4,674 research outputs found
Cross-Kerr nonlinearity between continuous-mode coherent states and single photons
Weak cross-Kerr nonlinearities between single photons and coherent states are
the basis for many applications in quantum information processing. These
nonlinearities have so far mainly been discussed in terms of highly idealized
single-mode models. We develop a general theory of the interaction between
continuous-mode photonic pulses and apply it to the case of a single photon
interacting with a coherent state. We quantitatively study the validity of the
usual single-mode approximation using the concepts of fidelity and conditional
phase. We show that high fidelities, non-zero conditional phases and high
photon numbers are compatible, under conditions where the pulses fully pass
through each other and where unwanted transverse-mode effects are suppressed.Comment: 8 pages, 2 figures, more general results in section V
Non-Markovian reduced dynamics and entanglement evolution of two coupled spins in a quantum spin environment
The exact quantum dynamics of the reduced density matrix of two coupled spin
qubits in a quantum Heisenberg XY spin star environment in the thermodynamic
limit at arbitrarily finite temperatures is obtained using a novel operator
technique. In this approach, the transformed Hamiltonian becomes effectively
Jaynes-Cumming like and thus the analysis is also relevant to cavity quantum
electrodynamics. This special operator technique is mathematically simple and
physically clear, and allows us to treat systems and environments that could
all be strongly coupled mutually and internally. To study their entanglement
evolution, the concurrence of the reduced density matrix of the two coupled
central spins is also obtained exactly. It is shown that the dynamics of the
entanglement depends on the initial state of the system and the coupling
strength between the two coupled central spins, the thermal temperature of the
spin environment and the interaction between the constituents of the spin
environment. We also investigate the effect of detuning which in our model can
be controlled by the strength of a locally applied external magnetic field. It
is found that the detuning has a significant effect on the entanglement
generation between the two spin qubits.Comment: 9 pages (two-coulumn), 6 figures. To appear in Phys. Rev.
Anomalous quantum confined Stark effects in stacked InAs/GaAs self-assembled quantum dots
Vertically stacked and coupled InAs/GaAs self-assembled quantum dots (SADs)
are predicted to exhibit a strong non-parabolic dependence of the interband
transition energy on the electric field, which is not encountered in single SAD
structures nor in other types of quantum structures. Our study based on an
eight-band strain-dependent Hamiltonian indicates that
this anomalous quantum confined Stark effect is caused by the three-dimensional
strain field distribution which influences drastically the hole states in the
stacked SAD structures.Comment: 4 pages, 4 figure
Limitations of practical multi-photon decoherence-free states
It is shown in this paper that decoherence-free subspace (DFS) of practical
multi-photon polarization can not avoid the exponential decoherence even in the
same extra-environment if the photons are frequency-anticorrelated. The reason
lies in that the condition of collective decoherence is not satisfied in this
case. As an example, the evolution of biphoton's decoherence-free state is
given. Possible solution for feasible multi-photon's DFS state is also given.Comment: 6 pages, no figur
3D Face Reconstruction from Light Field Images: A Model-free Approach
Reconstructing 3D facial geometry from a single RGB image has recently
instigated wide research interest. However, it is still an ill-posed problem
and most methods rely on prior models hence undermining the accuracy of the
recovered 3D faces. In this paper, we exploit the Epipolar Plane Images (EPI)
obtained from light field cameras and learn CNN models that recover horizontal
and vertical 3D facial curves from the respective horizontal and vertical EPIs.
Our 3D face reconstruction network (FaceLFnet) comprises a densely connected
architecture to learn accurate 3D facial curves from low resolution EPIs. To
train the proposed FaceLFnets from scratch, we synthesize photo-realistic light
field images from 3D facial scans. The curve by curve 3D face estimation
approach allows the networks to learn from only 14K images of 80 identities,
which still comprises over 11 Million EPIs/curves. The estimated facial curves
are merged into a single pointcloud to which a surface is fitted to get the
final 3D face. Our method is model-free, requires only a few training samples
to learn FaceLFnet and can reconstruct 3D faces with high accuracy from single
light field images under varying poses, expressions and lighting conditions.
Comparison on the BU-3DFE and BU-4DFE datasets show that our method reduces
reconstruction errors by over 20% compared to recent state of the art
An introduction to crowdsourcing for language and multimedia technology research
Language and multimedia technology research often relies on
large manually constructed datasets for training or evaluation of algorithms and systems. Constructing these datasets is often expensive with significant challenges in terms of recruitment of personnel to carry out the work. Crowdsourcing methods using scalable pools of workers available on-demand offers a flexible means of rapid low-cost construction of many of these datasets to support existing research requirements and potentially promote new research initiatives that would otherwise not be possible
Mean Free Path and Energy Fluctuations in Quantum Chaotic Billiards
The elastic mean free path of carriers in a recently introduced model of
quantum chaotic billiards in two and three dimensions is calculated. The model
incorporates surface roughness at a microscopic scale by randomly choosing the
atomic levels at the surface sites between -W/2 and W/2. Surface roughness
yields a mean free path l that decreases as L/W^2 as W increases, L being the
linear size of the system. But this diminution ceases when the surface layer
begins to decouple from the bulk for large enough values of W, leaving more or
less unperturbed states on the bulk. Consequently, the mean free path shows a
minimum of about L/2 for W of the order of the band width. Energy fluctuations
reflect the behavior of the mean free path. At small energy scales, strong
level correlations manifest themselves by small values of the number of levels
variance Sigma^2(E) that are close to Random Matrix Theory (RMT) in all cases.
At larger energy scales, fluctuations are below the logarithmic behavior of RMT
for l > L, and above RMT value when l < L.Comment: 8 twocolumn pages, seven figures, revtex and epsf macros. To be
published in Physical Review B
Coherent Matter Wave Transport in Speckle Potentials
This article studies multiple scattering of matter waves by a disordered
optical potential in two and in three dimensions. We calculate fundamental
transport quantities such as the scattering mean free path , the
Boltzmann transport mean free path \elltrb, and the Boltzmann diffusion
constant , using a diagrammatic Green functions approach. Coherent
multiple scattering induces interference corrections known as weak localization
which entail a reduced diffusion constant. We derive the corresponding
expressions for matter wave transport in an correlated speckle potential and
provide the relevant parameter values for a possible experimental study of this
coherent transport regime, including the critical crossover to the regime of
strong or Anderson localization.Comment: 33 pages, minor corrections, published versio
Integration of Lie 2-algebras and their morphisms
Given a strict Lie 2-algebra, we can integrate it to a strict Lie 2-group by
integrating the corresponding Lie algebra crossed module. On the other hand,
the integration procedure of Getzler and Henriques will also produce a 2-group.
In this paper, we show that these two integration results are Morita
equivalent. As an application, we integrate a non-strict morphism between Lie
algebra crossed modules to a generalized morphism between their corresponding
Lie group crossed modules.Comment: 19 pages, Lett. Math. Phys. 102 (2), (2012.11), 223-24
Tunable Electron Multibunch Production in Plasma Wakefield Accelerators
Synchronized, independently tunable and focused J-class laser pulses are
used to release multiple electron populations via photo-ionization inside an
electron-beam driven plasma wave. By varying the laser foci in the laboratory
frame and the position of the underdense photocathodes in the co-moving frame,
the delays between the produced bunches and their energies are adjusted. The
resulting multibunches have ultra-high quality and brightness, allowing for
hitherto impossible bunch configurations such as spatially overlapping bunch
populations with strictly separated energies, which opens up a new regime for
light sources such as free-electron-lasers
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