Add function to convert PennyLane objects to OpenFermion objects

[soranjh]

Feature details

⚠️ This issue is part of an internal assignment and not meant for external contributors.

PennyLane has several functions and classes for creating and manipulating qubit and fermionic operators. It will be useful to have functions that allow converting between PennyLane operations and OpenFermion objects. These new functions, called to_openfermion and from_openfermion , should allow users to

• Convert PennyLane Sum and LinearCombination objects to OpenFermion QubitOperator object.

>>> import pennylane as qml
>>> pl_term = 1.2 * qml.X(0) + 2.4 * qml.Z(1)
>>> of_term = to_openfermion(pl_term)
>>> of_term
1.2 [X0] + 2.4 [Z1]

• Convert OpenFermion QubitOperator object to PennyLane Sum and LinearCombination objects.

>>> import pennylane as qml
>>> of_term = QubitOperator('X0', 1.2) + QubitOperator('Z1', 2.4)
>>> pl_term = from_openfermion(of_term, wires = {0: 'a', 1: 'b'})
>>> pl_term
1.2 * X('a') + 2.4 * Z('b')

Implementation

The to_openfermion and from_openfermion functions should be placed in a new module called pennylane.qchem.convert_openfermion and be importable as

>>> import pennylane as qml
>>> qml.to_openfermion(...)
>>> qml.from_openfermion(...)

Requirements

1. The from_openfermion function should have a wires argument accepting a dictionary for custom wire mapping.
2. The from_openfermion function should have a tol argument for discarding the imaginary part of the coefficients.
3. Both functions should be properly tested.

[dariavh]

Hi @soranjh ,

Thank you for providing detailed guidelines for the assignment. I have implemented the requested features in PR #5773. Could you confirm if we only need the tol and wires arguments for the from_openfermion method, while these can also be implemented for to_openfermion?

Additionally, the codecov test keeps failing, even when the coverage of my two adjusted files qchem/convert.py and qchem/convert_openfermion.py has a 100% coverage.

[soranjh]

Hi @soranjh ,

Thank you for providing detailed guidelines for the assignment. I have implemented the requested features in PR #5773. Could you confirm if we only need the tol and wires arguments for the from_openfermion method, while these can also be implemented for to_openfermion?

Additionally, the codecov test keeps failing, even when the coverage of my two adjusted files qchem/convert.py and qchem/convert_openfermion.py has a 100% coverage.

Thanks @dariavh. You can also add tol and wires arguments for to_openfermion. We will explore the codecov test.