Example stacking DDD and ZNE

[jordandsullivan]

Description

Tutorial to combine ZNE and DDD.

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License

• I license this contribution under the terms of the GNU GPL, version 3 and grant Unitary Fund the right to provide additional permissions as described in section 7 of the GNU GPL, version 3.

Before opening the PR, please ensure you have completed the following where appropriate.

• I added unit tests for new code.
• I used type hints in function signatures.
• I used Google-style docstrings for functions.
• I updated the documentation where relevant.
• Added myself / the copyright holder to the AUTHORS file

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.22%. Comparing base (2ff8134) to head (b70e53f).
Report is 4 commits behind head on main.

Additional details and impacted files

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[nathanshammah]

Thanks @jordandsullivan. To solve the issue that the notebook doesn't render in the documentation, you just need to add it to the TOC tree in the examples file. Eventually we use md files, but as a temporary file, it should work fine also with .ipynb format. The jupytext extension allows you to edit the md file as a notebook.

[nathanshammah]

My suggestion is that the executor should be only called by ZNE. The DD part is a "pre-processing", with input the initial circuit (e.g., GHZ state) and output the DD compiled quantum circuit. This is fed to ZNE. This should remove incompatibility types.
ddd.insert_ddd_sequences should be used. See the documentation here and here.

[jordandsullivan]

My suggestion is that the executor should be only called by ZNE. The DD part is a "pre-processing", with input the initial circuit (e.g., GHZ state) and output the DD compiled quantum circuit. This is fed to ZNE. This should remove incompatibility types. ddd.insert_ddd_sequences should be used. See the documentation here and here.

Thanks for the tip @nathanshammah , that did it. Now the question is mainly how to find a combination of error levels and circuit depth for which using DDD and ZNE together is in fact useful. Unfortunately, DDD seems to do best when you insert a lot of idle time in the circuit (which I modeled by inserting identity gates), while ZNE does significantly worse. ZNE itself shines in lower depth circuits.

Perhaps it would be useful to do a characterization sweep of the range of noise levels or circuit depths at which each technique was useful?

[cosenal]

Left some preliminary comments, but I haven't run the notebook yet.

[jordandsullivan]

I'm not sure why the output does not seem to be getting included in the docs build :/ https://mitiq--2345.org.readthedocs.build/en/2345/examples/combine_ddd_zne.html

[cosenal]

@jordandsullivan hm? I can see the output in the doc build.

[jordandsullivan]

@cosenal Ah I just needed to do a hard reload (CMD+SHIFT+R) in my browser. It's showing up for me now :)

[cosenal]

The one at the bottom is the most important comment.

[cosenal]

LGTM!