qiskit/releasenotes/notes/0.23/fix-identity-simplification...

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---
fixes:
- |
Fixed an issue with the :func:`~.transpile` function when run with
``optimization_level`` set to ``1``, ``2``, or ``3`` and no
``backend``, ``basis_gates``, or ``target`` argument specified. If
the input circuit had runs of single qubit gates which could be simplified
the output circuit would not be as optimized as possible as those runs
of single qubit gates would not have been removed. This could have been
corrected previously by specifying either the ``backend``, ``basis_gates``,
or ``target`` arguments on the :func:`~.transpile` call, but now the output
will be as simplified as it can be without knowing the target gates allowed.
Fixed `#9217 <https://github.com/Qiskit/qiskit-terra/issues/9217>`__
- |
Fixed an issue with the :func:`~.transpile` function when run with
``optimization_level=3`` and no ``backend``, ``basis_gates``, or ``target``
argument specified. If the input circuit contained any 2 qubit blocks which
were equivalent to an identity matrix the output circuit would not be as
optimized as possible and and would still contain that identity block.
This could have been corrected previously by specifying either the
``backend``, ``basis_gates``, or ``target`` arguments on the
:func:`~.transpile` call, but now the output will be as simplified as it
can be without knowing the target gates allowed.
Fixed `#9217 <https://github.com/Qiskit/qiskit-terra/issues/9217>`__