get_probabilities: Checks that all qubits are known.

projectq/backends/_ibm/_ibm.py

@@ -167,11 +167,15 @@ def get_probabilities(self, qureg):
             probabilities.
 
         Raises:
-            Exception: If no data is available (i.e., if the circuit has not
-                been executed).
+            RuntimeError: If no data is available (i.e., if the circuit has
+                not been executed). Or if a qubit was supplied which was not
+                present in the circuit (might have gotten optimized away).
         """
         if len(self._probabilities) == 0:
             raise RuntimeError("Please, run the circuit first!")
+        if any(qb.id not in self._mapping for qb in qureg):
+            raise RuntimeError("Unknown qubit. Please, make sure that all "
+                               "provided qubits are present in the circuit.")
 
         probability_dict = dict()
 

projectq/backends/_ibm/_ibm_test.py

@@ -111,3 +111,6 @@ def mock_send(*args, **kwargs):
     prob_dict = eng.backend.get_probabilities([qureg[0], qureg[2], qureg[1]])
     assert prob_dict['111'] == pytest.approx(0.38671875)
     assert prob_dict['101'] == pytest.approx(0.0263671875)
+
+    with pytest.raises(RuntimeError):
+        eng.backend.get_probabilities(eng.allocate_qubit())