# This code is part of Qiskit.
#
# (C) Copyright IBM 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.
"""Two-qubit instruction representing a swap + single-qubit reset."""
from __future__ import annotations
from qiskit.circuit import QuantumCircuit, Instruction
[docs]
class Move(Instruction):
"""A two-qubit instruction representing a reset of the second qubit followed by a swap.
**Circuit Symbol:**
.. parsed-literal::
┌───────┐
q_0: ┤0 ├ q_0: ──────X─
│ Move │ = │
q_1: ┤1 ├ q_1: ─|0>──X─
└───────┘
The desired effect of this instruction, typically, is to move the state of
the first qubit to the second qubit. For this to work as expected, the
second incoming qubit must share no entanglement with the remainder of the
system. If this qubit *is* entangled, then performing the reset operation
will in turn implement a quantum channel on the other qubit(s) with which
it is entangled, resulting in the partial collapse of those qubits.
The simplest way to ensure that the second (i.e., destination) qubit shares
no entanglement with the remainder of the system is to use a fresh qubit
which has not been used since initialization.
Another valid way is to use, as a desination qubit, a qubit whose immediate
prior use was as the source (i.e., first) qubit of a preceding
:class:`Move` operation.
The following circuit contains two :class:`Move` operations, corresponding
to each of the aforementioned cases:
.. plot::
:include-source:
import numpy as np
from qiskit import QuantumCircuit
from circuit_knitting.cutting.instructions import Move
qc = QuantumCircuit(4)
qc.ryy(np.pi / 4, 0, 1)
qc.rx(np.pi / 4, 3)
qc.append(Move(), [1, 2])
qc.rz(np.pi / 4, 0)
qc.ryy(np.pi / 4, 2, 3)
qc.append(Move(), [2, 1])
qc.ryy(np.pi / 4, 0, 1)
qc.rx(np.pi / 4, 3)
qc.draw("mpl")
A full demonstration of the :class:`Move` instruction is available in `the
introductory tutorial on wire cutting
<../circuit_cutting/tutorials/03_wire_cutting_via_move_instruction.ipynb>`__.
"""
def __init__(self, label: str | None = None):
"""Create a :class:`Move` instruction."""
super().__init__("move", 2, 0, [], label=label)
def _define(self):
"""Set definition to equivalent circuit."""
qc = QuantumCircuit(2, name=self.name)
qc.reset(1)
qc.swap(0, 1)
self.definition = qc