Remove Tensor in devices (#204)

* remove Tensor in devices

* fix wrong variable name and format

* fix bug

* temp addition: where are we testing this?

* update changelog

* remove temp change

* trigger ci

CHANGELOG.md

@@ -9,6 +9,10 @@
 * The ``qml.QubitStateVector`` template has been removed. Instead, use :class:`~pennylane.StatePrep`.
   [(#203)](https://github.com/PennyLaneAI/pennylane-cirq/pull/203)
 
+* Support for the `pennylane.operation.Tensor` observable is removed. This observable was deprecated
+  in PennyLane 0.39, and is removed in PennyLane 0.40.
+  [(#204)](https://github.com/PennyLaneAI/pennylane-cirq/pull/204)
+
 ### Deprecations 👋
 
 ### Documentation 📝

pennylane_cirq/cirq_device.py

@@ -41,7 +41,6 @@
 import cirq
 import pennylane as qml
 from pennylane.devices import QubitDevice
-from pennylane.operation import Tensor
 from pennylane.ops import Prod
 from pennylane.wires import Wires
 
@@ -192,9 +191,8 @@ def supports_operation(self, operation):
 
     def to_paulistring(self, observable):
         """Convert an observable to a cirq.PauliString"""
-        if isinstance(observable, (Tensor, Prod)):
-            obs = observable.obs if isinstance(observable, Tensor) else observable.operands
-            obs = [self.to_paulistring(o) for o in obs]
+        if isinstance(observable, Prod):
+            obs = [self.to_paulistring(o) for o in observable.operands]
             return functools.reduce(operator.mul, obs)
         cirq_op = self._observable_map[observable.name]
         if cirq_op is None:

pennylane_cirq/qsim_device.py

@@ -81,16 +81,8 @@ def observables(self):
         return set(self._base_observable_map)
 
     def expval(self, observable, shot_range=None, bin_size=None):
-        is_tensor = isinstance(observable, (qml.operation.Tensor, qml.ops.Prod))
 
-        ob_names = (
-            [obs.name for obs in observable.operands]
-            if isinstance(observable, qml.ops.Prod)
-            else observable.name
-        )
-        if (
-            is_tensor and all(obs == "Identity" for obs in ob_names)
-        ) or observable.name == "Identity":
+        if isinstance(observable.simplify(), qml.Identity):
             return 1
 
         return super().expval(observable, shot_range, bin_size)

pennylane_cirq/simulator_device.py

@@ -155,49 +155,28 @@ def expval(self, observable, shot_range=None, bin_size=None):
         # pylint: disable=missing-function-docstring
         # Analytic mode
         if self.shots is None:
-            if not isinstance(observable, (qml.operation.Tensor, qml.ops.Prod)):
-                # Observable on a single wire
-                # Projector, Hermitian
-                if self._observable_map[observable.name] is None or observable.name == "Projector":
+            all_observables = (
+                list(observable.operands) if isinstance(observable, qml.ops.Prod) else [observable]
+            )
+
+            for obs in all_observables:
+                if self._observable_map[obs.name] is None or obs.name == "Projector":
                     return super().expval(observable, shot_range, bin_size)
 
-                if observable.name == "Hadamard":
-                    circuit = self.circuit
-                    obs = cirq.PauliSum() + self.to_paulistring(qml.PauliZ(wires=observable.wires))
-                else:
-                    circuit = self.pre_rotated_circuit
-                    obs = cirq.PauliSum() + self.to_paulistring(observable)
+            if "Hadamard" in [op.name for op in all_observables]:
+                list_obs = []
 
-            # Observables are in tensor form
-            else:
+                for obs in all_observables:
+                    list_obs.append(qml.PauliZ(wires=obs.wires))
 
-                ob_names = (
-                    [op.name for op in observable.operands]
-                    if isinstance(observable, qml.ops.Prod)
-                    else observable.name
-                )
-
-                # Projector, Hamiltonian, Hermitian
-                for name in ob_names:
-                    if self._observable_map[name] is None or name == "Projector":
-                        return super().expval(observable, shot_range, bin_size)
-
-                if "Hadamard" in ob_names:
-                    list_obs = []
-                    observables = (
-                        observable.operands
-                        if isinstance(observable, qml.ops.Prod)
-                        else observable.obs
-                    )
-                    for obs in observables:
-                        list_obs.append(qml.PauliZ(wires=obs.wires))
-
-                    T = qml.prod(*list_obs)
-                    circuit = self.circuit
-                    obs = cirq.PauliSum() + self.to_paulistring(T)
-                else:
-                    circuit = self.pre_rotated_circuit
-                    obs = cirq.PauliSum() + self.to_paulistring(observable)
+                T = qml.prod(*list_obs)
+
+                circuit = self.circuit
+                obs = cirq.PauliSum() + self.to_paulistring(T)
+
+            else:
+                circuit = self.pre_rotated_circuit
+                obs = cirq.PauliSum() + self.to_paulistring(observable)
 
             return self._simulator.simulate_expectation_values(
                 program=circuit,