358 research outputs found
Nuclear spin state narrowing via gate--controlled Rabi oscillations in a double quantum dot
We study spin dynamics for two electrons confined to a double quantum dot
under the influence of an oscillating exchange interaction. This leads to
driven Rabi oscillations between the --state and the
--state of the two--electron system. The width of the
Rabi resonance is proportional to the amplitude of the oscillating exchange. A
measurement of the Rabi resonance allows one to narrow the distribution of
nuclear spin states and thereby to prolong the spin decoherence time. Further,
we study decoherence of the two-electron states due to the hyperfine
interaction and give requirements on the parameters of the system in order to
initialize in the --state and to perform a
operation with unit fidelity.Comment: v1:9 pages, 1 figure; v2: 13 pages, 2 figures, added section on
measurement, to appear in Phys. Rev.
Spin Hall Effect
The intrinsic spin Hall effect in semiconductors has developed to a
remarkably lively and rapidly growing branch of research in the field of
semiconductor spintronics. In this article we give a pedagogical overview on
both theoretical and experimental accomplishments and challenges. Emphasis is
put on the the description of the intrinsic mechanisms of spin Hall transport
in III-V zinc-blende semiconductors, and on the effects of dissipation.Comment: 22 pages, minor adjustments, version as publishe
Optimal time-dependent polarized current pattern for fast domain wall propagation in nanowires: Exact solutions for biaxial and uniaxial anisotropies
One of the important issues in nanomagnetism is to lower the current needed
for a technologically useful domain wall (DW) propagation speed. Based on the
modified Landau-Lifshitz-Gilbert (LLG) equation with both Slonczewski
spin-transfer torque and the field-like torque, we derive the optimal spin
current pattern for fast DW propagation along nanowires. Under such conditions,
the DW velocity in biaxial wires can be enhanced as much as ten times compared
to the velocities achieved in experiments so far. Moreover, the fast variation
of spin polarization can help DW depinning. Possible experimental realizations
are discussed.Comment: 4 pages, 1 figur
Large N Scaling Behavior of the Lipkin-Meshkov-Glick Model
We introduce a novel semiclassical approach to the Lipkin model. In this way
the well-known phase transition arising at the critical value of the coupling
is intuitively understood. New results -- showing for strong couplings the
existence of a threshold energy which separates deformed from undeformed states
as well as the divergence of the density of states at the threshold energy --
are explained straightforwardly and in quantitative terms by the appearance of
a double well structure in a classical system corresponding to the Lipkin
model. Previously unnoticed features of the eigenstates near the threshold
energy are also predicted and found to hold.Comment: 4 pages, 2 figures, to appear in PR
The transfer dilemma
Abstract In this paper we provide an overview of research on transfer, highlighting its main tenets. Then we look at interviews of two fifth grade students learning about mathematical concepts regarding operations on positive and negative quantities. We attempt to focus on how their learning is influenced by their prior knowledge and experience. We take the position that transfer is a theory of learning and we attempt to show that it cannot provide a solid foundation for explaining such examples of learning
Quantum Entanglement in Fermionic Lattices
The Fock space of a system of indistinguishable particles is isomorphic (in a
non-unique way) to the state-space of a composite i.e., many-modes, quantum
system. One can then discuss quantum entanglement for fermionic as well as
bosonic systems. We exemplify the use of this notion -central in quantum
information - by studying some e.g., Hubbard,lattice fermionic models relevant
to condensed matter physics.Comment: 4 Pages LaTeX, 1 TeX Figure. Presentation improved, title changed. To
appear in PR
Entanglement in a two-identical-particle system
The definition of entanglement in identical-particle system is introduced.
The separability criterion in two-identical particle system is given. The
physical meaning of the definition is analysed. Applications to two-boson and
two-fermion systems are made. It is found new entanglement and correlation
phenomena in identical-boson systems exist, and they may have applications in
the field of quantum information.Comment: 4 page
Swapping and entangling hyperfine coupled nuclear spin baths
We numerically study the hyperfine induced nuclear spin dynamics in a system
of two coupled quantum dots in zero magnetic field. Each of the electron spins
is considered to interact with an individual bath of nuclear spins via
homogeneous coupling constants (all coupling coefficients being equal). In
order to lower the dimension of the problem, the two baths are approximated by
two single long spins. We demonstrate that the hyperfine interaction enables to
utilize the nuclear baths for quantum information purposes. In particular, we
show that it is possible to swap the nuclear ensembles on time scales of
seconds and indicate that it might even be possible to fully entangle them. As
a key result, it turns out that the larger the baths are, the more useful they
become as a resource of quantum information. Interestingly, the nuclear spin
dynamics strongly benefits from combining two quantum dots of different
geometry to a double dot set up.Comment: 6 pages, 7 figure
Non-Markovian dynamics of interacting qubit pair coupled to two independent bosonic baths
The dynamics of two interacting spins coupled to separate bosonic baths is
studied. An analytical solution in Born approximation for arbitrary spectral
density functions of the bosonic environments is found. It is shown that in the
non-Markovian cases concurrence "lives" longer or reaches greater values.Comment: 13 page
High Order Coherent Control Sequences of Finite-Width Pulses
The performance of sequences of designed pulses of finite length is
analyzed for a bath of spins and it is compared with that of sequences of
ideal, instantaneous pulses. The degree of the design of the pulse strongly
affects the performance of the sequences. Non-equidistant, adapted sequences of
pulses, which equal instantaneous ones up to , outperform
equidistant or concatenated sequences. Moreover, they do so at low energy cost
which grows only logarithmically with the number of pulses, in contrast to
standard pulses with linear growth.Comment: 6 pages, 5 figures, new figures, published versio
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