Default to one core

qermit/noise_model/transpiler_backend.py

@@ -37,7 +37,7 @@ def __init__(
         transpiler: BasePass,
         result_dict: Dict[ResultHandle, BackendResult] = {},
         max_batch_size: int = 1000,
-        n_cores: Optional[int] = None,
+        n_cores: int = 1,
     ):
         """Initialisation method.
 
@@ -52,9 +52,9 @@ def __init__(
             results within backend, defaults to {}
         :type result_dict: Dict[ResultHandle, BackendResult], optional
         :param n_cores: Shots will be taken in parallel. This parameter
-            specifies the number of cores to use.
-        :type n_cores: Optional[int]. Defaults to None, using all available
-            cores.
+            specifies the number of cores to use. The default is to use
+            one core.
+        :type n_cores: Optional[int]. Defaults to 1.
         """
 
         self.transpiler = transpiler
@@ -261,16 +261,28 @@ def get_counts(
         :rtype: Iterator[Counter]
         """
 
-        if cbits is not None:
-            cbits_list = [cbit.to_list() for cbit in cbits]
-        else:
-            cbits_list = None
+        if self.n_cores > 1:
 
-        with multiprocessing.Pool(self.n_cores) as pool:
-            processes = [
-                pool.apply_async(self._get_batch_counts, args=(circuit_list, cbits_list))
-                for circuit_list in self._gen_batches(circuit, n_shots)
-            ]
-            counter_list = [p.get() for p in processes]
+            if cbits is not None:
+                cbits_list = [cbit.to_list() for cbit in cbits]
+            else:
+                cbits_list = None
 
-        return sum(counter_list, Counter())
+            with multiprocessing.Pool(self.n_cores) as pool:
+                processes = [
+                    pool.apply_async(self._get_batch_counts, args=(circuit_list, cbits_list))
+                    for circuit_list in self._gen_batches(circuit, n_shots)
+                ]
+                counter_list = [p.get() for p in processes]
+
+            return sum(counter_list, Counter())
+
+        else:
+
+            counter: Counter = Counter()
+            for circuit_list in self._gen_batches(circuit, n_shots):
+                result_list = self.backend.run_circuits(circuit_list, n_shots=1)
+                counter += sum((result.get_counts(cbits=cbits)
+                            for result in result_list), Counter())
+
+            return counter

tests/zne_test.py

@@ -863,7 +863,10 @@ def test_end_to_end_noise_aware_zne_mitex_starting_from_ptm() -> None:
         }
     )
     transpiler = PauliErrorTranspile(noise_model=noise_model)
-    backend = TranspilerBackend(transpiler=transpiler)
+    backend = TranspilerBackend(
+        transpiler=transpiler,
+        n_cores=1,
+    )
 
     # Here we perform ZNE with some unevenly spaced
     # noise scaling values.
@@ -920,7 +923,10 @@ def test_end_to_end_noise_aware_zne_mitex():
         noise_model={OpType.CZ: error_distribution}
     )
     transpiler = PauliErrorTranspile(noise_model=noise_model)
-    backend = TranspilerBackend(transpiler=transpiler)
+    backend = TranspilerBackend(
+        transpiler=transpiler,
+        n_cores=1,
+    )
 
     zne_mitex = gen_ZNE_MitEx(
         backend=backend,