Source code for pennylane.qchem.convert_openfermion

# Copyright 2018-2024 Xanadu Quantum Technologies Inc.

# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at

#     http://www.apache.org/licenses/LICENSE-2.0

# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
"""
This module contains the functions for converting between OpenFermion and PennyLane objects.
"""

from functools import singledispatch
from typing import Union

# pylint: disable= import-outside-toplevel,no-member,unused-import
import pennylane as qml
from pennylane import numpy as np
from pennylane.fermi.fermionic import FermiSentence, FermiWord
from pennylane.ops import LinearCombination, Sum
from pennylane.qchem.convert import _openfermion_to_pennylane, _pennylane_to_openfermion


def _import_of():
    """Import openfermion."""
    try:
        # pylint: disable=import-outside-toplevel, unused-import, multiple-imports
        import openfermion
    except ImportError as Error:
        raise ImportError(
            "This feature requires openfermion. "
            "It can be installed with: pip install openfermion."
        ) from Error

    return openfermion


def from_openfermion(openfermion_op, wires=None, tol=1e-16):
    r"""Convert OpenFermion
    `FermionOperator <https://quantumai.google/reference/python/openfermion/ops/FermionOperator>`__
    to PennyLane :class:`~.fermi.FermiWord` or :class:`~.fermi.FermiSentence` and
    OpenFermion `QubitOperator <https://quantumai.google/reference/python/openfermion/ops/QubitOperator>`__
    to PennyLane :class:`~.LinearCombination`.

    Args:
        openfermion_op (FermionOperator, QubitOperator): OpenFermion operator.
        wires (dict): Custom wire mapping used to convert the external qubit
            operator to a PennyLane operator.
            Only dictionaries with integer keys (for qubit-to-wire conversion) are accepted.
            If ``None``, the identity map (e.g., ``0->0, 1->1, ...``) will be used.
        tol (float): Tolerance for discarding negligible coefficients.

    Returns:
        Union[FermiWord, FermiSentence, LinearCombination]: PennyLane operator.

    **Example**

    >>> import pennylane as qml
    >>> from openfermion import FermionOperator, QubitOperator
    >>> of_op = 0.5 * FermionOperator('0^ 2') + FermionOperator('0 2^')
    >>> pl_op = qml.from_openfermion(of_op)
    >>> print(pl_op)
    0.5 * a⁺(0) a(2)
    + 1.0 * a(0) a⁺(2)

    >>> of_op = QubitOperator('X0', 1.2) + QubitOperator('Z1', 2.4)
    >>> pl_op = qml.from_openfermion(of_op)
    >>> print(pl_op)
    1.2 * X(0) + 2.4 * Z(1)
    """
    openfermion = _import_of()

    if isinstance(openfermion_op, openfermion.FermionOperator):

        if wires:
            raise ValueError("Custom wire mapping is not supported for fermionic operators.")

        typemap = {0: "-", 1: "+"}

        fermi_words = []
        fermi_coeffs = []

        for ops, val in openfermion_op.terms.items():
            fw_dict = {(i, op[0]): typemap[op[1]] for i, op in enumerate(ops)}
            fermi_words.append(FermiWord(fw_dict))
            fermi_coeffs.append(val)

        if len(fermi_words) == 1 and fermi_coeffs[0] == 1.0:
            return fermi_words[0]

        pl_op = FermiSentence(dict(zip(fermi_words, fermi_coeffs)))
        pl_op.simplify(tol=tol)

        return pl_op

    coeffs, pl_ops = _openfermion_to_pennylane(openfermion_op, wires=wires, tol=tol)

    pennylane_op = qml.ops.LinearCombination(coeffs, pl_ops)

    return pennylane_op


def to_openfermion(
    pennylane_op: Union[Sum, LinearCombination, FermiWord, FermiSentence], wires=None, tol=1.0e-16
):
    r"""Convert a PennyLane operator to OpenFermion
    `QubitOperator <https://quantumai.google/reference/python/openfermion/ops/QubitOperator>`__ or
    `FermionOperator <https://quantumai.google/reference/python/openfermion/ops/FermionOperator>`__.

    Args:
        pennylane_op (~ops.op_math.Sum, ~ops.op_math.LinearCombination, FermiWord, FermiSentence):
            PennyLane operator
        wires (dict): Custom wire mapping used to convert a PennyLane qubit operator
            to the external operator.
            Only dictionaries with integer keys (for qubit-to-wire conversion) are accepted.
            If ``None``, the identity map (e.g., ``0->0, 1->1, ...``) will be used.

    Returns:
        (QubitOperator, FermionOperator): OpenFermion operator

    **Example**

    >>> import pennylane as qml
    >>> w1 = qml.fermi.FermiWord({(0, 0) : '+', (1, 1) : '-'})
    >>> w2 = qml.fermi.FermiWord({(0, 1) : '+', (1, 2) : '-'})
    >>> fermi_s = qml.fermi.FermiSentence({w1 : 1.2, w2: 3.1})
    >>> of_fermi_op = qml.to_openfermion(fermi_s)
    >>> of_fermi_op
    1.2 [0^ 1] +
    3.1 [1^ 2]

    >>> sum_op = 1.2 * qml.X(0) + 2.4 * qml.Z(1)
    >>> of_qubit_op = qml.to_openfermion(sum_op)
    >>> of_qubit_op
    (1.2+0j) [X0] +
    (2.4+0j) [Z1]
    """

    return _to_openfermion_dispatch(pennylane_op, wires=wires, tol=tol)


@singledispatch
def _to_openfermion_dispatch(pl_op, wires=None, tol=1.0e-16):
    """Dispatches to appropriate function if pl_op is a ``Sum``, ``LinearCombination, ``FermiWord`` or ``FermiSentence``."""
    raise ValueError(
        f"pl_op must be a Sum, LinearCombination, FermiWord or FermiSentence, got: {type(pl_op)}."
    )


@_to_openfermion_dispatch.register
def _(pl_op: Sum, wires=None, tol=1.0e-16):
    coeffs, ops = pl_op.terms()
    return _pennylane_to_openfermion(np.array(coeffs), ops, wires=wires, tol=tol)


# pylint: disable=unused-argument, protected-access
@_to_openfermion_dispatch.register
def _(ops: FermiWord, wires=None, tol=1.0e-16):
    openfermion = _import_of()

    if wires:
        raise ValueError("Custom wire mapping is not supported for fermionic operators.")

    return openfermion.ops.FermionOperator(qml.fermi.fermionic._to_string(ops, of=True))


@_to_openfermion_dispatch.register
def _(pl_op: FermiSentence, wires=None, tol=1.0e-16):
    openfermion = _import_of()

    if wires:
        raise ValueError("Custom wire mapping is not supported for fermionic operators.")

    fermion_op = openfermion.ops.FermionOperator()
    for fermi_word in pl_op:
        if np.abs(pl_op[fermi_word].imag) < tol:
            fermion_op += pl_op[fermi_word].real * to_openfermion(fermi_word)
        else:
            fermion_op += pl_op[fermi_word] * to_openfermion(fermi_word)

    return fermion_op

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