Fix memory handling in result formatting to include only measured bits (#245)

Fix memory handling in result formatting to include only measured bits

Author: jbrixon

qiskit_aqt_provider/aqt_job.py

@@ -523,7 +523,22 @@ def _partial_qiskit_result_dict(
     data: dict[str, Any] = {"counts": _format_counts(samples, meas_map)}
 
     if memory:
-        data["memory"] = ["".join(str(x) for x in reversed(states)) for states in samples]
+        # build per-shot classical memory strings using the measurement mapping
+        mem_slots = circuit.num_clbits
+        memory_list: list[str] = []
+        for states in samples:
+            # initialize classical register (all 0)
+            creg = ["0"] * mem_slots
+            for src_index, dest_indices in meas_map.items():
+                if src_index < len(states):
+                    src_bit = str(states[src_index])
+                    for dest_index in dest_indices:
+                        # place measured bit into the classical register position(s)
+                        if 0 <= dest_index < mem_slots:
+                            creg[dest_index] = src_bit
+            # Qiskit orders memory strings with bit 0 as least-significant (rightmost)
+            memory_list.append("".join(reversed(creg)))
+        data["memory"] = memory_list
 
     return {
         "shots": shots,

test/test_execution.py

@@ -239,6 +239,39 @@ def test_get_memory_ancilla_qubits(
     assert len(memory) == shots
 
 
+@pytest.mark.parametrize(
+    ("clbits", "flip", "measure", "expected"),
+    [
+        (5, [1], [0, 1, 2, 3, 4], "00010"),
+        (3, [1], [0, 1, 2], "010"),
+        (2, [2, 4], [3, 4], "10"),
+        (3, [2, 3], [1, 2, 3], "110"),
+        (3, [2, 4], [0, 2, 4], "110"),
+    ],
+)
+def test_memory_ignores_unmeasured_qubits(
+    clbits: int,
+    flip: list[int],
+    measure: list[int],
+    expected: str,
+    any_offline_simulator_no_noise: AnyAQTResource,
+) -> None:
+    """Regression test: memory should only contain measured classical bits."""
+    qc = QuantumCircuit(5, clbits)
+    for i in flip:
+        qc.x(i)
+    qc.measure(measure, range(clbits))
+
+    job = any_offline_simulator_no_noise.run(
+        qiskit.transpile(qc, any_offline_simulator_no_noise), shots=2, memory=True
+    )
+
+    memory = job.result().get_memory()
+
+    assert set(memory) == {expected}
+    assert len(memory) == 2
+
+
 @pytest.mark.parametrize("shots", [123])
 def test_get_memory_bit_ordering(
     shots: int, any_offline_simulator_no_noise: AnyAQTResource