remote: use SKSynthesisPlugin when possible

The remote/ backend now uses the SKSynthesisPlugin when (1)
rotations are absent from the gate set and (2) H and T are in the
gate set.
This will allow us to support logical qubit targets through the API.
The drawback is that the decision rule (1 + 2 above) may not be suitable
for some given situation in the future. Time will tell!

qiskit_alice_bob_provider/remote/backend.py

@@ -45,6 +45,7 @@ def __init__(self, api_client: ApiClient, target_description: Dict):
         self._api_client = api_client
         self._target = ab_target_to_qiskit_target(target_description)
         self._options = _options_from_ab_target(target_description)
+        self._translation_plugin = _determine_translation_plugin(self._target)
 
     def __repr__(self) -> str:
         return f'<AliceBobRemoteBackend(name={self.name})>'
@@ -64,7 +65,7 @@ def max_circuits(self):
     def get_translation_stage_plugin(self):
         """This hook tells Qiskit to run the transpilation passes contained
         in translation_plugin.StatePreparationPlugin"""
-        return 'state_preparation'
+        return self._translation_plugin
 
     def run(self, run_input: QuantumCircuit, **kwargs) -> AliceBobRemoteJob:
         """Run the quantum circuit on the Alice & Bob backend by calling the
@@ -131,3 +132,14 @@ def _ab_input_params_from_options(options: Options) -> Dict:
             name = 'nbShots'
         input_params[name] = value
     return input_params
+
+
+def _determine_translation_plugin(target: Target) -> str:
+    """Choose which translation plugin to apply, depending on the gate set of
+    the backend."""
+    instructions = {instr.name for instr, _ in target.instructions}
+    if any(
+        rotation in instructions for rotation in ['u', 'rx', 'ry', 'rz']
+    ) or any(gate not in instructions for gate in ['h', 't']):
+        return 'state_preparation'
+    return 'sk_synthesis'

tests/conftest.py

@@ -193,11 +193,61 @@ def all_instructions_target() -> dict:
     }
 
 
+@pytest.fixture
+def h_target() -> dict:
+    return {
+        'name': 'H',
+        'numQubits': 1,
+        'instructions': [
+            {'signature': '__quantum__qis__h__body:void (%Qubit*)'}
+        ],
+        'inputParams': {
+            'nbShots': {
+                'required': True,
+                'default': 1000,
+                'constraints': [{'min': 1, 'max': 10000000}],
+            },
+            'averageNbPhotons': {
+                'required': False,
+                'default': None,
+                'constraints': [],
+            },
+        },
+    }
+
+
+@pytest.fixture
+def h_t_target() -> dict:
+    return {
+        'name': 'H_T',
+        'numQubits': 1,
+        'instructions': [
+            {'signature': '__quantum__qis__h__body:void (%Qubit*)'},
+            {'signature': '__quantum__qis__t__body:void (%Qubit*)'},
+        ],
+        'inputParams': {
+            'nbShots': {
+                'required': True,
+                'default': 1000,
+                'constraints': [{'min': 1, 'max': 10000000}],
+            },
+            'averageNbPhotons': {
+                'required': False,
+                'default': None,
+                'constraints': [],
+            },
+        },
+    }
+
+
 @pytest.fixture
 def targets(
-    single_cat_target: Dict, all_instructions_target: Dict
+    single_cat_target: Dict,
+    all_instructions_target: Dict,
+    h_target: Dict,
+    h_t_target: Dict,
 ) -> List[Dict]:
-    return [single_cat_target, all_instructions_target]
+    return [single_cat_target, all_instructions_target, h_target, h_t_target]
 
 
 @pytest.fixture

tests/remote/test_backend.py

@@ -163,3 +163,30 @@ def test_translation_plugin_and_qir(mocked_targets) -> None:
         )
         in qir
     )
+
+
+def test_determine_translation_plugin(mocked_targets) -> None:
+    p = AliceBobRemoteProvider(api_key='foo')
+
+    # Rotations available
+    assert (
+        p.get_backend('ALL_INSTRUCTIONS').get_translation_stage_plugin()
+        == 'state_preparation'
+    )
+
+    # H and T missing
+    assert (
+        p.get_backend('EMU:1Q:LESCANNE_2020').get_translation_stage_plugin()
+        == 'state_preparation'
+    )
+
+    # T missing
+    assert (
+        p.get_backend('H').get_translation_stage_plugin()
+        == 'state_preparation'
+    )
+
+    # ok
+    assert (
+        p.get_backend('H_T').get_translation_stage_plugin() == 'sk_synthesis'
+    )