# Source code for qiskit.aqua.operators.expectations.aer_pauli_expectation

```
# This code is part of Qiskit.
#
# (C) Copyright IBM 2020, 2021.
#
# 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.
""" AerPauliExpectation Class """
import logging
from typing import Union
from qiskit.aqua import MissingOptionalLibraryError
from ..operator_base import OperatorBase
from .expectation_base import ExpectationBase
from ..list_ops.list_op import ListOp
from ..list_ops.composed_op import ComposedOp
from ..list_ops.summed_op import SummedOp
from ..primitive_ops.pauli_op import PauliOp
from ..state_fns.circuit_state_fn import CircuitStateFn
from ..state_fns.operator_state_fn import OperatorStateFn
logger = logging.getLogger(__name__)
[docs]class AerPauliExpectation(ExpectationBase):
r""" An Expectation converter for using Aer's operator snapshot to
take expectations of quantum state circuits over Pauli observables.
"""
[docs] def convert(self, operator: OperatorBase) -> OperatorBase:
""" Accept an Operator and return a new Operator with the Pauli measurements replaced by
AerSnapshot-based expectation circuits.
Args:
operator: The operator to convert.
Returns:
The converted operator.
"""
if isinstance(operator, OperatorStateFn) and operator.is_measurement:
return self._replace_pauli_sums(operator.primitive) * operator.coeff
elif isinstance(operator, ListOp):
return operator.traverse(self.convert)
else:
return operator
# pylint: disable=inconsistent-return-statements,import-outside-toplevel
@classmethod
def _replace_pauli_sums(cls, operator):
try:
from qiskit.providers.aer.extensions import SnapshotExpectationValue
except ImportError as ex:
raise MissingOptionalLibraryError(
libname='qiskit-aer',
name='AerPauliExpectation',
pip_install='pip install qiskit-aer') from ex
# The 'expval_measurement' label on the snapshot instruction is special - the
# CircuitSampler will look for it to know that the circuit is a Expectation
# measurement, and not simply a
# circuit to replace with a DictStateFn
# Change to Pauli representation if necessary
if not {'Pauli'} == operator.primitive_strings():
logger.warning('Measured Observable is not composed of only Paulis, converting to '
'Pauli representation, which can be expensive.')
# Setting massive=False because this conversion is implicit. User can perform this
# action on the Observable with massive=True explicitly if they so choose.
operator = operator.to_pauli_op(massive=False)
if isinstance(operator, SummedOp):
paulis = [[meas.coeff, meas.primitive] for meas in operator.oplist]
snapshot_instruction = SnapshotExpectationValue('expval_measurement', paulis)
snapshot_op = CircuitStateFn(snapshot_instruction, is_measurement=True)
return snapshot_op
if isinstance(operator, PauliOp):
paulis = [[operator.coeff, operator.primitive]]
snapshot_instruction = SnapshotExpectationValue('expval_measurement', paulis)
snapshot_op = CircuitStateFn(snapshot_instruction, is_measurement=True)
return snapshot_op
if isinstance(operator, ListOp):
return operator.traverse(cls._replace_pauli_sums)
[docs] def compute_variance(self, exp_op: OperatorBase) -> Union[list, float]:
r"""
Compute the variance of the expectation estimator. Because Aer takes this expectation
with matrix multiplication, the estimation is exact and the variance is always 0,
but we need to return those values in a way which matches the Operator's structure.
Args:
exp_op: The full expectation value Operator after sampling.
Returns:
The variances or lists thereof (if exp_op contains ListOps) of the expectation value
estimation, equal to 0.
"""
# Need to do this to mimic Op structure
def sum_variance(operator):
if isinstance(operator, ComposedOp):
return 0.0
elif isinstance(operator, ListOp):
return operator._combo_fn([sum_variance(op) for op in operator.oplist])
return sum_variance(exp_op)
```