Correcting documentation of Drag/LiftedDrag (Qiskit#7659)

[tobias-kehrer]

Summary

This PR partially fixes #7659.
As discussed in #7659, the documentation of the Drag/LiftedDrag pulse was not consistent with its implementation in qiskit.pulse.library.continuous.py.

A remaining problem is the explicit definition of the sampler strategy. This should be fixed after/with the update of parametric pulses (#7821).

Details and comments

This PR corrects the definition of f(x) (called Drag/LiftedDrag, https://qiskit.org/documentation/stubs/qiskit.pulse.library.Drag.html) up to explicitly defining the chosen sampler strategy.

Before the PR, the function f(x) was wrongly depending on (f'(x) - f'(-1))/(1 - f'(-1)). This dependency is corrected to (g(x) - g(-1))/(1 - g(-1)). Furthermore, the imaginary part of the f(x) is now correctly defined to be proportional to (g(x) - g(-1))/(1 - g(-1)).

The documentation of the Drag/LiftedDrag pulse now follows its implementation in qiskit.pulse.library.continuous.py.

As @nkanazawa1989 pointed out in #7659, an update of parametric pulses (#7821) will unify the sampler strategies on backend and frontend. After or with this update, the documentation can be adjusted (if needed) stating explicitly which sampler strategy is or can be used.

[CLAassistant]

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[nkanazawa1989]

Seems like this is correct description of the function. However, since f'(x) is no longer used, we can remove another lines as well or introduce something like g'(x) to get more compact expression?

[tobias-kehrer]

I implemented the following changes:

• Removal of f'(x)
• Added definition of g'(x)
• Typos corrected 'gaussian' -> 'Gaussian'
• Link to qiskit.pulse.library.Gaussian added

[javabster]

Hi @tobias-kehrer thanks for your work! Looks like you just have some minor linting errors to fix up in the following files:

qiskit/pulse/library/parametric_pulses.py:353:0: C0301: Line too long (127/105) (line-too-long)
qiskit/pulse/library/parametric_pulses.py:356:0: C0301: Line too long (129/105) (line-too-long)
qiskit/pulse/library/parametric_pulses.py:358:0: C0301: Line too long (114/105) (line-too-long)

[tobias-kehrer]

Thank you @javabster for pointing out these errors!
There is a remaining error

qiskit/pulse/library/parametric_pulses.py:357:31: C0303: Trailing whitespace (trailing-whitespace)

probably due to the 'long' link in line 358. But this link should not be split in half, right?

[tobias-kehrer]

Thank you @javabster for pointing out these errors!
There is a remaining error

qiskit/pulse/library/parametric_pulses.py:357:31: C0303: Trailing whitespace (trailing-whitespace)

probably due to the 'long' link in line 358. But this link should not be split in half, right?

[nkanazawa1989]

Thanks. This looks good to me.

[coveralls]

Pull Request Test Coverage Report for Build 2152553017

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.005%) to 83.945%

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Totals
Change from base Build 2151886427:	0.005%
Covered Lines:	54198
Relevant Lines:	64564

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💛 - Coveralls