437 research outputs found
Buckyball Quantum Computer: Realization of a Quantum Gate
We have studied a system composed by two endohedral fullerene molecules. We
have found that this system can be used as good candidate for the realization
of Quantum Gates Each of these molecules encapsules an atom carrying a
spin,therefore they interact through the spin dipole interaction. We show that
a phase gate can be realized if we apply on each encased spin static and time
dependent magnetic field. We have evaluated the operational time of a
-phase gate, which is of the order of ns. We made a comparison between the
theoretical estimation of the gate time and the experimental decoherence time
for each spin. The comparison shows that the spin relaxation time is much
larger than the -gate operational time. Therefore, this indicates that,
during the decoherence time, it is possible to perform some thousands of
quantum computational operations. Moreover, through the study of concurrence,
we get very good results for the entanglement degree of the two-qubit system.
This finding opens a new avenue for the realization of Quantum Computers.Comment: 13 pages, 5 figures. Submitted to Physical Review
Fractal and chaotic solutions of the discrete nonlinear Schr\"odinger equation in classical and quantum systems
We discuss stationary solutions of the discrete nonlinear Schr\"odinger
equation (DNSE) with a potential of the type which is generically
applicable to several quantum spin, electron and classical lattice systems. We
show that there may arise chaotic spatial structures in the form of
incommensurate or irregular quantum states. As a first (typical) example we
consider a single electron which is strongly coupled with phonons on a
chain of atoms --- the (Rashba)--Holstein polaron model. In the adiabatic
approximation this system is conventionally described by the DNSE. Another
relevant example is that of superconducting states in layered superconductors
described by the same DNSE. Amongst many other applications the typical example
for a classical lattice is a system of coupled nonlinear oscillators. We
present the exact energy spectrum of this model in the strong coupling limit
and the corresponding wave function. Using this as a starting point we go on to
calculate the wave function for moderate coupling and find that the energy
eigenvalue of these structures of the wave function is in exquisite agreement
with the exact strong coupling result. This procedure allows us to obtain
(numerically) exact solutions of the DNSE directly. When applied to our typical
example we find that the wave function of an electron on a deformable lattice
(and other quantum or classical discrete systems) may exhibit incommensurate
and irregular structures. These states are analogous to the periodic,
quasiperiodic and chaotic structures found in classical chaotic dynamics
Electron locking in semiconductor superlattices
We describe a novel state of electrons and phonons arising in semiconductor
superlattices (SSL) due to strong electron-phonon interactions. These states
are characterized by a localization of phonons and a self-trapping or locking
of electrons in one or several quantum wells due to an additional,
deformational potential arising around these locking wells in SSL. The effect
is enhanced in a longitudinal magnetic field.
Using the tight-binding and adiabatic approximations the whole energy
spectrum of the self-trapped states is found and accurate, analytic expressions
are included for strong electron-phonon coupling. Finally, we discuss possible
experiments which may detect these predicted self-trapped states.Comment: 8 pages, 2 figures. Please note that the published article has the
title 'Electron locking in layered structures by a longitudinal magnetic
field
Two-dimensional Ising model with competing interactions and its application to clusters and arrays of -rings and adiabatic quantum computing
We study planar clusters consisting of loops including a Josephson
-junction (-rings). Each -ring carries a persistent current and
behaves as a classical orbital moment. The type of particular state associated
with the orientation of orbital moments at the cluster depends on the
interaction between these orbital moments and can be easily controlled, i.e. by
a bias current or by other means. We show that these systems can be described
by the two-dimensional Ising model with competing nearest-neighbor and diagonal
interactions and investigate the phase diagram of this model. The
characteristic features of the model are analyzed based on the exact solutions
for small clusters such as a 5-site square plaquette as well as on a mean-field
type approach for the infinite square lattice of Ising spins. The results are
compared with spin patterns obtained by Monte Carlo simulations for the 100
100 square lattice and with experiment. We show that the -ring
clusters may be used as a new type of superconducting memory elements. The
obtained results may be verified in experiments and are applicable to adiabatic
quantum computing where the states are switched adiabatically with the slow
change of coupling constants.Comment: 32 pages, 22 figures, RevTe
Extraordinary Magnetoresistance in Hybrid Semiconductor-Metal Systems
We show that extraordinary magnetoresistance (EMR) arises in systems
consisting of two components; a semiconducting ring with a metallic inclusion
embedded. The im- portant aspect of this discovery is that the system must have
a quasi-two-dimensional character. Using the same materials and geometries for
the samples as in experiments by Solin et al.[1;2], we show that such systems
indeed exhibit a huge magnetoresistance. The magnetoresistance arises due to
the switching of electrical current paths passing through the metallic
inclusion. Diagrams illustrating the flow of the current density within the
samples are utilised in discussion of the mechanism responsible for the
magnetoresistance effect. Extensions are then suggested which may be applicable
to the silver chalcogenides. Our theory offers an excellent description and
explanation of experiments where a huge magnetoresistance has been
discovered[2;3].Comment: 12 Pages, 5 Figure
Fine Structure and Fractional Aharonov-Bohm Effect
We find a fine structure in the Aharonov-Bohm effect, characterized by the
appearence of a new type of periodic oscillations having smaller fractional
period and an amplitude, which may compare with the amplitude of the
conventional Aharonov-Bohm effect. Specifically, at low density or strong
coupling on a Hubbard ring can coexist along with the conventional Aaronov-Bohm
oscillations with the period equal to an integer, measured in units of the
elementary flux quantum, two additional oscillations with periods and
. The integers and are the particles number and the number of
down-spin particles, respectively. {}From a solution of the Bethe ansatz
equations for electrons located on a ring in a magnetic field we show that
the fine structure is due to electron-electron and Zeeman interactions. Our
results are valid in the dilute density limit and for an arbitrary value of the
Hubbard repulsion Comment: 40 pages (Latex,Revtex) 12 figures by request, in Technical Reports
of ISSP , Ser. A, No.2836 (1994
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