# -*- coding: utf-8 -*-
# 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.
# pylint: disable=no-member
"""Implementations of boolean logic quantum circuits."""
from typing import List, Optional
from qiskit.circuit import QuantumRegister, QuantumCircuit
from qiskit.circuit.library.standard_gates import MCXGate
[docs]class OR(QuantumCircuit):
r"""A circuit implementing the logical OR operation on a number of qubits.
For the OR operation the state :math:`|1\rangle` is interpreted as ``True``. The result
qubit is flipped, if the state of any variable qubit is ``True``. The OR is implemented using
a multi-open-controlled X gate (i.e. flips if the state is :math:`|0\rangle`) and
applying an X gate on the result qubit.
Using a list of flags, qubits can be skipped or negated.
The OR gate without special flags:
.. jupyter-execute::
:hide-code:
from qiskit.circuit.library import OR
import qiskit.tools.jupyter
circuit = OR(5)
%circuit_library_info circuit
Using flags we can negate qubits or skip them. For instance, if we have 5 qubits and want to
return ``True`` if the first qubit is ``False`` or one of the last two are ``True`` we use the
flags ``[-1, 0, 0, 1, 1]``.
.. jupyter-execute::
:hide-code:
from qiskit.circuit.library import OR
import qiskit.tools.jupyter
circuit = OR(5, flags=[-1, 0, 0, 1, 1])
%circuit_library_info circuit
"""
def __init__(self, num_variable_qubits: int, flags: Optional[List[int]] = None,
mcx_mode: str = 'noancilla') -> None:
"""Create a new logical OR circuit.
Args:
num_variable_qubits: The qubits of which the OR is computed. The result will be written
into an additional result qubit.
flags: A list of +1/0/-1 marking negations or omisiions of qubits.
mcx_mode: The mode to be used to implement the multi-controlled X gate.
"""
# store num_variables_qubits and flags
self.num_variable_qubits = num_variable_qubits
self.flags = flags
# add registers
qr_variable = QuantumRegister(num_variable_qubits, name='variable')
qr_result = QuantumRegister(1, name='result')
super().__init__(qr_variable, qr_result, name='or')
# determine the control qubits: all that have a nonzero flag
flags = flags or [1] * num_variable_qubits
control_qubits = [q for q, flag in zip(qr_variable, flags) if flag != 0]
# determine the qubits that need to be flipped (if a flag is > 0)
flip_qubits = [q for q, flag in zip(qr_variable, flags) if flag > 0]
# determine the number of ancillas
self.num_ancilla_qubits = MCXGate.get_num_ancilla_qubits(len(control_qubits), mode=mcx_mode)
if self.num_ancilla_qubits > 0:
qr_ancilla = QuantumRegister(self.num_ancilla_qubits, 'ancilla')
self.add_register(qr_ancilla)
else:
qr_ancilla = []
self.x(qr_result)
if len(flip_qubits) > 0:
self.x(flip_qubits)
self.mcx(control_qubits, qr_result[:], qr_ancilla[:], mode=mcx_mode)
if len(flip_qubits) > 0:
self.x(flip_qubits)