# -*- coding: utf-8 -*-
# This code is part of Qiskit.
#
# (C) Copyright IBM 2017.
#
# 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.
"""Swap gate."""
import numpy
from qiskit.circuit.controlledgate import ControlledGate
from qiskit.circuit.gate import Gate
from qiskit.circuit.quantumregister import QuantumRegister
from qiskit.circuit._utils import _compute_control_matrix
[docs]class SwapGate(Gate):
r"""The SWAP gate.
This is a symmetric and Clifford gate.
**Circuit symbol:**
.. parsed-literal::
q_0: ─X─
│
q_1: ─X─
**Matrix Representation:**
.. math::
SWAP =
\begin{pmatrix}
1 & 0 & 0 & 0 \\
0 & 0 & 1 & 0 \\
0 & 1 & 0 & 0 \\
0 & 0 & 0 & 1
\end{pmatrix}
The gate is equivalent to a state swap and is a classical logic gate.
.. math::
|a, b\rangle \rightarrow |b, a\rangle
"""
def __init__(self, label=None):
"""Create new SWAP gate."""
super().__init__('swap', 2, [], label=label)
def _define(self):
"""
gate swap a,b { cx a,b; cx b,a; cx a,b; }
"""
from .x import CXGate
definition = []
q = QuantumRegister(2, 'q')
rule = [
(CXGate(), [q[0], q[1]], []),
(CXGate(), [q[1], q[0]], []),
(CXGate(), [q[0], q[1]], [])
]
for inst in rule:
definition.append(inst)
self.definition = definition
[docs] def control(self, num_ctrl_qubits=1, label=None, ctrl_state=None):
"""Return a (multi-)controlled-SWAP gate.
One control returns a CSWAP (Fredkin) gate.
Args:
num_ctrl_qubits (int): number of control qubits.
label (str or None): An optional label for the gate [Default: None]
ctrl_state (int or str or None): control state expressed as integer,
string (e.g. '110'), or None. If None, use all 1s.
Returns:
ControlledGate: controlled version of this gate.
"""
if num_ctrl_qubits == 1:
gate = CSwapGate(label=label, ctrl_state=ctrl_state)
gate.base_gate.label = self.label
return gate
return super().control(num_ctrl_qubits=num_ctrl_qubits, label=label, ctrl_state=ctrl_state)
[docs] def inverse(self):
"""Return inverse Swap gate (itself)."""
return SwapGate() # self-inverse
[docs] def to_matrix(self):
"""Return a numpy.array for the SWAP gate."""
return numpy.array([[1, 0, 0, 0],
[0, 0, 1, 0],
[0, 1, 0, 0],
[0, 0, 0, 1]], dtype=complex)
class CSwapMeta(type):
"""A Metaclass to ensure that CSwapGate and FredkinGate are of the same type.
Can be removed when FredkinGate gets removed.
"""
@classmethod
def __instancecheck__(mcs, inst):
return type(inst) in {CSwapGate, FredkinGate} # pylint: disable=unidiomatic-typecheck
[docs]class CSwapGate(ControlledGate, metaclass=CSwapMeta):
r"""Controlled-X gate.
**Circuit symbol:**
.. parsed-literal::
q_0: ─X─
│
q_1: ─X─
│
q_2: ─■─
**Matrix representation:**
.. math::
CSWAP\ q_0, q_1, q_2 =
|0 \rangle \langle 0| \otimes I \otimes I +
|1 \rangle \langle 1| \otimes SWAP =
\begin{pmatrix}
1 & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\
0 & 1 & 0 & 0 & 0 & 0 & 0 & 0 \\
0 & 0 & 1 & 0 & 0 & 0 & 0 & 0 \\
0 & 0 & 0 & 0 & 0 & 1 & 0 & 0 \\
0 & 0 & 0 & 0 & 1 & 0 & 0 & 0 \\
0 & 0 & 0 & 1 & 0 & 0 & 0 & 0 \\
0 & 0 & 0 & 0 & 0 & 0 & 1 & 0 \\
0 & 0 & 0 & 0 & 0 & 0 & 0 & 1 \\
\end{pmatrix}
.. note::
In Qiskit's convention, higher qubit indices are more significant
(little endian convention). In many textbooks, controlled gates are
presented with the assumption of more significant qubits as control,
which in our case would be q_2. Thus a textbook matrix for this
gate will be:
.. parsed-literal::
q_0: ─■─
│
q_1: ─X─
│
q_2: ─X─
.. math::
CSWAP\ q_2, q_1, q_0 =
|0 \rangle \langle 0| \otimes I \otimes I +
|1 \rangle \langle 1| \otimes SWAP =
\begin{pmatrix}
1 & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\
0 & 1 & 0 & 0 & 0 & 0 & 0 & 0 \\
0 & 0 & 1 & 0 & 0 & 0 & 0 & 0 \\
0 & 0 & 0 & 1 & 0 & 0 & 0 & 0 \\
0 & 0 & 0 & 0 & 1 & 0 & 0 & 0 \\
0 & 0 & 0 & 0 & 0 & 0 & 1 & 0 \\
0 & 0 & 0 & 0 & 0 & 1 & 0 & 0 \\
0 & 0 & 0 & 0 & 0 & 0 & 0 & 1 \\
\end{pmatrix}
In the computational basis, this gate swaps the states of
the two target qubits if the control qubit is in the
:math:`|1\rangle` state.
.. math::
|0, b, c\rangle \rightarrow |0, b, c\rangle
|1, b, c\rangle \rightarrow |1, c, b\rangle
"""
def __init__(self, label=None, ctrl_state=None):
"""Create new CSWAP gate."""
super().__init__('cswap', 3, [], num_ctrl_qubits=1, label=label,
ctrl_state=ctrl_state)
self.base_gate = SwapGate()
def _define(self):
"""
gate cswap a,b,c
{ cx c,b;
ccx a,b,c;
cx c,b;
}
"""
from .x import CXGate, CCXGate
definition = []
q = QuantumRegister(3, 'q')
rule = [
(CXGate(), [q[2], q[1]], []),
(CCXGate(), [q[0], q[1], q[2]], []),
(CXGate(), [q[2], q[1]], [])
]
for inst in rule:
definition.append(inst)
self.definition = definition
[docs] def inverse(self):
"""Return inverse CSwap gate (itself)."""
return CSwapGate() # self-inverse
[docs] def to_matrix(self):
"""Return a numpy.array for the Fredkin (CSWAP) gate."""
return _compute_control_matrix(self.base_gate.to_matrix(),
self.num_ctrl_qubits,
ctrl_state=self.ctrl_state)
class FredkinGate(CSwapGate, metaclass=CSwapMeta):
"""The deprecated CSwapGate class."""
def __init__(self):
import warnings
warnings.warn('The class FredkinGate is deprecated as of 0.14.0, and '
'will be removed no earlier than 3 months after that release date. '
'You should use the class CSwapGate instead.',
DeprecationWarning, stacklevel=2)
super().__init__()