# 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.
"""Z2 Symmetry Tapering Converter Class"""
import logging
from typing import List, Tuple, Union, cast
from qiskit.opflow.converters.converter_base import ConverterBase
from qiskit.opflow.operator_base import OperatorBase
from qiskit.opflow.primitive_ops.pauli_sum_op import PauliSumOp
from qiskit.opflow.primitive_ops.tapered_pauli_sum_op import Z2Symmetries
from qiskit.quantum_info import Pauli
from qiskit.utils.deprecation import deprecate_func
logger = logging.getLogger(__name__)
[documentos]class TwoQubitReduction(ConverterBase):
"""
Deprecated: Two qubit reduction converter which eliminates the central and last
qubit in a list of Pauli that has diagonal operators (Z,I) at those positions.
Chemistry specific method:
It can be used to taper two qubits in parity and binary-tree mapped
fermionic Hamiltonians when the spin orbitals are ordered in two spin
sectors, (block spin order) according to the number of particles in the system.
"""
@deprecate_func(
since="0.24.0",
additional_msg="For code migration guidelines, visit https://qisk.it/opflow_migration.",
)
def __init__(self, num_particles: Union[int, List[int], Tuple[int, int]]):
"""
Args:
num_particles: number of particles, if it is a list,
the first number is alpha and the second number if beta.
"""
super().__init__()
if isinstance(num_particles, (tuple, list)):
num_alpha = num_particles[0]
num_beta = num_particles[1]
else:
num_alpha = num_particles // 2
num_beta = num_particles // 2
par_1 = 1 if (num_alpha + num_beta) % 2 == 0 else -1
par_2 = 1 if num_alpha % 2 == 0 else -1
self._tapering_values = [par_2, par_1]
[documentos] def convert(self, operator: OperatorBase) -> OperatorBase:
"""
Converts the Operator to tapered one by Z2 symmetries.
Args:
operator: the operator
Returns:
A new operator whose qubit number is reduced by 2.
"""
if not isinstance(operator, PauliSumOp):
return operator
operator = cast(PauliSumOp, operator)
if operator.is_zero():
logger.info(
"Operator is empty, can not do two qubit reduction. Return the empty operator back."
)
return PauliSumOp.from_list([("I" * (operator.num_qubits - 2), 0)])
num_qubits = operator.num_qubits
last_idx = num_qubits - 1
mid_idx = num_qubits // 2 - 1
sq_list = [mid_idx, last_idx]
# build symmetries, sq_paulis:
symmetries, sq_paulis = [], []
for idx in sq_list:
pauli_str = ["I"] * num_qubits
pauli_str[idx] = "Z"
z_sym = Pauli("".join(pauli_str)[::-1])
symmetries.append(z_sym)
pauli_str[idx] = "X"
sq_pauli = Pauli("".join(pauli_str)[::-1])
sq_paulis.append(sq_pauli)
z2_symmetries = Z2Symmetries(symmetries, sq_paulis, sq_list, self._tapering_values)
return z2_symmetries.taper(operator)