#16974 fix TwoQubitBasisDecomposer with approximation_degree=1.0 - #16975
#16974 fix TwoQubitBasisDecomposer with approximation_degree=1.0#16975Gavin King (gavinking) wants to merge 1 commit into
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TwoQubitBasisDecomposer was silently approximating small controlled rotations at optimization levels 2 and 3 with a hardcoded fidelity threshold of 1 - 1e-9, despite approximation_degree=1.0. The fix disables approximate Weyl specialization of the target when `TwoQubitBasisDecomposer` is called with `approximate=False`. Approximate mode retains its previous behavior.
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Jake Lishman (jakelishman)
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Jake Lishman (jakelishman)
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This is proposing to change the default behaviour of one of the most central passes of Qiskit synthesis - it may have potentially significant knock-on effects on how the complete compiler pipeline works. The approximation_degree docs here do say "no approximation", but similar documentation elsewhere says "up to numerical precision" (which is ambiguous - decompositions are complex and the absolute precision we can guarantee isn't reliable), and this default fidelity has been set like this for a long time.
We need to discuss whether the fault is in documentation, expectations, and whether this requires knock-on changes to other parts of the stack. Let me remand that to the ongoing discussion in #16864.
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TwoQubitBasisDecomposerwas silently approximating small controlled rotations at optimization levels 2 and 3 with a hardcoded fidelity threshold of 1 - 1e-9, despiteapproximation_degree=1.0.The fix disables approximate Weyl specialization of the target when
TwoQubitBasisDecomposeris called withapproximate=False. Approximate mode retains its previous behavior.Fixes #16974.
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