Enable enable_new_opmath for lightning.tensor

.github/CHANGELOG.md

@@ -83,6 +83,9 @@
 
 ### Bug fixes
 
+* Bug fix for `_pauli_word` of `QuantumScriptSerializer`. `_pauli_word` can process `PauliWord` object: `I`.
+  [(#919)](https://github.com/PennyLaneAI/pennylane-lightning/pull/919)
+
 * Bug fix for analytic `probs` in the `lightning.tensor` C++ layer.
   [(#906)](https://github.com/PennyLaneAI/pennylane-lightning/pull/906)
 

doc/lightning_tensor/device.rst

@@ -22,7 +22,7 @@ and greater. This device supports both exact and finite shots measurements. Curr
 
 The ``lightning.tensor`` device is designed for expectation value calculations. Measurements of ``qml.probs()`` or ``qml.state()`` return dense vectors of dimension :math:`2^{n_\text{qubits}}`, so they should only be used for small systems.
 
-.. note:: ``qml.Hermitian`` is currently only supported for single wires. You can use ``qml.pauli_decompose`` on smaller matrices to obtain a compatible Pauli decomposition in the meantime. As a result, it is advisable to disable ``new_opmath`` for the ``lightning.tensor`` device to prevent the implicit conversion of Pauli words to multi-wire Hermitian observables. This limitation will be addressed once multi-wires Hermitian observables can be supported with ``cutensornet``.
+.. note:: ``qml.Hermitian`` is currently only supported for single wires. You can use ``qml.pauli_decompose`` on smaller matrices to obtain a compatible Pauli decomposition in the meantime.
 
 Users also have the flexibility to customize these parameters according to their specific needs with:
 

pennylane_lightning/core/_serialize.py

@@ -285,6 +285,9 @@
         def map_wire(wire: int):
             return wires_map[wire] if wires_map else wire
 
+        if len(observable) == 0:
+            return self.named_obs(PAULI_NAME_MAP["I"], [0])
+
         if len(observable) == 1:
             wire, pauli = list(observable.items())[0]
             return self.named_obs(PAULI_NAME_MAP[pauli], [map_wire(wire)])

pennylane_lightning/core/_version.py

@@ -16,4 +16,4 @@
    Version number (major.minor.patch[-label])
 """
 
-__version__ = "0.39.0-dev30"
+__version__ = "0.39.0-dev31"

tests/test_execute.py

@@ -33,9 +33,6 @@ class TestQChem:
     def test_VQE_gradients(self, diff_method, tol):
         """Test if the VQE procedure returns the expected gradients."""
 
-        if qml.operation.active_new_opmath() and device_name == "lightning.tensor":
-            pytest.skip("The new operation math is not yet fully supported for lightning.tensor")
-
         symbols = ["H", "H"]
 
         geometry = np.array(