Add "Save and retrieve Qiskit objects" page

[frankharkins]

Continuation of #956

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[qiskit-bot]

One or more of the the following people are requested to review this:

• @beckykd
• @jyu00

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[abbycross]

Reads very well!! Couple of tweaks in my suggestions, but approving overall.

[jyu00]

It may be worth noting that QPY only provides backward compatibility but not forward. So it may not work if you are trying to use Qiskit version N-1 to load a payload dumped from version N.

[frankharkins]

@jyu00 does this sentence describe it correctly?

QPY files save the full Qiskit circuit object and are forward-compatible, meaning they'll be compatible with newer versions of Qiskit.

[jyu00]

@jyu00 does this sentence describe it correctly?

QPY files save the full Qiskit circuit object and are forward-compatible, meaning they'll be compatible with newer versions of Qiskit.

Yes, but the goal here is to warn users they cannot load a QPY file using a down level Qiskit. So I think the sentence would better worded as QPY being backward compatible but not forward compatible (since it cannot predict the future).

Also Qiskit 1.0 added versions to QPY. So one can look up the version of a QPY file and compare that to the QPY versions support by their Qiskit. @mtreinish can give you more details if needed.

[frankharkins]

I think the confusion comes from the file type vs. the QPY program. Does this remove the ambiguity?

QPY files store the full Qiskit circuit object and will be compatible with newer versions of Qiskit (although not necessarily with older versions of Qiskit).

It removes mention of forward/backwards compatibility and makes it clearer that older versions of Qiskit might not understand the file.

EDIT: Approved by Jessie via Slack