[WIP] Add is_hermitian property to Operator

doc/releases/changelog-dev.md

@@ -111,6 +111,8 @@
 * `BasisEmbedding` can accept an int as argument instead of a list of bits (optionally). Example: `qml.BasisEmbedding(4, wires = range(4))` is now equivalent to `qml.BasisEmbedding([0,1,0,0], wires = range(4))` (because 4=0b100). 
   [(#2601)](https://github.com/PennyLaneAI/pennylane/pull/2601)
 
+* Introduced a new `is_hermitian` property to determine if an operator can be used in a measurement process.
+  [(#2629)](https://github.com/PennyLaneAI/pennylane/pull/2629)
 <h3>Breaking changes</h3>
 
 * The `qml.queuing.Queue` class is now removed.

pennylane/measurements.py

@@ -463,7 +463,7 @@ def circuit(x):
     Raises:
         QuantumFunctionError: `op` is not an instance of :class:`~.Observable`
     """
-    if not isinstance(op, (qml.operation.Observable, qml.Hamiltonian)):
+    if not op.is_hermitian:
         raise qml.QuantumFunctionError(
             f"{op.name} is not an observable: cannot be used with expval"
         )
@@ -498,7 +498,7 @@ def circuit(x):
     Raises:
         QuantumFunctionError: `op` is not an instance of :class:`~.Observable`
     """
-    if not isinstance(op, qml.operation.Observable):
+    if not op.is_hermitian:
         raise qml.QuantumFunctionError(f"{op.name} is not an observable: cannot be used with var")
 
     return MeasurementProcess(Variance, obs=op, shape=(1,), numeric_type=float)
@@ -577,7 +577,7 @@ def circuit(x):
         observable ``obs``.
     """
     if (
-        not isinstance(op, qml.operation.Observable) and op is not None
+        op is not None and not op.is_hermitian
     ):  # None type is also allowed for op
         raise qml.QuantumFunctionError(
             f"{op.name} is not an observable: cannot be used with sample"

pennylane/operation.py

@@ -1000,6 +1000,11 @@ def hyperparameters(self):
         self._hyperparameters = {}
         return self._hyperparameters
 
+    @property
+    def is_hermitian(self):
+        """This property determines if an operator is hermitian."""
+        return False
+
     def decomposition(self):
         r"""Representation of the operator as a product of other operators.
 
@@ -1552,6 +1557,11 @@ def _queue_category(self):
         """
         return "_ops" if isinstance(self, Operation) else None
 
+    @property
+    def is_hermitian(self):
+        """All observables must be hermitian"""
+        return True
+
     # pylint: disable=abstract-method
     return_type = None
     """None or ObservableReturnTypes: Measurement type that this observable is called with."""

tests/test_operation.py

@@ -600,6 +600,17 @@ class DummyOp(qml.operation.Operation):
         op = DummyOp(1.0, wires=0, id="test")
         assert op.control_wires == qml.wires.Wires([])
 
+    def test_is_hermitian(self):
+        """Test that is_hermitian defaults to False for an Operator"""
+
+        class DummyOp(qml.operation.Operation):
+            r"""Dummy custom operation"""
+            num_wires = 1
+            grad_method = None
+
+        op = DummyOp(wires=0)
+        assert op.is_hermitian is False
+
 
 class TestObservableConstruction:
     """Test custom observables construction."""
@@ -711,6 +722,17 @@ class DummyObservable(qml.operation.Observable):
         with pytest.raises(Exception, match="Must specify the wires *"):
             DummyObservable()
 
+    def test_is_hermitian(self):
+        """Test that the id attribute of an observable can be set."""
+
+        class DummyObserv(qml.operation.Observable):
+            r"""Dummy custom observable"""
+            num_wires = 1
+            grad_method = None
+
+        op = DummyObserv(wires=0)
+        assert op.is_hermitian is True
+
 
 class TestOperatorIntegration:
     """Integration tests for the Operator class"""