Add a diff tool for comparing multiple `Cirq.Circuit`s in Jupyter Notebooks

[freyam]

Is your feature request related to a use case or problem? Please describe.

Tanuj has an amazing idea to visualize the differences between circuits.

Logger Design

• register_initial and register_final methods will be used to maintain the state of the circuits and compute the difference between initial and final circuits.
• The difference between initial and final circuits will be stored for each call. Also, common stats like circuit depth, number of one / two qubit gates, circuit duration etc. will be computed and stored for each time step.
• Finally, a plot method will bring up an interactive widget to plot the state of the initial and final circuits highlighting the difference for each time step of the optimization.

Source: RFC: Circuit Transformers API #Logger Design

We could adopt this same technique to also implement a new module that shows the difference between N given circuits (N >= 2) in a clean SVG format or a GIF transition. We could adopt to use 3rd party open-source tools for this or a simple SVG diff-tool also might suffice.

Describe the solution you'd like

Task 0

• Merge Follow up work on Circuit._repr_svg_ #4499 which enables us to visualize cirq.Circuit in SVG format.

Task 1

• Create a diff tool that takes 2 cirq.Circuits as input and prints the differences between them in a nice way (will share demos soon.)

Task 2

• Upgrade the tool made in Task 1 that takes N cirq.Circuits as input and prints the differences in a neat tabular form or a slider form.

Task 3

• Convert the tool into an IPython widget which can be directly installed in Jupyter Notebooks.

Completing all the 3 tasks would be really helpful to all the users as it would allow an easy way to work around optimizers (and transformers) that modify the circuits. Now, users would be able to track the optimization (or transformation).

What is the urgency from your perspective for this issue? Is it blocking important work?

P2 - we should do it in the next couple of quarters

cc @tanujkhattar

[tanujkhattar]

Thank you @freyam for opening this.

For reference, The Transformer API RFC is still under review but I don't expect the logging feature request to change.

However, There are some design questions that would need to be addressed, for example -- I'm not sure if the Task 2 mentioned above is necessary. Once we have a tool which can plot the diff of two circuits, we could simply use it from the transformer logger to plot all the N steps as needed.

I would suggest to use this issue to track one particular change, i.e. Task 1 mentioned above. It's a clear independent function / tool which can then be used in the larger context of improving visualizations for transformers.

Once the transformer design is finalized, we can create other issues to track the relevant features (like Task 2 and 3 mentioned above)

[freyam]

I would suggest to use this issue to track one particular change, i.e. Task 1 mentioned above. It's a clear independent function / tool which can then be used in the larger context of improving visualizations for transformers.

Once the transformer design is finalized, we can create other issues to track the relevant features (like Task 2 and 3 mentioned above)

Works ✔️