Change Conditional.then_op usage to Conditional.base (#752)

New work in Pennylane removed `Conditional.then_op` in favour of
`Conditional.base`. This PR updates lightning to use the new attribute.

---------

Co-authored-by: ringo-but-quantum <github-ringo-but-quantum@xanadu.ai>

.github/CHANGELOG.md

@@ -67,6 +67,9 @@
 
 ### Bug fixes
 
+* `lightning.qubit` and `lightning.kokkos` use `qml.ops.Conditional.base` instead of `qml.ops.Conditional.then_op`.
+  [(#752)](https://github.com/PennyLaneAI/pennylane-lightning/pull/752)
+
 * Fix AVX streaming operation support with newer GCC.
   [(#729)](https://github.com/PennyLaneAI/pennylane-lightning/pull/729)
 

pennylane_lightning/core/_version.py

@@ -16,4 +16,4 @@
    Version number (major.minor.patch[-label])
 """
 
-__version__ = "0.37.0-dev27"
+__version__ = "0.37.0-dev28"

pennylane_lightning/lightning_kokkos/lightning_kokkos.py

@@ -432,7 +432,7 @@ def apply_lightning(self, operations, mid_measurements=None, postselect_mode=Non
 
             if isinstance(ops, Conditional):
                 if ops.meas_val.concretize(mid_measurements):
-                    self.apply_lightning([ops.then_op])
+                    self.apply_lightning([ops.base])
             elif isinstance(ops, MidMeasureMP):
                 self._apply_lightning_midmeasure(ops, mid_measurements, postselect_mode)
             elif isinstance(ops, qml.ops.op_math.Controlled) and isinstance(

pennylane_lightning/lightning_qubit/_state_vector.py

@@ -311,7 +311,7 @@ def _apply_lightning(
 
             if isinstance(operation, Conditional):
                 if operation.meas_val.concretize(mid_measurements):
-                    self._apply_lightning([operation.then_op])
+                    self._apply_lightning([operation.base])
             elif isinstance(operation, MidMeasureMP):
                 self._apply_lightning_midmeasure(
                     operation, mid_measurements, postselect_mode=postselect_mode