Default set to turn off approximation_degree

qiskit/compiler/transpiler.py

@@ -71,7 +71,7 @@ def transpile(
     scheduling_method: Optional[str] = None,
     instruction_durations: Optional[InstructionDurationsType] = None,
     dt: Optional[float] = None,
-    approximation_degree: Optional[float] = None,
+    approximation_degree: Optional[float] = 1.0,
     timing_constraints: Optional[Dict[str, int]] = None,
     seed_transpiler: Optional[int] = None,
     optimization_level: Optional[int] = None,

releasenotes/notes/turn-off-approx-degree-df3d39eb69f7f09f.yaml

@@ -0,0 +1,16 @@
+---
+upgrade:
+  - |
+    The implicit use of ``approximation_degree!=1.0`` by default in
+    in the :func:`~.transpile` function when ``optimization_level=3`` is set has been disabled.  The transpiler should, by default,
+    preserve unitarity of the input up to known transformations such as one-sided permutations
+    and similarity transformations.  This was broken by the previous use of ``approximation_degree=None``
+    leading to incorrect results in cases such as Trotterized evolution with many time steps where
+    unitaries were being overly approximated leading to incorrect results.  It was decided that
+    transformations that break unitary equivalence should be explicitly activated by the user.
+    If you desire the previous default behavior where synthesized :class:`~UnitaryGate` instructions
+    are approximated up to the error rates of the target backend's native instructions you can explicitly
+    set ``approximation_degree=None` when calling :func:`~.transpile` with ``optimization_level=3`, for
+    example::
+
+        transpile(circuit, backend, approximation_degree=None, optimization_level=3)