should be used instead.
   [(#5911)](https://github.com/PennyLaneAI/pennylane/pull/5911)

diff --git a/pennylane/ops/op_math/linear_combination.py b/pennylane/ops/op_math/linear_combination.py
@@ -37,9 +37,6 @@ class LinearCombination(Sum):
     Args:
         coeffs (tensor_like): coefficients of the ``LinearCombination`` expression
         observables (Iterable[Observable]): observables in the ``LinearCombination`` expression, of same length as ``coeffs``
-        simplify (bool): Specifies whether the ``LinearCombination`` is simplified upon initialization
-            (like-terms are combined). The default value is `False`. Note that ``coeffs`` cannot
-            be differentiated when using the ``'torch'`` interface and ``simplify=True``. Use of this argument is deprecated.
         grouping_type (str): If not ``None``, compute and store information on how to group commuting
             observables upon initialization. This information may be accessed when a :class:`~.QNode` containing this
             ``LinearCombination`` is executed on devices. The string refers to the type of binary relation between Pauli words.
@@ -52,10 +49,6 @@ class LinearCombination(Sum):
     .. seealso:: `rustworkx.ColoringStrategy <https://www.rustworkx.org/apiref/rustworkx.ColoringStrategy.html#coloringstrategy>`_
         for more information on the ``('lf', 'dsatur', 'gis')`` strategies.
 
-    .. warning::
-        The ``simplify`` argument is deprecated and will be removed in a future release.
-        Instead, you can call ``qml.simplify`` on the constructed operator.
-
     **Example:**
 
     A ``LinearCombination`` can be created by simply passing the list of coefficients
@@ -124,7 +117,6 @@ def __init__(
         self,
         coeffs,
         observables: list[Operator],
-        simplify=False,
         grouping_type=None,
         method="lf",
         _grouping_indices=None,
@@ -143,23 +135,6 @@ def __init__(
         if _pauli_rep is None:
             _pauli_rep = self._build_pauli_rep_static(coeffs, observables)
 
-        if simplify:
-
-            warnings.warn(
-                "The simplify argument in qml.Hamiltonian and qml.ops.LinearCombination is deprecated. "
-                "Instead, you can call qml.simplify on the constructed operator.",
-                qml.PennyLaneDeprecationWarning,
-            )
-
-            # simplify upon initialization changes ops such that they wouldnt be removed in self.queue() anymore
-            if qml.QueuingManager.recording():
-                for o in observables:
-                    qml.QueuingManager.remove(o)
-
-            coeffs, observables, _pauli_rep = self._simplify_coeffs_ops(
-                coeffs, observables, _pauli_rep
-            )
-
         self._coeffs = coeffs
 
         self._ops = [convert_to_opmath(op) for op in observables]

diff --git a/pennylane/ops/qubit/hamiltonian.py b/pennylane/ops/qubit/hamiltonian.py
@@ -81,8 +81,6 @@ class Hamiltonian(Observable):
     Args:
         coeffs (tensor_like): coefficients of the Hamiltonian expression
         observables (Iterable[Observable]): observables in the Hamiltonian expression, of same length as coeffs
-        simplify (bool): Specifies whether the Hamiltonian is simplified upon initialization
-                         (like-terms are combined). The default value is `False`. Use of this argument is deprecated.
         grouping_type (str): If not None, compute and store information on how to group commuting
             observables upon initialization. This information may be accessed when QNodes containing this
             Hamiltonian are executed on devices. The string refers to the type of binary relation between Pauli words.
@@ -91,10 +89,6 @@ class Hamiltonian(Observable):
             can be ``'lf'`` (Largest First) or ``'rlf'`` (Recursive Largest First). Ignored if ``grouping_type=None``.
         id (str): name to be assigned to this Hamiltonian instance
 
-    .. warning::
-        The ``simplify`` argument is deprecated and will be removed in a future release.
-        Instead, you can call ``qml.simplify`` on the constructed operator.
-
     **Example:**
 
     .. note::
@@ -254,7 +248,6 @@ def __init__(
         self,
         coeffs: TensorLike,
         observables: Iterable[Observable],
-        simplify: bool = False,
         grouping_type: Literal[None, "qwc", "commuting", "anticommuting"] = None,
         _grouping_indices: Optional[list[list[int]]] = None,
         method: Literal["lf", "rlf"] = "rlf",
@@ -293,23 +286,6 @@ def __init__(
         # commuting observables, since recomputation is costly
         self._grouping_indices = _grouping_indices
 
-        if simplify:
-
-            warn(
-                "The simplify argument in qml.Hamiltonian and qml.ops.LinearCombination is deprecated. "
-                "Instead, you can call qml.simplify on the constructed operator.",
-                qml.PennyLaneDeprecationWarning,
-            )
-
-            # simplify upon initialization changes ops such that they wouldnt be
-            # removed in self.queue() anymore, removing them here manually.
-            if qml.QueuingManager.recording():
-                for o in observables:
-                    qml.QueuingManager.remove(o)
-
-            with qml.QueuingManager.stop_recording():
-                self.simplify()
-
         if grouping_type is not None:
             with qml.QueuingManager.stop_recording():
                 self._grouping_indices = _compute_grouping_indices(

diff --git a/tests/ops/op_math/test_linear_combination.py b/tests/ops/op_math/test_linear_combination.py
@@ -580,14 +580,6 @@ def circuit2(param):
 class TestLinearCombination:
     """Test the LinearCombination class"""
 
-    def test_deprecation_simplify_argument(self):
-        """Test that a deprecation warning is raised if the simplify argument is True."""
-        with pytest.warns(
-            qml.PennyLaneDeprecationWarning,
-            match="deprecated",
-        ):
-            _ = qml.ops.LinearCombination([1.0], [qml.X(0)], simplify=True)
-
     def test_error_if_observables_operator(self):
         """Test thatt an error is raised if an operator is provided to observables."""
 
@@ -613,13 +605,9 @@ def test_error_if_observables_operator(self):
     def test_pauli_rep(self, coeffs, ops, true_pauli, simplify):
         """Test the pauli rep is correctly constructed"""
         if simplify:
-            with pytest.warns(
-                qml.PennyLaneDeprecationWarning,
-                match="deprecated",
-            ):
-                H = qml.ops.LinearCombination(coeffs, ops, simplify=simplify)
+            H = qml.ops.LinearCombination(coeffs, ops).simplify()
         else:
-            H = qml.ops.LinearCombination(coeffs, ops, simplify=simplify)
+            H = qml.ops.LinearCombination(coeffs, ops)
         pr = H.pauli_rep
         if simplify:
             pr.simplify()
@@ -1659,31 +1647,13 @@ def test_simplify_reduces_tape_parameters(self):
         @qml.qnode(device)
         def circuit():
             qml.RY(0.1, wires=0)
-            return qml.expval(qml.ops.LinearCombination([1.0, 2.0], [X(1), X(1)], simplify=True))
+            return qml.expval(qml.simplify(qml.ops.LinearCombination([1.0, 2.0], [X(1), X(1)])))
 
-        with pytest.warns(
-            qml.PennyLaneDeprecationWarning,
-            match="deprecated",
-        ):
-            circuit()
+        circuit()
         pars = circuit.qtape.get_parameters(trainable_only=False)
         # simplify worked and added 1. and 2.
         assert pars == [0.1, 3.0]
 
-    @pytest.mark.usefixtures("use_legacy_and_new_opmath")
-    def test_queuing_behaviour(self):
-        """Tests that the base observables are correctly dequeued with simplify=True"""
-
-        with qml.queuing.AnnotatedQueue() as q:
-            with pytest.warns(
-                qml.PennyLaneDeprecationWarning,
-                match="deprecated",
-            ):
-                obs = qml.Hamiltonian([1, 1, 1], [qml.X(0), qml.X(0), qml.Z(0)], simplify=True)
-
-        assert len(q) == 1
-        assert q.queue[0] == obs
-
 
 class TestLinearCombinationDifferentiation:
     """Test that the LinearCombination coefficients are differentiable"""
@@ -1702,23 +1672,13 @@ def circuit(coeffs, param):
             qml.RX(param, wires=0)
             qml.RY(param, wires=0)
             return qml.expval(
-                qml.ops.LinearCombination(
-                    coeffs,
-                    [X(0), Z(0)],
-                    simplify=simplify,
-                    grouping_type=group,
-                )
+                qml.simplify(qml.ops.LinearCombination(coeffs, [X(0), Z(0)], grouping_type=group))
+                if simplify
+                else qml.ops.LinearCombination(coeffs, [X(0), Z(0)], grouping_type=group)
             )
 
         grad_fn = qml.grad(circuit)
-        if simplify:
-            with pytest.warns(
-                qml.PennyLaneDeprecationWarning,
-                match="deprecated",
-            ):
-                grad = grad_fn(coeffs, param)
-        else:
-            grad = grad_fn(coeffs, param)
+        grad = grad_fn(coeffs, param)
 
         # differentiating a cost that combines circuits with
         # measurements expval(Pauli)
@@ -1783,23 +1743,13 @@ def circuit(coeffs, param):
             qml.RX(param, wires=0)
             qml.RY(param, wires=0)
             return qml.expval(
-                qml.ops.LinearCombination(
-                    coeffs,
-                    [X(0), Z(0)],
-                    simplify=simplify,
-                    grouping_type=group,
-                )
+                qml.simplify(qml.ops.LinearCombination(coeffs, [X(0), Z(0)], grouping_type=group))
+                if simplify
+                else qml.ops.LinearCombination(coeffs, [X(0), Z(0)], grouping_type=group)
             )
 
         grad_fn = qml.grad(circuit)
-        if simplify:
-            with pytest.warns(
-                qml.PennyLaneDeprecationWarning,
-                match="deprecated",
-            ):
-                grad = grad_fn(coeffs, param)
-        else:
-            grad = grad_fn(coeffs, param)
+        grad = grad_fn(coeffs, param)
 
         # differentiating a cost that combines circuits with
         # measurements expval(Pauli)
@@ -1860,24 +1810,13 @@ def circuit(coeffs, param):
             qml.RX(param, wires=0)
             qml.RY(param, wires=0)
             return qml.expval(
-                qml.ops.LinearCombination(
-                    coeffs,
-                    [X(0), Z(0)],
-                    simplify=simplify,
-                    grouping_type=group,
-                )
+                qml.simplify(qml.ops.LinearCombination(coeffs, [X(0), Z(0)], grouping_type=group))
+                if simplify
+                else qml.ops.LinearCombination(coeffs, [X(0), Z(0)], grouping_type=group)
             )
 
         grad_fn = jax.grad(circuit)
-
-        if simplify:
-            with pytest.warns(
-                qml.PennyLaneDeprecationWarning,
-                match="deprecated",
-            ):
-                grad = grad_fn(coeffs, param)
-        else:
-            grad = grad_fn(coeffs, param)
+        grad = grad_fn(coeffs, param)
 
         # differentiating a cost that combines circuits with
         # measurements expval(Pauli)
@@ -1938,22 +1877,12 @@ def circuit(coeffs, param):
             qml.RX(param, wires=0)
             qml.RY(param, wires=0)
             return qml.expval(
-                qml.ops.LinearCombination(
-                    coeffs,
-                    [X(0), Z(0)],
-                    simplify=simplify,
-                    grouping_type=group,
-                )
+                qml.simplify(qml.ops.LinearCombination(coeffs, [X(0), Z(0)], grouping_type=group))
+                if simplify
+                else qml.ops.LinearCombination(coeffs, [X(0), Z(0)], grouping_type=group)
             )
 
-        if simplify:
-            with pytest.warns(
-                qml.PennyLaneDeprecationWarning,
-                match="deprecated",
-            ):
-                res = circuit(coeffs, param)
-        else:
-            res = circuit(coeffs, param)
+        res = circuit(coeffs, param)
         res.backward()  # pylint:disable=no-member
         grad = (coeffs.grad, param.grad)
 
@@ -2032,23 +1961,13 @@ def circuit(coeffs, param):
             qml.RX(param, wires=0)
             qml.RY(param, wires=0)
             return qml.expval(
-                qml.ops.LinearCombination(
-                    coeffs,
-                    [X(0), Z(0)],
-                    simplify=simplify,
-                    grouping_type=group,
-                )
+                qml.simplify(qml.ops.LinearCombination(coeffs, [X(0), Z(0)], grouping_type=group))
+                if simplify
+                else qml.ops.LinearCombination(coeffs, [X(0), Z(0)], grouping_type=group)
             )
 
         with tf.GradientTape() as tape:
-            if simplify:
-                with pytest.warns(
-                    qml.PennyLaneDeprecationWarning,
-                    match="deprecated",
-                ):
-                    res = circuit(coeffs, param)
-            else:
-                res = circuit(coeffs, param)
+            res = circuit(coeffs, param)
         grad = tape.gradient(res, [coeffs, param])
 
         # differentiating a cost that combines circuits with