Fix get_results() when process_circuits() given circuits of different sizes

docs/changelog.md

@@ -8,6 +8,7 @@
 
 - Revert a change made in release v0.59.0 where users are warned about implicit qubit permutations in {py:func}`tk_to_qiskit`. This avoids spamming the user with unhelpful warnings when using pytket-qiskit backends. These backends handle implicit permutations automatically.
 - Add new level 3 optimisation that uses `GreedyPauliSimp`
+- Fix `get_results()` for Aer backends when circuits with different numbers of qubits or postprocessing circuits submitted together.
 
 ## 0.59.0 (November 2024)
 

pytket/extensions/qiskit/backends/aer.py

@@ -143,6 +143,10 @@ class _AerBaseBackend(Backend):
     _has_arch: bool = False
     _needs_transpile: bool = False
 
+    # Map from (job ID, circuit index) to (number of qubits, postprocessing circuit),
+    # i.e. from the first two components of the ResultHandle to the last two.
+    _circuit_data: dict[tuple[str, int], tuple[int, str]] = {}
+
     @property
     def required_predicates(self) -> list[Predicate]:
         return self._required_predicates
@@ -327,6 +331,7 @@ def process_circuits(
             for i, ind in enumerate(indices):
                 handle = ResultHandle(jobid, i, tkc_qubits_count[i], ppcirc_strs[i])
                 handle_list[ind] = handle
+                self._circuit_data[(jobid, i)] = (tkc_qubits_count[i], ppcirc_strs[i])
                 self._cache[handle] = {"job": job}
         return cast(list[ResultHandle], handle_list)
 
@@ -346,7 +351,7 @@ def get_result(self, handle: ResultHandle, **kwargs: KwargTypes) -> BackendResul
         try:
             return super().get_result(handle)
         except CircuitNotRunError:
-            jobid, _, qubit_n, ppc = handle
+            jobid = handle[0]
             try:
                 job: AerJob = self._cache[handle]["job"]
             except KeyError:
@@ -362,6 +367,7 @@ def get_result(self, handle: ResultHandle, **kwargs: KwargTypes) -> BackendResul
                 include_density_matrix=self._supports_density_matrix,
             )
             for circ_index, backres in enumerate(backresults):
+                qubit_n, ppc = self._circuit_data[(jobid, circ_index)]
                 self._cache[ResultHandle(jobid, circ_index, qubit_n, ppc)][
                     "result"
                 ] = backres

tests/backend_test.py

@@ -1541,3 +1541,13 @@ def test_optimisation_level_3_serialisation() -> None:
         },
     )
     assert p_dict == passlist.to_dict()
+
+
+def test_process_circuits_n_qubits() -> None:
+    # https://github.com/CQCL/pytket-qiskit/issues/420
+    circs = [Circuit(1).X(0).measure_all(), Circuit(2).X(0).measure_all()]
+    b = AerBackend()
+    hs = b.process_circuits(circs, n_shots=10)
+    rs = b.get_results(hs)
+    assert rs[0].get_counts() == Counter({(1,): 10})
+    assert rs[1].get_counts() == Counter({(1, 0): 10})