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Código fuente para qiskit.opflow.expectations.expectation_factory

# This code is part of Qiskit.
#
# (C) Copyright IBM 2020.
#
# 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.

""" ExpectationFactory Class """

import logging
from typing import Optional, Union

from qiskit import BasicAer
from qiskit.opflow.expectations.aer_pauli_expectation import AerPauliExpectation
from qiskit.opflow.expectations.expectation_base import ExpectationBase
from qiskit.opflow.expectations.matrix_expectation import MatrixExpectation
from qiskit.opflow.expectations.pauli_expectation import PauliExpectation
from qiskit.opflow.operator_base import OperatorBase
from qiskit.providers import Backend
from qiskit.utils.backend_utils import has_aer, is_aer_qasm, is_statevector_backend
from qiskit.utils.quantum_instance import QuantumInstance

logger = logging.getLogger(__name__)


[documentos]class ExpectationFactory: """A factory class for convenient automatic selection of an Expectation based on the Operator to be converted and backend used to sample the expectation value. """
[documentos] @staticmethod def build( operator: OperatorBase, backend: Optional[Union[Backend, QuantumInstance]] = None, include_custom: bool = True, ) -> ExpectationBase: """ A factory method for convenient automatic selection of an Expectation based on the Operator to be converted and backend used to sample the expectation value. Args: operator: The Operator whose expectation value will be taken. backend: The backend which will be used to sample the expectation value. include_custom: Whether the factory will include the (Aer) specific custom expectations if their behavior against the backend might not be as expected. For instance when using Aer qasm_simulator with paulis the Aer snapshot can be used but the outcome lacks shot noise and hence does not intuitively behave overall as people might expect when choosing a qasm_simulator. It is however fast as long as the more state vector like behavior is acceptable. Returns: The expectation algorithm which best fits the Operator and backend. Raises: ValueError: If operator is not of a composition for which we know the best Expectation method. """ backend_to_check = backend.backend if isinstance(backend, QuantumInstance) else backend # pylint: disable=cyclic-import primitives = operator.primitive_strings() if primitives in ({"Pauli"}, {"SparsePauliOp"}): if backend_to_check is None: # If user has Aer but didn't specify a backend, use the Aer fast expectation if has_aer(): from qiskit import Aer backend_to_check = Aer.get_backend("qasm_simulator") # If user doesn't have Aer, use statevector_simulator # for < 16 qubits, and qasm with warning for more. else: if operator.num_qubits <= 16: backend_to_check = BasicAer.get_backend("statevector_simulator") else: logger.warning( "%d qubits is a very large expectation value. " "Consider installing Aer to use " "Aer's fast expectation, which will perform better here. We'll use " "the BasicAer qasm backend for this expectation to avoid having to " "construct the %dx%d operator matrix.", operator.num_qubits, 2**operator.num_qubits, 2**operator.num_qubits, ) backend_to_check = BasicAer.get_backend("qasm_simulator") # If the user specified Aer qasm backend and is using a # Pauli operator, use the Aer fast expectation if we are including such # custom behaviors. if is_aer_qasm(backend_to_check) and include_custom: return AerPauliExpectation() # If the user specified a statevector backend (either Aer or BasicAer), # use a converter to produce a # Matrix operator and compute using matmul elif is_statevector_backend(backend_to_check): if operator.num_qubits >= 16: logger.warning( "Note: Using a statevector_simulator with %d qubits can be very expensive. " "Consider using the Aer qasm_simulator instead to take advantage of Aer's " "built-in fast Pauli Expectation", operator.num_qubits, ) return MatrixExpectation() # All other backends, including IBMQ, BasicAer QASM, go here. else: return PauliExpectation() elif primitives == {"Matrix"}: return MatrixExpectation() else: raise ValueError("Expectations of Mixed Operators not yet supported.")