Fix incorrect stop time bug #8729

qiskit/circuit/quantumcircuit.py

@@ -6875,7 +6875,10 @@ def qubit_stop_time(self, *qubits: Union[Qubit, int]) -> float:
             if len(qubits) == len([done for done in dones.values() if done]):  # all done
                 return max(stop for stop in stops.values())
 
-        return 0  # If there are no instructions over bits
+        if len(stops) > 0:  # not all but some qubits has instructions
+            return max(stop for stop in stops.values())
+        else:
+            return 0  # If there are no instructions over bits
 
 
 class _OuterCircuitScopeInterface(CircuitScopeInterface):

releasenotes/notes/fix-incorrect-qubit-stop-time-d0e056f60a01dd52.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    Fixed an issue where :meth:`.QuantumCircuit.qubit_stop_time` and 
+    :meth:`.QuantumCircuit.qubit_duration` returned incorrect time (duration)
+    when some qubits have instructions but the others idle.
+    Fixed `#8729 <https://github.com/Qiskit/qiskit/issues/8729>`__.

test/python/circuit/test_scheduled_circuit.py

@@ -355,6 +355,7 @@ def test_per_qubit_durations_loose_constrain(self):
         self.assertEqual(sc.qubit_stop_time(2), 0)
         self.assertEqual(sc.qubit_start_time(0, 1), 300)
         self.assertEqual(sc.qubit_stop_time(0, 1), 1400)
+        self.assertEqual(sc.qubit_stop_time(0, 1, 2), 1400)
 
         qc.measure_all()