Qiskit / qiskit / 14663307542

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github

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Commit Message

Improve 2q block collection via 1q quaternion-based collection (#13649)

* Add versor-based representation of 1q gates

1q gates are members of the group U(2), which we can represent as a
scalar phase term and a member of SU(2).  The members of SU(2) can be
represented by versors (also called unit quaternions, but I got tired of
typing that all the time...).

This adds a representation of versors and the group action to the Rust
code, and ways to convert from matrix-based forms to the them.

This commit introduces `nalgebra` as a dependency, to use its
quaternion logic.  This is a relatively heavy dependency, especially for
something as simple as quaternions, but some of this is in anticipation
of moving more matrix code to the static matrices of `nalgebra`, rather
than the too-dynamic-for-our-needs ones of `ndarray`; `faer` also offers
static matrices, but its APIs continue to heavily fluctuate between
versions, and it requires ever higher MSRVs.

* Use quaternions in 1q block collection

Switch the inner algorithm of `ConsolidateBlocks` to use the quaternion
form for single-qubit matrix multiplications.  This offered a few
percentage-points speedup for the block collection for typical
`rz-sx-rz-sx-rz`-type runs in quantum-volume-like collections.

* Avoid unnecessary allocations in qargs lookup

Switch the lookup logic for the qargs in the block collector to
determine the qargs ordering without additional heap allocations.  This
offers another modest (~4%) improvement in collection performance for
large runs.

* Avoid allocations in simple matrix operations

Producing the Kronecker product of the two single-qubit matrices from
the versor representation is trivially calculable, and can be written
into an existing allocation.  Similarly, switching the qubit order of a
2q matrix involves only six swaps, and does not need to allocate a new
matrix if one is already available.

There are lots of places remaining in this code where more matrix
allocations ... (continued)

Run Details

261 of 315 new or added lines in 3 files covered. (82.86%)

10 existing lines in 4 files now uncovered.

74356 of 84577 relevant lines covered (87.92%)

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