5,913 research outputs found
Metric space analysis of systems immersed in a magnetic field
Understanding the behavior of quantum systems subject to magnetic fields is
of fundamental importance and underpins quantum technologies. However, modeling
these systems is a complex task, because of many-body interactions and because
many-body approaches such as density functional theory get complicated by the
presence of a vector potential into the system Hamiltonian. We use the metric
space approach to quantum mechanics to study the effects of varying the
magnetic vector potential on quantum systems. The application of this technique
to model systems in the ground state provides insight into the fundamental
mapping at the core of current density functional theory, which relates the
many-body wavefunction, particle density and paramagnetic current density. We
show that the role of the paramagnetic current density in this relationship
becomes crucial when considering states with different magnetic quantum
numbers, . Additionally, varying the magnetic field uncovers a richer
complexity for the "band structure" present in ground state metric spaces, as
compared to previous studies varying scalar potentials. The robust nature of
the metric space approach is strengthened by demonstrating the gauge invariance
of the related metric for the paramagnetic current density. We go beyond ground
state properties and apply this approach to excited states. The results suggest
that, under specific conditions, a universal behavior may exist for the
relationships between the physical quantities defining the system
Feasibility of approximating spatial and local entanglement in long-range interacting systems using the extended Hubbard model
We investigate the extended Hubbard model as an approximation to the local
and spatial entanglement of a one-dimensional chain of nanostructures where the
particles interact via a long range interaction represented by a `soft' Coulomb
potential. In the process we design a protocol to calculate the
particle-particle spatial entanglement for the Hubbard model and show that, in
striking contrast with the loss of spatial degrees of freedom, the predictions
are reasonably accurate. We also compare results for the local entanglement
with previous results found using a contact interaction (PRA, 81 (2010) 052321)
and show that while the extended Hubbard model recovers a better agreement with
the entanglement of a long-range interacting system, there remain realistic
parameter regions where it fails to predict the quantitative and qualitative
behaviour of the entanglement in the nanostructure system.Comment: 6 pages, 5 figures and 1 table; added results with correlated hopping
term; accepted by EP
Effect of matrix parameters on mesoporous matrix based quantum computation
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. We study the feasibility of performing
quantum computation with different mesoporous matrices. We analyse which matrix
materials ensure that each individual stack of quantum dots can be considered
isolated from the rest of the ensemble-a key requirement of our scheme. This
requirement is satisfied for all matrix materials for feasible structure
parameters and GaN/AlN based quantum dots. We also show that one dimensional
ensembles substantially improve performances, even of CdSe/CdS based quantum
dots
Entanglement distribution for a practical quantum-dot-based quantum processor architecture
We propose a quantum dot (QD) architecture for enabling universal quantum information processing. Quantum registers, consisting of arrays of vertically stacked self-assembled semiconductor QDs, are connected by chains of in-plane self-assembled dots. We propose an entanglement distributor, a device for producing and distributing maximally entangled qubits on demand, communicated through in-plane dot chains. This enables the transmission of entanglement to spatially separated register stacks, providing a resource for the realization of a sizeable quantum processor built from coupled register stacks of practical size. Our entanglement distributor could be integrated into many of the present proposals for self-assembled QD-based quantum computation (QC). Our device exploits the properties of simple, relatively short, spin-chains and does not require microcavities. Utilizing the properties of self-assembled QDs, after distribution the entanglement can be mapped into relatively long-lived spin qubits and purified, providing a flexible, distributed, off-line resource. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft
Telescope performance and image simulations of the balloon-borne coded-mask protoMIRAX experiment
In this work we present the results of imaging simulations performed with the
help of the GEANT4 package for the protoMIRAX hard X-ray balloon experiment.
The instrumental background was simulated taking into account the various
radiation components and their angular dependence, as well as a detailed mass
model of the experiment. We modeled the meridian transits of the Crab Nebula
and the Galatic Centre region during balloon flights in Brazil ( of latitude and an altitude of km) and
introduced the correspondent spectra as inputs to the imaging simulations. We
present images of the Crab and of three sources in the Galactic Centre region:
1E 1740.7-2942, GRS 1758-258 and GX 1+4. The results show that the protoMIRAX
experiment is capable of making spectral and timing observations of bright hard
X-ray sources as well as important imaging demonstrations that will contribute
to the design of the MIRAX satellite mission.Comment: 9 figure
The entanglement of few-particle systems when using the local-density approximation
In this chapter we discuss methods to calculate the entanglement of a system
using density-functional theory. We firstly introduce density-functional theory
and the local-density approximation (LDA). We then discuss the concept of the
`interacting LDA system'. This is characterised by an interacting many-body
Hamiltonian which reproduces, uniquely and exactly, the ground state density
obtained from the single-particle Kohn-Sham equations of density-functional
theory when the local-density approximation is used. We motivate why this idea
can be useful for appraising the local-density approximation in many-body
physics particularly with regards to entanglement and related quantum
information applications. Using an iterative scheme, we find the Hamiltonian
characterising the interacting LDA system in relation to the test systems of
Hooke's atom and helium-like atoms. The interacting LDA system ground state
wavefunction is then used to calculate the spatial entanglement and the results
are compared and contrasted with the exact entanglement for the two test
systems. For Hooke's atom we also compare the entanglement to our previous
estimates of an LDA entanglement. These were obtained using a combination of
evolutionary algorithm and gradient descent, and using an LDA-based
perturbative approach. We finally discuss if the position-space information
entropy of the density---which can be obtained directly from the system density
and hence easily from density-functional theory methods---can be considered as
a proxy measure for the spatial entanglement for the test systems.Comment: 12 pages and 5 figures
Confirming the thermal Comptonization model for black hole X-ray emission in the low-hard state
Hard X-ray spectra of black hole binaries in the low/hard state are well
modeled by thermal Comptonization of soft seed photons by a corona-type region
with \thinspace{\thinspace}keV and optical depth around 1.
Previous spectral studies of 1E{\thinspace}1740.72942, including both the
soft and the hard X-ray bands, were always limited by gaps in the spectra or by
a combination of observations with imaging and non-imaging instruments. In this
study, we have used three rare nearly-simultaneous observations of
1E{\thinspace}1740.71942 by both XMM-Newton and INTEGRAL satellites to
combine spectra from four different imaging instruments with no data gaps, and
we successfully applied the Comptonization scenario to explain the broadband
X-ray spectra of this source in the low/hard state. For two of the three
observations, our analysis also shows that, models including Compton reflection
can adequately fit the data, in agreement with previous reports. We show that
the observations can also be modeled by a more detailed Comptonization scheme.
Furthermore, we find the presence of an iron K-edge absorption feature in one
occasion, which confirms what had been previously observed by Suzaku. Our
broadband analysis of this limited sample shows a rich spectral variability in
1E{\thinspace}1740.72942 at the low/hard state, and we address the possible
causes of these variations. More simultaneous soft/hard X-ray observations of
this system and other black-hole binaries would be very helpful in constraining
the Comptonization scenario and shedding more light on the physics of these
systems.Comment: 6 pages, two figures, accepted for publication in A&
Quantum test of the equivalence principle for atoms in superpositions of internal energy eigenstates
The Einstein Equivalence Principle (EEP) has a central role in the
understanding of gravity and space-time. In its weak form, or Weak Equivalence
Principle (WEP), it directly implies equivalence between inertial and
gravitational mass. Verifying this principle in a regime where the relevant
properties of the test body must be described by quantum theory has profound
implications. Here we report on a novel WEP test for atoms. A Bragg atom
interferometer in a gravity gradiometer configuration compares the free fall of
rubidium atoms prepared in two hyperfine states and in their coherent
superposition. The use of the superposition state allows testing genuine
quantum aspects of EEP with no classical analogue, which have remained
completely unexplored so far. In addition, we measure the Eotvos ratio of atoms
in two hyperfine levels with relative uncertainty in the low ,
improving previous results by almost two orders of magnitude.Comment: Accepted for publication in Nature Communicatio
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