Post-selection

[wakuwaku414]

Added a very basic post_selection() and functions that generate Sampler performs post-selection behind the scenes.

• post_selection()
• create_general_post_selection_sampler()
• functions that create sampler with post-selection, specialized for JW and BK mappings
  • create_jw_electron_number_post_selection_sampler()
  • create_bk_electron_number_post_selection_sampler()
• Filter functions specialized for JW and BK mappings
  • create_jw_electron_number_post_selection_filter_fn()
  • create_bk_electron_number_post_selection_filter_fn

The implementation provides necessary functionalities but I'm not sure if it follows the quri-parts's development policy. I would love to discuss with organizers what the implementation should be 😉

[kwkbtr]

Thank you for your contribution! I think implementation itself looks good.

On the other hand, we are bit worried that created_jw/bk_electron_number_post_selection_sampler() have rather limited use cases, since they cannot be applied to cases where an operation not conserving electron number is applied before measurement (e.g. an H gate to measure X).
Considering that, I may suggest exposing filter_fns in those methods as public functions and removing created_jw/bk_electron_number_post_selection_sampler(), since a user can easily create such a sampler with the filter_fn and create_general_post_selection_sampler().

In addition, those filter_fn are related to Fermion-qubit mappings, so they may be placed in quri-parts-chem.

[wakuwaku414]

Thanks!

On the other hand, we are bit worried that created_jw/bk_electron_number_post_selection_sampler() have rather limited use cases, since they cannot be applied to cases where an operation not conserving electron number is applied before measurement (e.g. an H gate to measure X).

I see. That makes sense.

Considering that, I may suggest exposing filter_fns in those methods as public functions and removing created_jw/bk_electron_number_post_selection_sampler(), since a user can easily create such a sampler with the filter_fn and create_general_post_selection_sampler().

Agree. Then I'll put the filter functions in quri-parts-chem 👍

[kwkbtr]

Sorry for a belated review.
I added a few minor comments but overall it looks good to me.
After merging this, we would like to consider unifying similar implementation in #154.

[kwkbtr]

Thank you!