qiskit/opflow/evolutions/matrix_evolution.py

# This code is part of Qiskit.
#
# (C) Copyright IBM 2020, 2023.
#
# This code is licensed under the Apache License, Version 2.0. You may
# obtain a copy of this license in the LICENSE.txt file in the root directory
# of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
#
# Any modifications or derivative works of this code must retain this
# copyright notice, and modified files need to carry a notice indicating
# that they have been altered from the originals.

"""MatrixEvolution Class"""

import logging

from qiskit.opflow.evolutions.evolution_base import EvolutionBase
from qiskit.opflow.evolutions.evolved_op import EvolvedOp
from qiskit.opflow.list_ops.list_op import ListOp
from qiskit.opflow.operator_base import OperatorBase
from qiskit.opflow.primitive_ops.matrix_op import MatrixOp
from qiskit.opflow.primitive_ops.pauli_op import PauliOp
from qiskit.utils.deprecation import deprecate_func

logger = logging.getLogger(__name__)


class MatrixEvolution(EvolutionBase):
    r"""
    Deprecated: Performs Evolution by classical matrix exponentiation, constructing a circuit with
    ``UnitaryGates`` or ``HamiltonianGates`` containing the exponentiation of the Operator.
    """

    @deprecate_func(
        since="0.24.0",
        additional_msg="For code migration guidelines, visit https://qisk.it/opflow_migration.",
    )
    def __init__(self) -> None:
        super().__init__()

    def convert(self, operator: OperatorBase) -> OperatorBase:
        r"""
        Traverse the operator, replacing ``EvolvedOps`` with ``CircuitOps`` containing
        ``UnitaryGates`` or ``HamiltonianGates`` (if self.coeff is a ``ParameterExpression``)
        equalling the exponentiation of -i * operator. This is done by converting the
        ``EvolvedOp.primitive`` to a ``MatrixOp`` and simply calling ``.exp_i()`` on that.

        Args:
            operator: The Operator to convert.

        Returns:
            The converted operator.
        """
        if isinstance(operator, EvolvedOp):
            if not {"Matrix"} == operator.primitive_strings():
                logger.warning(
                    "Evolved Hamiltonian is not composed of only MatrixOps, converting "
                    "to Matrix representation, which can be expensive."
                )
                # Setting massive=False because this conversion is implicit. User can perform this
                # action on the Hamiltonian with massive=True explicitly if they so choose.
                # TODO explore performance to see whether we should avoid doing this repeatedly
                matrix_ham = operator.primitive.to_matrix_op(massive=False)
                operator = EvolvedOp(matrix_ham, coeff=operator.coeff)

            if isinstance(operator.primitive, ListOp):
                return operator.primitive.exp_i() * operator.coeff
            elif isinstance(operator.primitive, (MatrixOp, PauliOp)):
                return operator.primitive.exp_i()
        elif isinstance(operator, ListOp):
            return operator.traverse(self.convert).reduce()

        return operator