Add default translation stage plugin

pyproject.toml

@@ -122,6 +122,7 @@ sk = "qiskit.transpiler.passes.synthesis.solovay_kitaev_synthesis:SolovayKitaevS
 default = "qiskit.transpiler.preset_passmanagers.builtin_plugins:DefaultInitPassManager"
 
 [project.entry-points."qiskit.transpiler.translation"]
+default = "qiskit.transpiler.preset_passmanagers.builtin_plugins:DefaultTranslationPassManager"
 synthesis = "qiskit.transpiler.preset_passmanagers.builtin_plugins:UnitarySynthesisPassManager"
 translator = "qiskit.transpiler.preset_passmanagers.builtin_plugins:BasisTranslatorPassManager"
 

qiskit/compiler/transpiler.py

@@ -191,10 +191,10 @@ def transpile(  # pylint: disable=too-many-return-statements
             This can also be the external plugin name to use for the ``routing`` stage.
             You can see a list of installed plugins by using :func:`~.list_stage_plugins` with
             ``"routing"`` for the ``stage_name`` argument.
-        translation_method: Name of translation pass (``"translator"`` or ``"synthesis"``)
-            This can also be the external plugin name to use for the ``translation`` stage.
-            You can see a list of installed plugins by using :func:`~.list_stage_plugins` with
-            ``"translation"`` for the ``stage_name`` argument.
+        translation_method: Name of translation pass (``"default"``, ``"translator"`` or
+            ``"synthesis"``). This can also be the external plugin name to use for the
+            ``translation`` stage.  You can see a list of installed plugins by using
+            :func:`~.list_stage_plugins` with ``"translation"`` for the ``stage_name`` argument.
         scheduling_method: Name of scheduling pass.
             * ``'as_soon_as_possible'``: Schedule instructions greedily, as early as possible
             on a qubit resource. (alias: ``'asap'``)

qiskit/transpiler/__init__.py

@@ -643,13 +643,25 @@
     * - Method
       - Summary
 
+    * - :ref:`default <transpiler-preset-stage-translation-translator>`
+      - Use a Qiskit-chosen default translation method.
+
     * - :ref:`translator <transpiler-preset-stage-translation-translator>`
       - Symbolic translation of gates to the target basis using known equivalences.
 
     * - :ref:`synthesis <transpiler-preset-stage-translation-synthesis>`
       - Collect each run of one- and two-qubit gates into a matrix representation, and resynthesize
         from there.
 
+.. _transpiler-preset-stage-translation-default:
+
+Built-in ``default`` plugin
+...........................
+
+Use a Qiskit-chosen default method for translation.  As of Qiskit 2.0, this is the same as
+:ref:`transpiler-preset-stage-translation-translator`, but the chosen algorithm might change during
+the 2.x series, either for all targets, or only for certain classes of target.
+
 .. _transpiler-preset-stage-translation-synthesis:
 
 Built-in ``synthesis`` plugin

qiskit/transpiler/preset_passmanagers/builtin_plugins.py

@@ -197,6 +197,17 @@ def pass_manager(self, pass_manager_config, optimization_level=None) -> PassMana
         return init
 
 
+class DefaultTranslationPassManager(PassManagerStagePlugin):
+    """Plugin class for the default-method translation stage."""
+
+    def pass_manager(self, pass_manager_config, optimization_level=None) -> PassManager:
+        # For now, this is just a wrapper around the `BasisTranslator`.  It might expand in the
+        # future if we want to change the default method to do more context-aware switching, or to
+        # start transitioning the default method without breaking the semantics of the default
+        # string referring to the `BasisTranslator`.
+        return BasisTranslatorPassManager().pass_manager(pass_manager_config, optimization_level)
+
+
 class BasisTranslatorPassManager(PassManagerStagePlugin):
     """Plugin class for translation stage with :class:`~.BasisTranslator`"""
 
@@ -535,7 +546,7 @@ class OptimizationPassManager(PassManagerStagePlugin):
     def pass_manager(self, pass_manager_config, optimization_level=None) -> PassManager:
         """Build pass manager for optimization stage."""
         # Obtain the translation method required for this pass to work
-        translation_method = pass_manager_config.translation_method or "translator"
+        translation_method = pass_manager_config.translation_method or "default"
         optimization = PassManager()
         if optimization_level != 0:
             plugin_manager = PassManagerStagePluginManager()

qiskit/transpiler/preset_passmanagers/common.py

@@ -60,15 +60,17 @@
 # Any method neither known good nor known bad (i.e. not a Terra-internal pass) is passed through
 # without error, since it is being supplied by a plugin and we don't have any knowledge of these.
 _CONTROL_FLOW_STATES = {
-    "layout_method": _ControlFlowState(working={"trivial", "dense", "sabre"}, not_working=set()),
+    "layout_method": _ControlFlowState(
+        working={"default", "trivial", "dense", "sabre"}, not_working=set()
+    ),
     "routing_method": _ControlFlowState(
         working={"none", "stochastic", "sabre"}, not_working={"lookahead", "basic"}
     ),
     "translation_method": _ControlFlowState(
-        working={"translator", "synthesis"},
+        working={"default", "translator", "synthesis"},
         not_working=set(),
     ),
-    "optimization_method": _ControlFlowState(working=set(), not_working=set()),
+    "optimization_method": _ControlFlowState(working={"default"}, not_working=set()),
     "scheduling_method": _ControlFlowState(working=set(), not_working={"alap", "asap"}),
 }
 

qiskit/transpiler/preset_passmanagers/level0.py

@@ -49,7 +49,7 @@ def level_0_pass_manager(pass_manager_config: PassManagerConfig) -> StagedPassMa
     init_method = pass_manager_config.init_method or "default"
     layout_method = pass_manager_config.layout_method or "default"
     routing_method = pass_manager_config.routing_method or "sabre"
-    translation_method = pass_manager_config.translation_method or "translator"
+    translation_method = pass_manager_config.translation_method or "default"
     optimization_method = pass_manager_config.optimization_method or "default"
     scheduling_method = pass_manager_config.scheduling_method or "default"
     target = pass_manager_config.target

qiskit/transpiler/preset_passmanagers/level1.py

@@ -53,7 +53,7 @@ def level_1_pass_manager(pass_manager_config: PassManagerConfig) -> StagedPassMa
     # based on whether the input circuit has control flow.
     layout_method = pass_manager_config.layout_method or "default"
     routing_method = pass_manager_config.routing_method or "sabre"
-    translation_method = pass_manager_config.translation_method or "translator"
+    translation_method = pass_manager_config.translation_method or "default"
     optimization_method = pass_manager_config.optimization_method or "default"
     scheduling_method = pass_manager_config.scheduling_method or "default"
     target = pass_manager_config.target

qiskit/transpiler/preset_passmanagers/level2.py

@@ -54,7 +54,7 @@ def level_2_pass_manager(pass_manager_config: PassManagerConfig) -> StagedPassMa
     init_method = pass_manager_config.init_method or "default"
     layout_method = pass_manager_config.layout_method or "default"
     routing_method = pass_manager_config.routing_method or "sabre"
-    translation_method = pass_manager_config.translation_method or "translator"
+    translation_method = pass_manager_config.translation_method or "default"
     optimization_method = pass_manager_config.optimization_method or "default"
     scheduling_method = pass_manager_config.scheduling_method or "default"
     target = pass_manager_config.target

qiskit/transpiler/preset_passmanagers/level3.py

@@ -54,7 +54,7 @@ def level_3_pass_manager(pass_manager_config: PassManagerConfig) -> StagedPassMa
     init_method = pass_manager_config.init_method or "default"
     layout_method = pass_manager_config.layout_method or "default"
     routing_method = pass_manager_config.routing_method or "sabre"
-    translation_method = pass_manager_config.translation_method or "translator"
+    translation_method = pass_manager_config.translation_method or "default"
     scheduling_method = pass_manager_config.scheduling_method
     optimization_method = pass_manager_config.optimization_method or "default"
     scheduling_method = pass_manager_config.scheduling_method or "default"

qiskit/transpiler/preset_passmanagers/plugin.py

@@ -71,7 +71,7 @@
 
    * - :ref:`translation <transpiler-preset-stage-translation>`
      - ``qiskit.transpiler.translation``
-     - ``translator``, ``synthesis``
+     - ``default``, ``translator``, ``synthesis``
      - Rewrite all gates outside the target ISA to use only gates within the ISA.
 
    * - :ref:`optimization <transpiler-preset-stage-optimization>`

releasenotes/notes/default-translation-stage-9d0335e354751af0.yaml

@@ -0,0 +1,18 @@
+---
+features_transpiler:
+  - |
+    A new :ref:`translation plugin stage <transpiler-preset-stage-translation>` is added, called
+    ``"default"``.  In Qiskit 2.0, this is simply an alias for the previous default ``"translator"``.
+    The underlying default algorithm may change over the course of the Qiskit 2.x series for some or
+    all targets, but you can always set ``translation_method=="translator"`` explicitly to maintain
+    the current behavior.
+upgrade_transpiler:
+  - |
+    The :ref:`translation plugin stage <transpiler-preset-stage-translation>` name ``default`` is
+    now reserved for the Qiskit built-in plugin of the same name.
+  - |
+    The default :ref:`translation plugin stage <transpiler-preset-stage-translation>` is now
+    ``"default"``.  In Qiskit 2.0, this is simply an alias for the previous default ``"translator"``.
+    The underlying default algorithm may change over the course of the Qiskit 2.x series for some or
+    all targets, but you can always set ``translation_method=="translator"`` explicitly to maintain
+    the current behavior.