fix: multi-target observable target bug (#132)

Fixes a bug where a circuit is still valid if two multi-target observables are applied to the same set of target qubits, but the qubits are in different orders; these observables should conflict with one another.

src/braket/circuits/circuit.py

@@ -110,6 +110,7 @@ def __init__(self, addable: AddableTypes = None, *args, **kwargs):
         self._moments: Moments = Moments()
         self._result_types: List[ResultType] = []
         self._qubit_observable_mapping: Dict[Union[int, Circuit._ALL_QUBITS], Observable] = {}
+        self._qubit_target_mapping: Dict[int, List[int]] = {}
 
         if addable is not None:
             self.add(addable, *args, **kwargs)
@@ -274,13 +275,23 @@ def _add_to_qubit_observable_mapping(self, result_type: ResultType) -> None:
 
         for target in targets:
             current_observable = all_qubits_observable or self._qubit_observable_mapping.get(target)
+            current_target = self._qubit_target_mapping.get(target)
             if current_observable and current_observable != observable:
                 raise ValueError(
                     f"Existing result type for observable {current_observable} for target {target}"
                     f" conflicts with observable {observable} for new result type"
                 )
+
             if result_type.target:
+                # The only way this can happen is if the observables (acting on multiple target
+                # qubits) and target qubits are the same, but the new target is the wrong order;
+                if current_target and current_target != targets:
+                    raise ValueError(
+                        f"Target order {current_target} of existing result type with observable"
+                        f" {current_observable} conflicts with order {targets} of new result type"
+                    )
                 self._qubit_observable_mapping[target] = observable
+                self._qubit_target_mapping[target] = targets
 
         if not result_type.target:
             self._qubit_observable_mapping[Circuit._ALL_QUBITS] = observable

test/unit_tests/braket/circuits/test_circuit.py

@@ -193,6 +193,15 @@ def test_add_result_type_observable_no_conflict_state_vector_obs_return_value():
     assert circ.result_types == expected
 
 
+@pytest.mark.xfail(raises=ValueError)
+def test_add_result_type_same_observable_wrong_target_order():
+    Circuit().add_result_type(
+        ResultType.Expectation(observable=Observable.Y() @ Observable.X(), target=[0, 1])
+    ).add_result_type(
+        ResultType.Variance(observable=Observable.Y() @ Observable.X(), target=[1, 0])
+    )
+
+
 @pytest.mark.xfail(raises=TypeError)
 def test_add_result_type_with_target_and_mapping(prob):
     Circuit().add_result_type(prob, target=[10], target_mapping={0: 10})