# Source code for qiskit.circuit.library.standard_gates.swap

# This code is part of Qiskit.
#
#
# obtain a copy of this license in the LICENSE.txt file in the root directory
#
# 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."""

from typing import Optional, Union
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 with_gate_array, with_controlled_gate_array

_SWAP_ARRAY = numpy.array([[1, 0, 0, 0], [0, 0, 1, 0], [0, 1, 0, 0], [0, 0, 0, 1]])

[docs]@with_gate_array(_SWAP_ARRAY)
class SwapGate(Gate):
r"""The SWAP gate.

This is a symmetric and Clifford gate.

Can be applied to a :class:~qiskit.circuit.QuantumCircuit
with the :meth:~qiskit.circuit.QuantumCircuit.swap method.

**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: Optional[str] = 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; }
"""
# pylint: disable=cyclic-import
from qiskit.circuit.quantumcircuit import QuantumCircuit
from .x import CXGate

q = QuantumRegister(2, "q")
qc = QuantumCircuit(q, name=self.name)
rules = [
(CXGate(), [q[0], q[1]], []),
(CXGate(), [q[1], q[0]], []),
(CXGate(), [q[0], q[1]], []),
]
for instr, qargs, cargs in rules:
qc._append(instr, qargs, cargs)

self.definition = qc

[docs]    def control(
self,
num_ctrl_qubits: int = 1,
label: Optional[str] = None,
ctrl_state: Optional[Union[str, int]] = 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]@with_controlled_gate_array(_SWAP_ARRAY, num_ctrl_qubits=1)
class CSwapGate(ControlledGate):
r"""Controlled-SWAP gate, also known as the Fredkin gate.

Can be applied to a :class:~qiskit.circuit.QuantumCircuit
with the :meth:~qiskit.circuit.QuantumCircuit.cswap and
:meth:~qiskit.circuit.QuantumCircuit.fredkin methods.

**Circuit symbol:**

.. parsed-literal::

q_0: ─■─
│
q_1: ─X─
│
q_2: ─X─

**Matrix representation:**

.. math::

CSWAP\ q_0, q_1, q_2 =
I \otimes I \otimes |0 \rangle \langle 0| +
SWAP \otimes |1 \rangle \langle 1| =
\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: ─X─
│
q_1: ─X─
│
q_2: ─■─

.. 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: Optional[str] = None, ctrl_state: Optional[Union[str, int]] = None):
"""Create new CSWAP gate."""
super().__init__(
"cswap",
3,
[],
num_ctrl_qubits=1,
label=label,
ctrl_state=ctrl_state,
base_gate=SwapGate(),
)

def _define(self):
"""
gate cswap a,b,c
{ cx c,b;
ccx a,b,c;
cx c,b;
}
"""
# pylint: disable=cyclic-import
from qiskit.circuit.quantumcircuit import QuantumCircuit
from .x import CXGate, CCXGate

q = QuantumRegister(3, "q")
qc = QuantumCircuit(q, name=self.name)
rules = [
(CXGate(), [q[2], q[1]], []),
(CCXGate(), [q[0], q[1], q[2]], []),
(CXGate(), [q[2], q[1]], []),
]
for instr, qargs, cargs in rules:
qc._append(instr, qargs, cargs)

self.definition = qc

[docs]    def inverse(self):
"""Return inverse CSwap gate (itself)."""
return CSwapGate(ctrl_state=self.ctrl_state)  # self-inverse