Allow tuple-valued control_values with qml.ctrl on a Controlled…

… instance. (#5725)

**Context:**
By default, `control_values` of `Controlled` instances are lists.
`qml.defer_measurements` passes a tuple as `control_values` to `ctrl`.
`create_controlled_op`, called by `ctrl`, tries to merge
`control_values` of the base operation and the provided
`control_values`.
The consequence is an attempted addition of `list` and `tuple`, if
`qml.defer_measurements` hits a `qml.cond` of a `Controlled` instance,
leading to #5717

**Description of the Change:**
This PR fixes the above problem in two (redundant!) ways: 
- tuples are allowed to be passed as `control_values` to `ctrl`, which
casts them to a `list`.
- `defer_measurements` passes a `list` itself.

Clearly those changes are redundant for the bug #5717, with the former
also providing users with the option of using `tuple`s for
`control_values`. We could skip the change to `defer_measurements`,
thus.

**Benefits:**

**Possible Drawbacks:**

**Related GitHub Issues:**
Fixes #5717 

[sc-63645]

doc/releases/changelog-dev.md

@@ -9,6 +9,9 @@
 
 <h3>Improvements 🛠</h3>
 
+* `ctrl` now works with tuple-valued `control_values` when applied to any already controlled operation.
+  [(#5725)](https://github.com/PennyLaneAI/pennylane/pull/5725)
+
 * Add support for 3 new pytest markers: `unit`, `integration` and `system`.
   [(#5517)](https://github.com/PennyLaneAI/pennylane/pull/5517)
 
@@ -141,6 +144,9 @@
 
 <h3>Bug fixes 🐛</h3>
 
+* `qml.cond` can now be applied to `ControlledOp` operations when deferring measurements.
+  [(#5725)](https://github.com/PennyLaneAI/pennylane/pull/5725)
+
 * The legacy `Tensor` class can now handle a `Projector` with abstract tracer input.
   [(#5720)](https://github.com/PennyLaneAI/pennylane/pull/5720)
 

pennylane/ops/op_math/controlled.py

@@ -140,6 +140,8 @@ def create_controlled_op(op, control, control_values=None, work_wires=None):
         control_values = [control_values]
     elif control_values is None:
         control_values = [True] * len(control)
+    elif isinstance(control_values, tuple):
+        control_values = list(control_values)
 
     ctrl_op = _try_wrap_in_custom_ctrl_op(
         op, control, control_values=control_values, work_wires=work_wires

tests/ops/op_math/test_controlled.py

@@ -163,6 +163,17 @@ def test_zero_one_control_values(self):
         op = Controlled(self.temp_op, (0, 1), control_values=[0, 1])
         assert op.control_values == [False, True]
 
+    @pytest.mark.parametrize("control_values", [True, False, 0, 1])
+    def test_scalar_control_values(self, control_values):
+        """Test assignment of provided control_values."""
+        op = Controlled(self.temp_op, 0, control_values=control_values)
+        assert op.control_values == [control_values]
+
+    def test_tuple_control_values(self):
+        """Test assignment of provided control_values."""
+        op = Controlled(self.temp_op, (0, 1), control_values=(0, 1))
+        assert op.control_values == [False, True]
+
     def test_non_boolean_control_values(self):
         """Test control values are converted to booleans."""
         op = Controlled(self.temp_op, (0, 1, 2), control_values=["", None, 5])

tests/ops/op_math/test_controlled_ops.py

@@ -737,3 +737,15 @@ def test_controlled_method(base, cbase):
     """Tests the _controlled method for parametric ops."""
     # pylint: disable=protected-access
     assert qml.equal(base._controlled("a"), cbase)
+
+
+@pytest.mark.parametrize(
+    "control, control_values",
+    [([0, 1], [True, False]), ([10, "a"], (0, 0)), ([2], 1), (2, (True,))],
+)
+@pytest.mark.parametrize(
+    "base_op", [qml.CRX(0.2, [21, 22]), qml.CNOT([21, 22]), qml.CPhase(0.6, [21, 22])]
+)
+def test_controlling_a_controlled_operation(control, control_values, base_op):
+    """Test that a controlled op can be controlled again."""
+    qml.ctrl(base_op, control=control, control_values=control_values)

tests/transforms/test_defer_measurements.py

@@ -1063,7 +1063,7 @@ def qnode():
     )
     def test_cond_qfunc(self, device):
         """Test that a qfunc can also used with qml.cond."""
-        dev = qml.device(device, wires=3)
+        dev = qml.device(device, wires=4)
 
         r = 2.324
 
@@ -1074,20 +1074,22 @@ def normal_circuit(rads):
             qml.CNOT(wires=[0, 1])
             qml.CRY(rads, wires=[0, 1])
             qml.CZ(wires=[0, 1])
-            return qml.probs(wires=1)
+            qml.ctrl(qml.CRX, control=0, control_values=[1])(0.5, [1, 2])
+            return qml.probs(wires=[1, 2])
 
         def f(x):
             qml.PauliX(1)
             qml.RY(x, wires=1)
             qml.PauliZ(1)
+            qml.CRX(0.5, [1, 2])
 
         @qml.defer_measurements
         @qml.qnode(dev)
         def quantum_control_circuit(r):
             qml.Hadamard(0)
             m_0 = qml.measure(0)
             qml.cond(m_0, f)(r)
-            return qml.probs(wires=1)
+            return qml.probs(wires=[1, 2])
 
         exp = normal_circuit(r)
         cond_probs = quantum_control_circuit(r)