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Fix determinism in ConsolidateBlocks #16237

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Fix determinism in ConsolidateBlocks #16237

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18 changes: 10 additions & 8 deletions qiskit/transpiler/passes/optimization/consolidate_blocks.py
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Original file line number Diff line number Diff line change
Expand Up @@ -117,19 +117,21 @@ def __init__(
self.decomposer = TwoQubitBasisDecomposer(kak_basis_gate)
self.basis_gate_name = kak_basis_gate.name
elif basis_gates is not None:
kak_gates = KAK_GATE_NAMES.keys() & (basis_gates or [])
kak_param_gates = KAK_GATE_PARAM_NAMES.keys() & (basis_gates or [])
if kak_param_gates:
kak_gate = next((gate for gate in KAK_GATE_NAMES if gate in basis_gates), None)
kak_param_gate = next(
(gate for gate in KAK_GATE_PARAM_NAMES if gate in basis_gates), None
)
Comment on lines -120 to +123
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@jakelishman jakelishman May 22, 2026

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Incredibly minor, but since KAK_GATE_NAMES and KAK_GATE_PARAM_NAMES are reliably dict, but basis_gates is a list, you might want to drive the iteration from basis_gates and lookup into KAK_GATE_NAMES instead, especially if you're in the habit of passing huge basis_gates lists.

if kak_param_gate is not None:
self.decomposer = TwoQubitControlledUDecomposer(
KAK_GATE_PARAM_NAMES[next(iter(kak_param_gates))]
KAK_GATE_PARAM_NAMES[kak_param_gate]
)
self.basis_gate_name = next(iter(kak_param_gates))
elif kak_gates:
self.basis_gate_name = kak_param_gate
elif kak_gate is not None:
self.decomposer = TwoQubitBasisDecomposer(
KAK_GATE_NAMES[next(iter(kak_gates))],
KAK_GATE_NAMES[kak_gate],
basis_fidelity=approximation_degree or 1.0,
)
self.basis_gate_name = next(iter(kak_gates))
self.basis_gate_name = kak_gate
else:
self.decomposer = None
self.basis_gate_name = "cx"
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5 changes: 5 additions & 0 deletions releasenotes/notes/fix-determinism-in-consolidate-blocks-6c96b3c19f474150.yaml
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Original file line number Diff line number Diff line change
@@ -0,0 +1,5 @@
---
fixes:
- |
Fixed a problem in :class:`.ConsolidateBlocks` where basis gate selection was not always
deterministic, which could lead to different transpiled circuits across runs.
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@jakelishman jakelishman May 22, 2026

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Can you add a quick mention of the configuration needed to make it non-deterministic too? I think most "common" use of the pass was always deterministic.

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