CNOT decomposition method returns DecompositionUndefinedError

doc/releases/changelog-dev.md

@@ -108,6 +108,9 @@
  Hamiltonians.
  [(#5950)](https://github.com/PennyLaneAI/pennylane/pull/5950)
 
+* The `CNOT` operator no longer decomposes to itself. Instead, it raises a `qml.DecompositionUndefinedError`.
+ [(#6039)](https://github.com/PennyLaneAI/pennylane/pull/6039)
+
 <h4>Community contributions 🥳</h4>
 
 * `DefaultQutritMixed` readout error has been added using parameters `readout_relaxation_probs` and 

pennylane/ops/functions/assert_valid.py

@@ -56,6 +56,9 @@ def _check_decomposition(op, skip_wire_mapping):
 
  assert isinstance(decomp, list), "decomposition must be a list"
  assert isinstance(compute_decomp, list), "decomposition must be a list"
+ assert op.__class__ not in [
+ decomp_op.__class__ for decomp_op in decomp
+ ], "an operator should not be included in its own decomposition"
 
  for o1, o2, o3 in zip(decomp, compute_decomp, processed_queue):
  assert o1 == o2, "decomposition must match compute_decomposition"
@@ -67,12 +70,14 @@ def _check_decomposition(op, skip_wire_mapping):
  # Check that mapping wires transitions to the decomposition
  wire_map = {w: ascii_lowercase[i] for i, w in enumerate(op.wires)}
  mapped_op = op.map_wires(wire_map)
- mapped_decomp = mapped_op.decomposition()
- orig_decomp = op.decomposition()
- for mapped_op, orig_op in zip(mapped_decomp, orig_decomp):
- assert (
- mapped_op.wires == qml.map_wires(orig_op, wire_map).wires
- ), "Operators in decomposition of wire-mapped operator must have mapped wires."
+ # I don't think this is a good solution, try to think of a better one
+ if mapped_op.has_decomposition:
+ mapped_decomp = mapped_op.decomposition()
+ orig_decomp = op.decomposition()
+ for mapped_op, orig_op in zip(mapped_decomp, orig_decomp):
+ assert (
+ mapped_op.wires == qml.map_wires(orig_op, wire_map).wires
+ ), "Operators in decomposition of wire-mapped operator must have mapped wires."
  else:
  failure_comment = "If has_decomposition is False, then decomposition must raise a ``DecompositionUndefinedError``."
  _assert_error_raised(

pennylane/ops/op_math/controlled_ops.py

@@ -749,10 +749,10 @@ class CNOT(ControlledOp):
  The controlled-NOT operator
 
  .. math:: CNOT = \begin{bmatrix}
-  1 & 0 & 0 & 0 \\
-  0 & 1 & 0 & 0\\
-  0 & 0 & 0 & 1\\
-  0 & 0 & 1 & 0
+ 1 & 0 & 0 & 0 \\
+ 0 & 1 & 0 & 0\\
+ 0 & 0 & 0 & 1\\
+ 0 & 0 & 1 & 0
  \end{bmatrix}.
 
  .. note:: The first wire provided corresponds to the **control qubit**.
@@ -791,6 +791,32 @@ def _primitive_bind_call(cls, wires, id=None):
  def __init__(self, wires, id=None):
  super().__init__(qml.PauliX(wires=wires[1:]), wires[:1], id=id)
 
+ @property
+ def has_decomposition(self):
+ return False
+
+ @staticmethod
+ def compute_decomposition(*params, wires=None, **hyperparameters): # -> List["Operator"]:
+ r"""Representation of the operator as a product of other operators (static method).
+
+ .. math:: O = O_1 O_2 \dots O_n.
+
+ .. note::
+ Operations making up the decomposition should be queued within the
+ ``compute_decomposition`` method.
+
+ .. seealso:: :meth:`~.Operator.decomposition`.
+
+ Args:
+ *params (list): trainable parameters of the operator, as stored in the ``parameters`` attribute
+ wires (Iterable[Any], Wires): wires that the operator acts on
+ **hyperparams (dict): non-trainable hyperparameters of the operator, as stored in the ``hyperparameters`` attribute
+
+ Raises:
+ qml.DecompositionUndefinedError
+ """
+ raise qml.operation.DecompositionUndefinedError
+
  def __repr__(self):
  return f"CNOT(wires={self.wires.tolist()})"
 

pennylane/transforms/tape_expand.py

@@ -297,14 +297,17 @@ def _custom_decomposition(obj, fn):
  obj = getattr(qml, obj)
 
  original_decomp_method = obj.compute_decomposition
+ original_has_decomp_property = obj.has_decomposition
 
  try:
  # Explicitly set the new compute_decomposition method
  obj.compute_decomposition = staticmethod(fn)
+ obj.has_decomposition = lambda obj: True
  yield
 
  finally:
  obj.compute_decomposition = staticmethod(original_decomp_method)
+ obj.has_decomposition = original_has_decomp_property
 
  # Loop through the decomposition dictionary and create all the contexts
  try:

tests/ops/functions/test_assert_valid.py

@@ -80,6 +80,17 @@ def compute_decomposition(wires):
  with pytest.raises(AssertionError, match="If has_decomposition is False"):
  assert_valid(BadDecomp(wires=0), skip_pickle=True)
 
+ def test_decomposition_must_not_contain_op(self):
+ """Test that the decomposition of an operator doesn't include the operator itself"""
+
+ class BadDecomp(Operator):
+ @staticmethod
+ def compute_decomposition(wires):
+ return [BadDecomp(wires)]
+
+ with pytest.raises(AssertionError, match="should not be included in its own decomposition"):
+ assert_valid(BadDecomp(wires=0), skip_pickle=True)
+
 
 class TestBadMatrix:
  """Tests involving matrix validation."""

tests/ops/op_math/test_controlled_ops.py

@@ -763,3 +763,14 @@ def test_tuple_control_wires_parametric_ops(op_type):
  """Test that tuples can be provided as control wire labels."""
 
  assert op_type(0.123, [(0, 1), 2]).wires == qml.wires.Wires([(0, 1), 2])
+
+
+def test_CNOT_decomposition():
+ """Test that CNOT raises a DecompositionUndefinedError instead of using the
+ controlled_op decomposition functions"""
+
+ with pytest.raises(qml.operation.DecompositionUndefinedError):
+ qml.CNOT.compute_decomposition()
+
+ with pytest.raises(qml.operation.DecompositionUndefinedError):
+ qml.CNOT([0, 1]).decomposition()