Fix bug causing slices with depth > 1 to be forward-prop'd (#62)

* Fix bug, add test

* Update qiskit_addon_obp/backpropagation.py

Co-authored-by: Max Rossmannek <oss@zurich.ibm.com>

* peer review

* Update logger msg

* reno

* Cast to list for reversed

* Make op_nodes a list

---------

Co-authored-by: Max Rossmannek <oss@zurich.ibm.com>

qiskit_addon_obp/backpropagation.py

@@ -164,7 +164,8 @@ def handle_timeout(signum, frame):
             # PERF: we will likely need to parallelize this loop
             for i in range(num_observables):
                 non_trivial_slice = False
-                for op_idx, op_node in enumerate(circuit_to_dag(slice_).topological_op_nodes()):
+                op_nodes = list(circuit_to_dag(slice_).topological_op_nodes())[::-1]
+                for op_idx, op_node in enumerate(op_nodes):
                     # Ignore barriers within slices
                     if op_node.name == "barrier":
                         continue
@@ -214,7 +215,7 @@ def handle_timeout(signum, frame):
                         )
 
                     LOGGER.debug(
-                        f"Size of the observable after backpropagating the {op_idx}-th gate in "
+                        f"Size of the observable after backpropagating gate id {len(op_nodes) - op_idx - 1} in "
                         f"the current layer: {len(observables_tmp[i])}"
                     )
 

releasenotes/notes/slice-bug-1aa7af1b87d4a9df.yaml

@@ -0,0 +1,4 @@
+---
+fixes:
+  - |
+    Fixed a bug in :func:`qiskit_addon_obp.backpropagate` which caused slices with depth > 1 to be incorrectly forward-propagated. Gates from a given slice will now be correctly propagated into the observable in reverse order (i.e., from the back of the slice).

test/test_backpropagation.py

@@ -61,7 +61,15 @@ def test_backpropagate(self):
             self.assertEqual({Pauli("IX"), Pauli("IY")}, set(new_obs[0].paulis))
             self.assertEqual(0, len(slices))
             self.assertEqual([], slices)
-
+        with self.subTest("Depth-2"):
+            qc = QuantumCircuit(2)
+            qc.h(0)
+            qc.cx(0, 1)
+            obs = SparsePauliOp("ZZ")
+            target_obs = SparsePauliOp("ZI")
+            new_obs, slices, _ = backpropagate(obs, [qc])
+            self.assertEqual(target_obs, new_obs)
+            self.assertEqual([], slices)
         with self.subTest("Scattered qargs"):
             qc = QuantumCircuit(5)
             obs = SparsePauliOp("XIYIZ")
@@ -512,47 +520,35 @@ def test_backpropagate(self):
             new_obs, _, _ = backpropagate(
                 obs, [qc], truncation_error_budget=setup_budget(max_error_total=0.1, p_norm=2)
             )
-            self.assertEqual(7, len(new_obs.paulis))
+            self.assertEqual(3, len(new_obs.paulis))
             self.assertEqual(
                 {
                     Pauli("XX"),
-                    Pauli("YX"),
-                    Pauli("ZX"),
+                    Pauli("XZ"),
                     Pauli("XY"),
-                    Pauli("YY"),
-                    Pauli("ZY"),
-                    Pauli("IZ"),
                 },
                 set(new_obs.paulis),
             )
             # Budget to truncate smallest term
             new_obs, _, _ = backpropagate(
-                obs, [qc], truncation_error_budget=setup_budget(max_error_total=0.2, p_norm=2)
+                obs, [qc], truncation_error_budget=setup_budget(max_error_total=0.26, p_norm=2)
             )
-            self.assertEqual(6, len(new_obs.paulis))
+            self.assertEqual(2, len(new_obs.paulis))
             self.assertEqual(
                 {
                     Pauli("XX"),
-                    Pauli("ZX"),
                     Pauli("XY"),
-                    Pauli("YY"),
-                    Pauli("ZY"),
-                    Pauli("IZ"),
                 },
                 set(new_obs.paulis),
             )
             # Budget to truncate 2 smallest terms
             new_obs, _, _ = backpropagate(
-                obs, [qc], truncation_error_budget=setup_budget(max_error_total=0.29, p_norm=2)
+                obs, [qc], truncation_error_budget=setup_budget(max_error_total=0.501, p_norm=2)
             )
-            self.assertEqual(5, len(new_obs.paulis))
+            self.assertEqual(1, len(new_obs.paulis))
             self.assertEqual(
                 {
-                    Pauli("XX"),
                     Pauli("XY"),
-                    Pauli("YY"),
-                    Pauli("ZY"),
-                    Pauli("IZ"),
                 },
                 set(new_obs.paulis),
             )