qiskit/releasenotes/notes/0.19/add-gate-error-objective-00...

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---
features:
- |
The values ``"gate_error"`` and ``"balanced"`` are now available for the
``objective`` option in the construction of the
:class:`~qiskit.transpiler.passes.BIPMapping` object, and ``"balanced"`` is
now the default.
The ``"gate_error"`` objective requires passing a
:obj:`.BackendProperties` instance in the ``backend_prop``
kwarg, which contains the 2q-gate gate errors used in the computation of the
objectives. The ``"balanced"`` objective will use the
:obj:`.BackendProperties` instance if it is given, but otherwise will assume
a CX error rate as given in the new parameter ``default_cx_error_rate``.
The relative weights of the gate-error and depth components of the balanced
objective can be controlled with the new ``depth_obj_weight`` parameter.
upgrade:
- |
The default method for :obj:`.BIPMapping` is now ``balanced`` rather than
``depth``. This new objective generally achieves a better result, as it
factors in both the circuit depth and the gate error.