8,422 research outputs found
Two-electron quantum dots as scalable qubits
We show that two electrons confined in a square semiconductor quantum dot
have two isolated low-lying energy eigenstates, which have the potential to
form the basis of scalable computing elements (qubits). Initialisation,
one-qubit and two-qubit universal gates, and readout are performed using
electrostatic gates and magnetic fields. Two-qubit transformations are
performed via the Coulomb interaction between electrons on adjacent dots.
Choice of initial states and subsequent asymmetric tuning of the tunnelling
energy parameters on adjacent dots control the effect of this interaction.Comment: Revised version, accepted by PR
Quantum Information processing by NMR: Implementation of Inversion-on-equality gate, Parity gate and Fanout gate
While quantum information processing by nuclear magnetic resonance (NMR) with
small number of qubits is well established, implementation of lengthy
computations have proved to be difficult due to decoherence/relaxation. In such
circumstances, shallow circuits (circuits using parallel computation) may prove
to be realistic. Parity and fanout gates are essential to create shallow
circuits. In this article we implement inversion-on-equality gate, followed by
parity gate and fanout gate in 3-qubit systems by NMR, using evolution under
indirect exchange coupling Hamiltonian.Comment: 24 pages, 7 figure
Coupled quantum dots as quantum gates
We consider a new quantum gate mechanism based on electron spins in coupled
semiconductor quantum dots. Such gates provide a general source of spin
entanglement and can be used for quantum computers. We determine the exchange
coupling J in the effective Heisenberg model as a function of magnetic (B) and
electric fields, and of the inter-dot distance (a) within the Heitler-London
approximation of molecular physics. This result is refined by using
sp-hybridization, and by the Hund-Mulliken molecular-orbit approach which leads
to an extended Hubbard description for the two-dot system that shows a
remarkable dependence on B and a due to the long-range Coulomb interaction. We
find that the exchange J changes sign at a finite field (leading to a
pronounced jump in the magnetization) and then decays exponentially. The
magnetization and the spin susceptibilities of the coupled dots are calculated.
We show that the dephasing due to nuclear spins in GaAs can be strongly
suppressed by dynamical nuclear spin polarization and/or by magnetic fields.Comment: 10 pages, 4 figures. v2: minor corrections, appendix added. to be
published in Phys.Rev.
QuEST and High Performance Simulation of Quantum Computers
We introduce QuEST, the Quantum Exact Simulation Toolkit, and compare it to
ProjectQ, qHipster and a recent distributed implementation of Quantum++. QuEST
is the first open source, OpenMP and MPI hybridised, GPU accelerated simulator
of universal quantum circuits. Embodied as a C library, it is designed so that
a user's code can be deployed seamlessly to any platform from a laptop to a
supercomputer. QuEST is capable of simulating generic quantum circuits of
general single-qubit gates and multi-qubit controlled gates, on pure and mixed
states, represented as state-vectors and density matrices, and under the
presence of decoherence. Using the ARCUS Phase-B and ARCHER supercomputers, we
benchmark QuEST's simulation of random circuits of up to 38 qubits, distributed
over up to 2048 compute nodes, each with up to 24 cores. We directly compare
QuEST's performance to ProjectQ's on single machines, and discuss the
differences in distribution strategies of QuEST, qHipster and Quantum++. QuEST
shows excellent scaling, both strong and weak, on multicore and distributed
architectures.Comment: 8 pages, 8 figures; fixed typos; updated QuEST URL and fixed typo in
Fig. 4 caption where ProjectQ and QuEST were swapped in speedup subplot
explanation; added explanation of simulation algorithm, updated bibliography;
stressed technical novelty of QuEST; mentioned new density matrix suppor
Electron Spin for Classical Information Processing: A Brief Survey of Spin-Based Logic Devices, Gates and Circuits
In electronics, information has been traditionally stored, processed and
communicated using an electron's charge. This paradigm is increasingly turning
out to be energy-inefficient, because movement of charge within an
information-processing device invariably causes current flow and an associated
dissipation. Replacing charge with the "spin" of an electron to encode
information may eliminate much of this dissipation and lead to more
energy-efficient "green electronics". This realization has spurred significant
research in spintronic devices and circuits where spin either directly acts as
the physical variable for hosting information or augments the role of charge.
In this review article, we discuss and elucidate some of these ideas, and
highlight their strengths and weaknesses. Many of them can potentially reduce
energy dissipation significantly, but unfortunately are error-prone and
unreliable. Moreover, there are serious obstacles to their technological
implementation that may be difficult to overcome in the near term.
This review addresses three constructs: (1) single devices or binary switches
that can be constituents of Boolean logic gates for digital information
processing, (2) complete gates that are capable of performing specific Boolean
logic operations, and (3) combinational circuits or architectures (equivalent
to many gates working in unison) that are capable of performing universal
computation.Comment: Topical Revie
Variable energy, high flux, ground-state atomic oxygen source
A variable energy, high flux atomic oxygen source is described which is comprised of a means for producing a high density beam of molecules which will emit O(-) ions when bombarded with electrons; a means of producing a high current stream of electrons at a low energy level passing through the high density beam of molecules to produce a combined stream of electrons and O(-) ions; means for accelerating the combined stream to a desired energy level; means for producing an intense magnetic field to confine the electrons and O(-) ions; means for directing a multiple pass laser beam through the combined stream to strip off the excess electrons from a plurality of the O(-) ions to produce ground-state O atoms within the combined stream; electrostatic deflection means for deflecting the path of the O(-) ions and the electrons in the combined stream; and, means for stopping the O(-) ions and the electrons and for allowing only the ground-state O atoms to continue as the source of the atoms of interest. The method and apparatus are also adaptable for producing other ground-state atoms and/or molecules
Spin interactions and switching in vertically tunnel-coupled quantum dots
We determine the spin exchange coupling J between two electrons located in
two vertically tunnel-coupled quantum dots, and its variation when magnetic (B)
and electric (E) fields (both in-plane and perpendicular) are applied. We
predict a strong decrease of J as the in-plane B field is increased, mainly due
to orbital compression. Combined with the Zeeman splitting, this leads to a
singlet-triplet crossing, which can be observed as a pronounced jump in the
magnetization at in-plane fields of a few Tesla, and perpendicular fields of
the order of 10 Tesla for typical self-assembled dots. We use harmonic
potentials to model the confining of electrons, and calculate the exchange J
using the Heitler-London and Hund-Mulliken technique, including the long-range
Coulomb interaction. With our results we provide experimental criteria for the
distinction of singlet and triplet states and therefore for microscopic spin
measurements. In the case where dots of different sizes are coupled, we present
a simple method to switch on and off the spin coupling with exponential
sensitivity using an in-plane electric field. Switching the spin coupling is
essential for quantum computation using electronic spins as qubits.Comment: 13 pages, 9 figure
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