Add function to load Qiskit operators

[Mandrenkov]

Context:

The next release of the PennyLane-Qiskit plugin will include PennyLaneAI/pennylane-qiskit#401 which features a new function for converting Qiskit SparsePauliOp instances into PennyLane operators.

Description of the Change:

• Added a from_qiskit_op() function to the IO module for loading Qiskit operators.

In a nutshell,

>>> import pennylane as qml
>>> from qiskit.quantum_info import SparsePauliOp
>>> qiskit_op = SparsePauliOp(["II", "XY"])
>>> qml.from_qiskit_op(qiskit_op)
I(0) + X(1) @ Y(0)

Benefits:

• Qiskit operators can be directly converted into PennyLane operators.

Possible Drawbacks:

None.

Related GitHub Issues:

None.

[codecov]

Codecov Report

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Additional details and impacted files

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[trbromley]

Thanks @Mandrenkov!

[Mandrenkov]

Thanks for the review, @trbromley!

[lillian542]

Looking good to me, and all seems to be working well in a Jupyter notebook. Just let me know when its ready for a final review!

[trbromley]

Thanks @Mandrenkov!!

[Mandrenkov]

Thanks for all the feedback, @trbromley!

[lillian542]

Looks good to me, thanks @Mandrenkov! 🚀