No backprop devices (#61)

* Getting lighting

* revert test hack

* formatting

* Revert "formatting"

This reverts commit b2a082c.

* formatting

* Revert "revert test hack"

This reverts commit e410e3b.

* test no backprop

* qfunc

* Update tests/test_apply.py

Co-authored-by: Josh Izaac <josh146@gmail.com>

* test best diff method

* changelog

Co-authored-by: Josh Izaac <josh146@gmail.com>

CHANGELOG.md

@@ -10,10 +10,17 @@
 
 ### Bug fixes
 
+* Fixes a bug where the `QNode` would swap `LightningQubit` to
+  `DefaultQubitAutograd` on device execution due to the inherited
+  `passthru_devices` entry of the `capabilities` dictionary.
+  [(#61)](https://github.com/PennyLaneAI/pennylane-lightning/pull/61)
+
 ### Contributors
 
 This release contains contributions from (in alphabetical order):
 
+Josh Izaac, Antal Száva
+
 ---
 
 # Release 0.14.0

pennylane_lightning/lightning_qubit.py

@@ -107,6 +107,7 @@ def capabilities(cls):
             supports_analytic_computation=True,
             returns_state=True,
         )
+        capabilities.pop("passthru_devices", None)
         return capabilities
 
     def apply(self, operations, rotations=None, **kwargs):

tests/test_apply.py

@@ -454,6 +454,29 @@ def test_load_default_qubit_device(self):
         assert dev.analytic
         assert dev.short_name == "lightning.qubit"
 
+    def test_no_backprop(self):
+        """Test that lightning.qubit does not support the backprop
+        differentiation method."""
+
+        dev = qml.device("lightning.qubit", wires=2)
+        def circuit():
+            """Simple quantum function."""
+            return qml.expval(qml.PauliZ(0))
+
+        with pytest.raises(qml.QuantumFunctionError):
+            qml.QNode(circuit, dev, diff_method="backprop")
+
+    def test_best_gets_lightning(self):
+        """Test that the best differentiation method returns lightning
+        qubit."""
+        dev = qml.device("lightning.qubit", wires=2)
+        def circuit():
+            """Simple quantum function."""
+            return qml.expval(qml.PauliZ(0))
+
+        qnode = qml.QNode(circuit, dev, diff_method="best")
+        assert isinstance(qnode.device, LightningQubit)
+
     def test_args(self):
         """Test that the plugin requires correct arguments"""