1,666 research outputs found
Knitting distributed cluster state ladders with spin chains
There has been much recent study on the application of spin chains to quantum
state transfer and communication. Here we discuss the utilisation of spin
chains (set up for perfect quantum state transfer) for the knitting of
distributed cluster state structures, between spin qubits repeatedly injected
and extracted at the ends of the chain. The cluster states emerge from the
natural evolution of the system across different excitation number sectors. We
discuss the decohering effects of errors in the injection and extraction
process as well as the effects of fabrication and random errors.Comment: To be published in PRA. v2 includes minor corrections as well as an
added discussion on refocussin
Spin injection and electric field effect in degenerate semiconductors
We analyze spin-transport in semiconductors in the regime characterized by
(intermediate to degenerate), where is the Fermi
temperature. Such a regime is of great importance since it includes the lightly
doped semiconductor structures used in most experiments; we demonstrate that,
at the same time, it corresponds to the regime in which carrier-carrier
interactions assume a relevant role. Starting from a general formulation of the
drift-diffusion equations, which includes many-body correlation effects, we
perform detailed calculations of the spin injection characteristics of various
heterostructures, and analyze the combined effects of carrier density
variation, applied electric field and Coulomb interaction. We show the
existence of a degenerate regime, peculiar to semiconductors, which strongly
differs, as spin-transport is concerned, from the degenerate regime of metals.Comment: Version accepted for publication in Phys. Rev.
Effect of perturbations on information transfer in spin chains
Spin chains have been proposed as a reliable and convenient way of
transferring information and entanglement in a quantum computational context.
Nonetheless, it has to be expected that any physical implementation of these
systems will be subject to several perturbative factors which could potentially
diminish the transfer quality. In this paper, we investigate a number of
possible fabrication defects in the spin chains themselves as well as the
effect of non-synchronous or imperfect input operations, with a focus on the
case of multiple excitation/qubit transfer. We consider both entangled and
unentangled states, and in particular the transfer of an entangled pair of
adjacent spins at one end of a chain under the mirroring rule and also the
creation of entanglement resulting from injection at both end spins.Comment: Journal version fixes last typo
Spin Coulomb drag in the two-dimensional electron liquid
We calculate the spin-drag transresistivity
in a two-dimensional electron gas at temperature in the random phase
approximation. In the low-temperature regime we show that, at variance with the
three-dimensional low-temperature result [], the spin transresistivity of a two-dimensional {\it spin unpolarized}
electron gas has the form . In the
spin-polarized case the familiar form is
recovered, but the constant of proportionality diverges logarithmically as
the spin-polarization tends to zero. In the high-temperature regime we obtain
(where
is the effective Rydberg energy) {\it independent} of the density.
Again, this differs from the three-dimensional result, which has a logarithmic
dependence on the density. Two important differences between the spin-drag
transresistivity and the ordinary Coulomb drag transresistivity are pointed
out: (i) The singularity at low temperature is smaller, in the Coulomb
drag case, by a factor where is the Fermi wave vector and
is the separation between the layers. (ii) The collective mode contribution
to the spin-drag transresistivity is negligible at all temperatures. Moreover
the spin drag effect is, for comparable parameters, larger than the ordinary
Coulomb drag effect.Comment: 6 figures; various changes; version accepted for publicatio
Mesoporous matrices for quantum computation with improved response through redundance
We present a solid state implementation of quantum computation, which improves previously proposed optically driven schemes. Our proposal is based on vertical arrays of quantum dots embedded in a mesoporous material which can be fabricated with present technology. The redundant encoding typical of the chosen hardware protects the computation against gate errors and the effects of measurement induced noise. The system parameters required for quantum computation applications are calculated for II-VI and III-V materials and found to be within the experimental range. The proposed hardware may help minimize errors due to polydispersity of dot sizes, which is at present one of the main problems in relation to quantum dot-based quantum computation. (c) 2007 American Institute of Physics
Freezing distributed entanglement in spin chains
We show how to freeze distributed entanglement that has been created from the
natural dynamics of spin chain systems. The technique that we propose simply
requires single-qubit operations and isolates the entanglement in specific
qubits at the ends of branches. Such frozen entanglement provides a useful
resource, for example for teleportation or distributed quantum processing. The
scheme can be applied to a wide range of systems -- including actual spin
systems and alternative qubit embodiments in strings of quantum dots, molecules
or atoms.Comment: 5 pages, to appear in Phys. Rev. A (Rapid Communication
Geometry induced entanglement transitions in nanostructures
We model quantum dot nanostructures using a one-dimensional system of two
interacting electrons. We show that strong and rapid variations may be induced
in the spatial entanglement by varying the nanostructure geometry. We
investigate the position-space information entropy as an indicator of the
entanglement in this system. We also consider the expectation value of the
Coulomb interaction and the ratio of this expectation to the expectation of the
confining potential and their link to the entanglement. We look at the first
derivative of the entanglement and the position-space information entropy to
infer information about a possible quantum phase transition.Comment: 3 pages, 2 figures, to appear in Journal of Applied Physic
Hubbard model as an approximation to the entanglement in nanostructures
We investigate how well the one-dimensional Hubbard model describes the entanglement of particles trapped in a string of quantum wells. We calculate the average single-site entanglement for two particles interacting via a contact interaction and consider the effect of varying the interaction strength and the interwell distance. We compare the results with the ones obtained within the one-dimensional Hubbard model with on-site interaction. We suggest an upper bound for the average single-site entanglement for two electrons in M wells and discuss analytical limits for very large repulsive and attractive interactions. We investigate how the interplay between interaction and potential shape in the quantum-well system dictates the position and size of the entanglement maxima and the agreement with the theoretical limits. Finally, we calculate the spatial entanglement for the quantum-well system and compare it to its average single-site entanglement
Effect of confinement potential geometry on entanglement in quantum dot-based nanostructures
We calculate the spatial entanglement between two electrons trapped in a
nanostructure for a broad class of confinement potentials, including single and
double quantum dots, and core-shell quantum dot structures.
By using a parametrized confinement potential, we are able to switch from one
structure to the others with continuity and to analyze how the entanglement is
influenced by the changes in the confinement geometry. We calculate the
many-body wave function by `exact' diagonalization of the time independent
Schr\"odinger equation. We discuss the relationship between the entanglement
and specific cuts of the wave function, and show that the wave function at a
single highly symmetric point could be a good indicator for the entanglement
content of the system. We analyze the counterintuitive relationship between
spatial entanglement and Coulomb interaction, which connects maxima (minima) of
the first to minima (maxima) of the latter. We introduce a potential quantum
phase transition which relates quantum states characterized by different
spatial topology. Finally we show that by varying shape, range and strength of
the confinement potential, it is possible to induce strong and rapid variations
of the entanglement between the two electrons. This property may be used to
tailor nanostructures according to the level of entanglement required by a
specific application.Comment: 10 pages, 8 figures and 1 tabl
Entanglement and density-functional theory: testing approximations on Hooke's atom
We present two methods of calculating the spatial entanglement of an
interacting electron system within the framework of density-functional theory.
These methods are tested on the model system of Hooke's atom for which the
spatial entanglement can be calculated exactly. We analyse how the strength of
the confining potential affects the spatial entanglement and how accurately the
methods that we introduced reproduce the exact trends. We also compare the
results with the outcomes of standard first-order perturbation methods. The
accuracies of energies and densities when using these methods are also
considered.Comment: 14 pages with 18 figures; corrected typos, corrected expression for
first-order energy in section VI and consequently Fig.13, conclusions and
other results unaffecte
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