1,627 research outputs found
Angular Momentum of a Brane-world Model
In this paper we discuss the properties of the general covariant angular
momentum of a five-dimensional brane-world model. Through calculating the total
angular momentum of this model, we are able to analyze the properties of the
total angular momentum in the inflationary RS model. We show that the
space-like components of the total angular momentum of are all zero while the
others are non-zero, which agrees with the results from ordinary RS model.Comment: 8 pages; accepted by Chinese Physics
From Ground States to Local Hamiltonians
Traditional quantum physics solves ground states for a given Hamiltonian,
while quantum information science asks for the existence and construction of
certain Hamiltonians for given ground states. In practical situations, one
would be mainly interested in local Hamiltonians with certain interaction
patterns, such as nearest neighbour interactions on some type of lattices. A
necessary condition for a space to be the ground-state space of some local
Hamiltonian with a given interaction pattern, is that the maximally mixed state
supported on is uniquely determined by its reduced density matrices
associated with the given pattern, based on the principle of maximum entropy.
However, it is unclear whether this condition is in general also sufficient. We
examine the situations for the existence of such a local Hamiltonian to have
satisfying the necessary condition mentioned above as its ground-state
space, by linking to faces of the convex body of the local reduced states. We
further discuss some methods for constructing the corresponding local
Hamiltonians with given interaction patterns, mainly from physical points of
view, including constructions related to perturbation methods, local
frustration-free Hamiltonians, as well as thermodynamical ensembles.Comment: 11 pages, 2 figures, to be published in PR
Ground-State Spaces of Frustration-Free Hamiltonians
We study the ground-state space properties for frustration-free Hamiltonians.
We introduce a concept of `reduced spaces' to characterize local structures of
ground-state spaces. For a many-body system, we characterize mathematical
structures for the set of all the -particle reduced spaces, which
with a binary operation called join forms a semilattice that can be interpreted
as an abstract convex structure. The smallest nonzero elements in ,
called atoms, are analogs of extreme points. We study the properties of atoms
in and discuss its relationship with ground states of -local
frustration-free Hamiltonians. For spin-1/2 systems, we show that all the atoms
in are unique ground states of some 2-local frustration-free
Hamiltonians. Moreover, we show that the elements in may not be the
join of atoms, indicating a richer structure for beyond the convex
structure. Our study of deepens the understanding of ground-state
space properties for frustration-free Hamiltonians, from a new angle of reduced
spaces.Comment: 23 pages, no figur
Ozone predictabilities due to meteorological uncertainties in the Mexico City basin using ensemble forecasts
The purpose of the present study is to investigate the sensitivity of ozone (O<sub>3</sub>) predictions in the Mexico City Metropolitan Area (MCMA) to meteorological initial uncertainties and planetary boundary layer (PBL) parameterization schemes using state-of-the-art meteorological and photochemical prediction models through ensemble forecasts. The simulated periods (3, 9, 15 and 29 March 2006) represent four typical meteorological episodes ("South-Venting", "O<sub>3</sub>-North", "O<sub>3</sub>-South" and "Convection-North", respectively) in the Mexico City basin during the MCMA-2006/MILAGRO campaign. Our results demonstrate that the uncertainties in meteorological initial conditions have significant impacts on O<sub>3</sub> predictions, including peak time O<sub>3</sub> concentrations ([O<sub>3</sub>]), horizontal and vertical O<sub>3</sub> distributions, and temporal variations. The ensemble spread of the simulated peak [O<sub>3</sub>] averaged over the city's ambient monitoring sites can reach up to 10 ppb. The increasing uncertainties in meteorological fields during peak O<sub>3</sub> period contribute to the largest unpredictability in O<sub>3</sub> simulations, while the impacts of wind speeds and PBL height on [O<sub>3</sub>] are more straightforward and important. The magnitude of the ensemble spreads varies with different PBL schemes and meteorological episodes. The uncertainties in O<sub>3</sub> predictions caused by PBL schemes mainly come from their ability to represent the mixing layer height; but overall, these uncertainties are smaller than those from the uncertainties in meteorological initial conditions
Complete Characterization of the Ground Space Structure of Two-Body Frustration-Free Hamiltonians for Qubits
The problem of finding the ground state of a frustration-free Hamiltonian
carrying only two-body interactions between qubits is known to be solvable in
polynomial time. It is also shown recently that, for any such Hamiltonian,
there is always a ground state that is a product of single- or two-qubit
states. However, it remains unclear whether the whole ground space is of any
succinct structure. Here, we give a complete characterization of the ground
space of any two-body frustration-free Hamiltonian of qubits. Namely, it is a
span of tree tensor network states of the same tree structure. This
characterization allows us to show that the problem of determining the ground
state degeneracy is as hard as, but no harder than, its classical analog.Comment: 5pages, 3 figure
Quantum Capacity Approaching Codes for the Detected-Jump Channel
The quantum channel capacity gives the ultimate limit for the rate at which
quantum data can be reliably transmitted through a noisy quantum channel.
Degradable quantum channels are among the few channels whose quantum capacities
are known. Given the quantum capacity of a degradable channel, it remains
challenging to find a practical coding scheme which approaches capacity. Here
we discuss code designs for the detected-jump channel, a degradable channel
with practical relevance describing the physics of spontaneous decay of atoms
with detected photon emission. We show that this channel can be used to
simulate a binary classical channel with both erasures and bit-flips. The
capacity of the simulated classical channel gives a lower bound on the quantum
capacity of the detected-jump channel. When the jump probability is small, it
almost equals the quantum capacity. Hence using a classical capacity
approaching code for the simulated classical channel yields a quantum code
which approaches the quantum capacity of the detected-jump channel
Estimating the Material Properties of Fabric from Video
Passively estimating the intrinsic material properties of deformable objects moving in a natural environment is essential for scene understanding. We present a framework to automatically analyze videos of fabrics moving under various unknown wind forces, and recover two key material properties of the fabric: stiffness and area weight. We extend features previously developed to compactly represent static image textures to describe video textures, such as fabric motion. A discriminatively trained regression model is then used to predict the physical properties of fabric from these features. The success of our model is demonstrated on a new, publicly available database of fabric videos with corresponding measured ground truth material properties. We show that our predictions are well correlated with ground truth measurements of stiffness and density for the fabrics. Our contributions include: (a) a database that can be used for training and testing algorithms for passively predicting fabric properties from video, (b) an algorithm for predicting the material properties of fabric from a video, and (c) a perceptual study of humans' ability to estimate the material properties of fabric from videos and images.National Science Foundation (U.S.) (CGV-1111415)National Science Foundation (U.S.) (CGV-1212928)National Science Foundation (U.S.). Graduate Research FellowshipMassachusetts Institute of Technology (Intelligent Initiative Postdoctoral Fellowship)United States. Intelligence Advanced Research Projects Activity (D10PC20023
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