Qiskit / qiskit / 23185212546

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github

Committed by web-flow

Commit Message

Fix approximate-by-default behaviour of `UnitarySynthesis` (#15807)

* Fix approximate-by-default behaviour of `UnitarySynthesis`

In gh-15492[^1], we accidentally swapped the behaviour of
`approximation_degree` such that `None` was treated as "exact" and `1.0`
as "up to gate error".  Despite all _other_ explicit float values
meaning up-to-gate-fidelity multiplied by the value,
`approximation_degree=1.0` has historically _actually_ meant "exact
synthesis", and `None` is "up to gate error" instead.

While the oversight is easy to correct (and now encapsulated on entry to
`UnitarySynthesis` rather than dealing with two different systems), the
more worrying aspect was that our test suite did not catch the swap; we
came unfortunately close to releasing Qiskit 2.4 with approximate
synthesis turned on by default.

[^1]: 0b8bceb0: Rewrite default `UnitarySynthesis` to cache decomposers

* Improve documentation comments

* Fix typo

Run Details

59 of 67 new or added lines in 5 files covered. (88.06%)

23 existing lines in 4 files now uncovered.

101145 of 115574 relevant lines covered (87.52%)

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