Use Sabre by default for optimization levels 1 and 2

This commit updates the preset pass manager construction to use the
SabreLayout and SabreSwap passes by default for optimization level 1 and
level 2. With the recently merged Qiskit#7977 the performance of the sabre
swap pass has improved significantly enough to be considered for use by
default with optimization levels 1 and 2. While for small numbers of
target device qubits (< 30) the SabreLayout/SabreSwap pass doesn't quite
match the runtime performance of DenseLayout/StochasticSwap it typically
has better runtime performance for larger target devices. Additionally,
the runtime performance of Sabre should also improve further after Qiskit#8388
is finished. However, the output quality from the sabre passes is
typically better resulting in fewer swap gates being inserted. With the
combination of better quality and comparable runtime performance it
makes sense to use sabre as the default for optimization levels 1 and 2.
For optimization level 0 stochastic swap is still used there because we
want to continue to leverage TrivialLayout for that level and to get
the full quality advantages SabreSwap and SabreLayout should be used
together.

qiskit/transpiler/preset_passmanagers/level1.py

@@ -69,8 +69,8 @@ def level_1_pass_manager(pass_manager_config: PassManagerConfig) -> StagedPassMa
     inst_map = pass_manager_config.inst_map
     coupling_map = pass_manager_config.coupling_map
     initial_layout = pass_manager_config.initial_layout
-    layout_method = pass_manager_config.layout_method or "dense"
-    routing_method = pass_manager_config.routing_method or "stochastic"
+    layout_method = pass_manager_config.layout_method or "sabre"
+    routing_method = pass_manager_config.routing_method or "sabre"
     translation_method = pass_manager_config.translation_method or "translator"
     scheduling_method = pass_manager_config.scheduling_method
     instruction_durations = pass_manager_config.instruction_durations
@@ -154,7 +154,7 @@ def _vf2_match_not_found(property_set):
     elif routing_method == "lookahead":
         routing_pass = LookaheadSwap(coupling_map, search_depth=4, search_width=4)
     elif routing_method == "sabre":
-        routing_pass = SabreSwap(coupling_map, heuristic="lookahead", seed=seed_transpiler)
+        routing_pass = SabreSwap(coupling_map, heuristic="decay", seed=seed_transpiler)
     elif routing_method == "toqm":
         HAS_TOQM.require_now("TOQM-based routing")
         from qiskit_toqm import ToqmSwap, ToqmStrategyO1, latencies_from_target

qiskit/transpiler/preset_passmanagers/level2.py

@@ -71,8 +71,8 @@ def level_2_pass_manager(pass_manager_config: PassManagerConfig) -> StagedPassMa
     inst_map = pass_manager_config.inst_map
     coupling_map = pass_manager_config.coupling_map
     initial_layout = pass_manager_config.initial_layout
-    layout_method = pass_manager_config.layout_method or "dense"
-    routing_method = pass_manager_config.routing_method or "stochastic"
+    layout_method = pass_manager_config.layout_method or "sabre"
+    routing_method = pass_manager_config.routing_method or "sabre"
     translation_method = pass_manager_config.translation_method or "translator"
     scheduling_method = pass_manager_config.scheduling_method
     instruction_durations = pass_manager_config.instruction_durations

releasenotes/notes/sabres-for-everyone-3148ccf2064ccb0d.yaml

@@ -0,0 +1,17 @@
+---
+upgrade:
+  - |
+    The preset pass managers for levels 1 and 2, which will be used when
+    ``optimization_level=1`` or ``optimization_level=2`` with
+    :func:`~.transpile` or :func:`~.generate_preset_pass_manager` and output
+    from :func:`~.level_1_pass_manager` and :func:`~.level_2_pass_manager`,
+    will now use :class:`~.SabreLayout` and :func:`~SabreSwap` by default
+    instead of the previous defaults :class:`~.DenseLayout` and
+    :class:`~.StochasticSwap`. This change was made to improve the output
+    quality of the transpiler, the :class:`~.SabreLayout` and
+    :func:`~SabreSwap` combination typically results in fewer
+    :class:`~.SwapGate` objects being inserted into the output circuit.
+    If you would like to use the previous default passes you can set
+    ``layout_method='dense'`` and ``routing_method='stochastic'`` on
+    :func:`~.transpile` or :func:`~.generate_preset_pass_manager to
+    leverage :class:`~.DenseLayout` and :class:`~.StochasticSwap` respectively.