Circuit cutting: Add input validation checks

doc/releases/changelog-dev.md

@@ -99,6 +99,7 @@
   A suite of integration tests has been added.
   [(#2231)](https://github.com/PennyLaneAI/pennylane/pull/2231)
   [(#2234)](https://github.com/PennyLaneAI/pennylane/pull/2234)
+  [(#2251)](https://github.com/PennyLaneAI/pennylane/pull/2251)
 
 <h3>Improvements</h3>
 

pennylane/transforms/qcut.py

@@ -34,7 +34,6 @@
 from pennylane.operation import Expectation, Operation, Operator, Tensor
 from pennylane.ops.qubit.non_parametric_ops import WireCut
 from pennylane.tape import QuantumTape
-from pennylane.transforms import batch_transform
 from pennylane.wires import Wires
 
 from .batch_transform import batch_transform
@@ -941,6 +940,33 @@ def circuit(x):
         >>> qml.grad(cut_circuit)(x)
         -0.506395895364911
     """
+    if len(tape.measurements) != 1:
+        raise ValueError(
+            "The circuit cutting workflow only supports circuits with a single output "
+            "measurement"
+        )
+
+    if not all(m.return_type is Expectation for m in tape.measurements):
+        raise ValueError(
+            "The circuit cutting workflow only supports circuits with expectation "
+            "value measurements"
+        )
+
+    if use_opt_einsum:
+        try:
+            import opt_einsum  # pylint: disable=import-outside-toplevel,unused-import
+        except ImportError as e:
+            raise ImportError(
+                "The opt_einsum package is required when use_opt_einsum is set to "
+                "True in the cut_circuit function. This package can be "
+                "installed using:\npip install opt_einsum"
+            ) from e
+
+    num_cut = len([op for op in tape.operations if isinstance(op, WireCut)])
+    if num_cut == 0:
+        raise ValueError(
+            "Cannot apply the circuit cutting workflow to a circuit without any cuts"
+        )
 
     g = tape_to_graph(tape)
     replace_wire_cut_nodes(g)

tests/transforms/test_qcut.py

@@ -2106,6 +2106,57 @@ def circuit(x):
         assert np.isclose(grad, grad_expected)
 
 
+class TestCutCircuitTransformValidation:
+    """Tests of validation checks in the cut_circuit function"""
+
+    def test_multiple_measurements_raises(self):
+        """Tests if a ValueError is raised when a tape with multiple measurements is requested
+        to be cut"""
+
+        with qml.tape.QuantumTape() as tape:
+            qml.expval(qml.PauliZ(0))
+            qml.expval(qml.PauliZ(1))
+
+        with pytest.raises(ValueError, match="The circuit cutting workflow only supports circuits"):
+            qcut.cut_circuit(tape)
+
+    def test_no_measurements_raises(self):
+        """Tests if a ValueError is raised when a tape with no measurement is requested
+        to be cut"""
+        with pytest.raises(ValueError, match="The circuit cutting workflow only supports circuits"):
+            qcut.cut_circuit(qml.tape.QuantumTape())
+
+    def test_non_expectation_raises(self):
+        """Tests if a ValueError is raised when a tape with measurements that are not expectation
+        values is requested to be cut"""
+
+        with qml.tape.QuantumTape() as tape:
+            qml.var(qml.PauliZ(0))
+
+        with pytest.raises(ValueError, match="workflow only supports circuits with expectation"):
+            qcut.cut_circuit(tape)
+
+    def test_fail_import(self, monkeypatch):
+        """Test if an ImportError is raised when opt_einsum is requested but not installed"""
+        with qml.tape.QuantumTape() as tape:
+            qml.expval(qml.PauliZ(0))
+
+        with monkeypatch.context() as m:
+            m.setitem(sys.modules, "opt_einsum", None)
+
+            with pytest.raises(ImportError, match="The opt_einsum package is required"):
+                qcut.cut_circuit(tape, use_opt_einsum=True)
+
+    def test_no_cuts_raises(self):
+        """Tests if a ValueError is raised when circuit cutting is to be applied to a circuit
+        without cuts"""
+        with qml.tape.QuantumTape() as tape:
+            qml.expval(qml.PauliZ(0))
+
+        with pytest.raises(ValueError, match="to a circuit without any cuts"):
+            qcut.cut_circuit(tape)
+
+
 class TestCutStrategy:
     """Tests for class CutStrategy"""