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Improve performance and randomness of QuantumVolume
#12097
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Improve performance and randomness of `QuantumVolume`
jakelishman f235854
Make test explicitly test reproducibility
jakelishman f269237
Protect best-effort seed retrieval against old Numpy
jakelishman b7240cf
Merge remote-tracking branch 'ibm/main' into fast-qv
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Original file line number | Diff line number | Diff line change |
---|---|---|
@@ -0,0 +1,27 @@ | ||
--- | ||
features_circuits: | ||
- | | ||
Construction time for :class:`.QuantumVolume` circuits has been significantly improved, on the | ||
order of 10x or a bit more. The internal SU4 gates will now also use more bits of randomness | ||
during their generation, leading to more representative volume circuits, especially at large | ||
widths and depths. | ||
- | | ||
:class:`.QuantumVolume` now has a ``flatten`` keyword argument. This defaults to ``False``, | ||
where the constructed circuit contains a single instruction that in turn contains the actual | ||
volume structure. If set ``True``, the circuit will directly have the volumetric SU4 matrices. | ||
- | | ||
:class:`.UnitaryGate` now accepts an optional ``num_qubits`` argument. The only effect of this | ||
is to skip the inference of the qubit count, which can be helpful for performance when many | ||
gates are being constructed. | ||
upgrade_circuits: | ||
- | | ||
The random-number usage of :class:`.QuantumVolume` has changed, so you will get a different | ||
circuit for a fixed seed between older versions of Qiskit and this version. The random-unitary | ||
generation now uses more bits of entropy, so large circuits will be less biased. | ||
- | | ||
The internal :class:`.UnitaryGate` instances in the definition of a :class:`.QuantumVolume` | ||
circuit will no longer have a :attr:`~.Instruction.label` field set. Previously this was set | ||
to the string ``su4_<seed>`` where ``<seed>`` was a three-digit number denoting the seed of an | ||
internal Numpy pRNG instance for that gate. Doing this was a serious performance problem, and | ||
the seed ought not to have been useful; if you need to retrieve the matrix from the gate, simply | ||
use the :meth:`.Gate.to_matrix` method. |
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This
num_qubits
optimization seems a bit unecessary (30ns vs 120ns on my machine, but fine).There was a problem hiding this comment.
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That's fair - I did this when it was still
np.log2
and it was a huge overhead, and I didn't really think to switch it from the Numpy overhead at the time.