Add single-qubit wire cutting how-to and expand_observables function

[garrison]

I put the how-to and the new expand_observables function together in the same PR because the how-to requires the function. I think it's a decent name, but I'd be open to something better. (We can always choose a new name later, too.) This is the PR promised in #367.

[garrison]

I actually quite like the name expand_observables now that it is used alongside cut_wires (#370).

[caleb-johnson]

Going to take a look at notebooks, but a few small comments here. Looking great

[caleb-johnson]

So the original and final circuits need to be comprised of the same Qubit instances, is that right?

If that's the case, we could add a sentence here telling users explicitly to build their two circuit using the same qubit instances. Or maybe this sentence is clear enough? What do you think?

[garrison]

The change in a5ebfb8 is subtle, but I think it helps to clarify this. Since this function is meant to be used with cut_wires and cut_wires already maintains this condition, I don't think we need to belabor it IMO.

[caleb-johnson]

Notes on the how-to:

• Link to arXiv, rather than just text
• If we're going to finish out the workflow, we should move it to tutorials. Otherwise, I think you can cut it off after cell 7

[caleb-johnson]

Notes on the how-to:

• Link to arXiv, rather than just text
• If we're going to finish out the workflow, we should move it to tutorials. Otherwise, I think you can cut it off after cell 7

I know there is value in showing the rest of the workflow is the same. I'll think more about how to make this more "how-to-y" and less "tutorial-y"

[garrison]

I know there is value in showing the rest of the workflow is the same. I'll think more about how to make this more "how-to-y" and less "tutorial-y"

Yeah, the compromise I made was just to put all the "regular" steps in a single section at the end.

[coveralls]

Pull Request Test Coverage Report for Build 5895817405

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.06%) to 91.226%

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Totals
Change from base Build 5885129429:	0.06%
Covered Lines:	2724
Relevant Lines:	2986

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💛 - Coveralls

[garrison]

• Link to arXiv, rather than just text

thanks, not sure how I missed this (copy and paste fail)

[garrison]

I think I've addressed everything that is immediately actionable here. I'm going to merge it, and we can continue the discussion on anything I missed at #370, since it will mean taking a second pass over the how-to. If there are minor tweaks to expand_observables, too, we can add them there as well.