Qiskit / qiskit / 34096520551

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Commit Message

Fix identity one-qubit gates in the U and U3 Euler bases (#16834)

* Fix identity one-qubit gates in U/U3 Euler decomposition

At zero theta, `U` and `U3` are diagonal and only depend on `phi + lam`,
but the emitters for those two bases normalised `phi` and `lam`
separately and then tested them individually. A result such as
`U(0, -pi, -pi)` is the identity, yet both angles are non-zero, so the
gate was kept. Normalise `phi + lam` first, the way the `U321` basis
already does, before deciding whether to emit anything.

This is what left a stray one-qubit gate behind when `TwoQubitBasisDecomposer`
used the `U` or `U3` basis, e.g. decomposing CX in the CX basis.

Fixes #9315

* Update C transpiler test for the removed identity gate

test_transpile_idle_qubits transpiles a CRZ onto a CX + U target and
checks the instruction count at each optimisation level. With the
identity one-qubit gate no longer emitted, optimisation level 1 now
produces 7 instructions rather than 8, which is what levels 2 and 3
already expected.

* Address review feedback on synthesis test

Coverage Stats

14 of 14 new or added lines in 1 file covered. (100.0%)

11 existing lines in 3 files now uncovered.

116080 of 132186 relevant lines covered (87.82%)

988377.85 hits per line