#793 support Quantum Post-Selection for processing counts

[richardyoung00]

Summary

Add support for Quantum Post-Selection for processing counts as per issue #793

Details and comments

Created a utility method get_subsystems_post_selected which returns a list of dictionaries of counts of measured bit strings for all the other registers for when the indication_register matches the flag_string bit string.

[claassistantio]

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[claassistantio]

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[woodsp-ibm]

Hi, thanks for the contribution. I have a thought....

As the new post select method code is pretty much identical to the prior get_subsystems_counts code, but for an if statement, maybe we could combine the new function into the prior method. It would help with the naming too since the new method name had to be trimmed to fit the max length and lost a little of its description in the process. How about adding two new optional fields to the existing method such that the default behavior is as before, but if set can have post selection something like

def get_subsystems_counts(complete_system_counts, post_select_index=None, select_flag=None):

then the if statement would be included and look something like this

            if post_select_index is None or subsystem_measurements[post_select_index] == select_flag:
                d_l[k] += count

So with the post_select_index default of None it works as before but with a value will the do post selection. Of course this would be explained in docstring that this got the counts with optional post selection. Users of existing get_subsytem_counts would be unaffected since the default behavior is unchanged.

You might have more meaningful names than what I chose above for the parameters, but this was the idea. Any thoughts?

[richardyoung00]

Sounds like a good suggestion. I will make the code changes.

[woodsp-ibm]

Thanks again for the submission and the suggested refactor.