Add test_templates module + Fix LQ decomposition of StatePrep Ops + Fix LQ decomposition strategy of QFT and GroverOperator

.github/CHANGELOG.md

@@ -52,6 +52,9 @@
 
 ### Improvements
 
+* Add `test_templates.py` module where Grover and QSVT are tested.
+  [(#684)](https://github.com/PennyLaneAI/pennylane-lightning/pull/684)
+
 * Create `cuda_utils` for common usage of CUDA related backends.
   [(#676)](https://github.com/PennyLaneAI/pennylane-lightning/pull/676)
 

pennylane_lightning/core/_serialize.py

@@ -255,6 +255,9 @@
 
     def _pauli_sentence(self, observable, wires_map: dict = None):
         """Serialize a :class:`pennylane.pauli.PauliSentence` into a Hamiltonian."""
+        # Trivial Pauli sentences' items is empty, cannot unpack
+        if not observable:
+            return self.hamiltonian_obs(np.array([0.0]).astype(self.rtype), [self._ob(Identity(0))])
         pwords, coeffs = zip(*observable.items())
         terms = [self._pauli_word(pw, wires_map) for pw in pwords]
         coeffs = np.array(coeffs).astype(self.rtype)

pennylane_lightning/core/_version.py

@@ -16,4 +16,4 @@
    Version number (major.minor.patch[-label])
 """
 
-__version__ = "0.36.0-dev29"
+__version__ = "0.36.0-dev30"

tests/test_execute.py

@@ -87,40 +87,3 @@ def dev_l_execute(t):
 
         assert np.allclose(grad_dev_l, grad_qml_l, tol)
         assert np.allclose(grad_dev_l, grad_qml_d, tol)
-
-
-class TestGrover:
-    """Test Grover's algorithm (multi-controlled gates, decomposition, etc.)"""
-
-    @pytest.mark.parametrize("num_qubits", range(4, 8))
-    def test_grover(self, num_qubits):
-        np.random.seed(42)
-        omega = np.random.rand(num_qubits) > 0.5
-        dev = qml.device(device_name, wires=num_qubits)
-        wires = list(range(num_qubits))
-
-        @qml.qnode(dev, diff_method=None)
-        def circuit(omega):
-            iterations = int(np.round(np.sqrt(2**num_qubits) * np.pi / 4))
-
-            # Initial state preparation
-            for wire in wires:
-                qml.Hadamard(wires=wire)
-
-            # Grover's iterator
-            for _ in range(iterations):
-                qml.FlipSign(omega, wires=wires)
-                qml.templates.GroverOperator(wires)
-
-            return qml.probs(wires=wires)
-
-        prob = circuit(omega)
-        index = omega.astype(int)
-        index = functools.reduce(
-            lambda sum, x: sum + (1 << x[0]) * x[1],
-            zip([i for i in range(len(index) - 1, -1, -1)], index),
-            0,
-        )
-        assert np.allclose(np.sum(prob), 1.0)
-        assert prob[index] > 0.95
-        assert np.sum(prob) - prob[index] < 0.05