Qiskit / qiskit / 21854460743

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Commit Message

Add fixedbitset clifford for simd bitwise composition (#15325)

* Add fixedbitset clifford for simd bitwise composition

This commit adds a custom clifford implementation for the litinski
transform that is simd optimized by using fixedbitset. This makes most
of the operations operating on the clifford use SIMD bitwise
instructions rather than iterating over arrays manually. This improves
the runtime performance of doing gate composition on the clifford. There
is likely a bit more overhead for iterating over the tableau but this
shouldn't be significant compared the the runtime gains constructing the
tableau.

For this commit the clifford class is duplicated because the other place
the existing struct in the quantum_info is used, clifford synthesis, is
based on the current tableau structure. This opted to copy the code
because I didn't want to change the synthesis code too. We should be
able to unify the implementations eventually, but it would require
adjusting anything directly accessing the clifford tableau which I
viewed as outside the scope of this PR.

* Unify the clifford implementations

In earlier iterations of this PR the fixedbitset based clifford
implementation was kept as a private struct inside the litinski
transform module. This was because what initial inspired the
implementation was speeding up the litinski transform. However, there is
no reason to keep it seperate and we can use the fixedbitset based
implementation for all the internal rust clifford struct usage. This
commit replaces `qiskit_quantum_info::clifford::Clifford` with the new
fixedbitset implementation and then updates all the usage of it based on
those changes. This includes moving the litinski transform
implementation to go back to using the quantum_info version which was
changed in earlier commits on this PR branch.

* Add reusable scratch space to Clifford struct

In an attempt to reduce the memory pressure caused by append operations
this commit adds a new private fie... (continued)

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