31,601 research outputs found
Quantum ether: photons and electrons from a rotor model
We give an example of a purely bosonic model -- a rotor model on the 3D cubic
lattice -- whose low energy excitations behave like massless U(1) gauge bosons
and massless Dirac fermions. This model can be viewed as a ``quantum ether'': a
medium that gives rise to both photons and electrons. It illustrates a general
mechanism for the emergence of gauge bosons and fermions known as ``string-net
condensation.'' Other, more complex, string-net condensed models can have
excitations that behave like gluons, quarks and other particles in the standard
model. This suggests that photons, electrons and other elementary particles may
have a unified origin: string-net condensation in our vacuum.Comment: 10 pages, 6 figures, RevTeX4. Home page http://dao.mit.edu/~we
A system for learning statistical motion patterns
Analysis of motion patterns is an effective approach for anomaly detection and behavior prediction. Current approaches for the analysis of motion patterns depend on known scenes, where objects move in predefined ways. It is highly desirable to automatically construct object motion patterns which reflect the knowledge of the scene. In this paper, we present a system for automatically learning motion patterns for anomaly detection and behavior prediction based on a proposed algorithm for robustly tracking multiple objects. In the tracking algorithm, foreground pixels are clustered using a fast accurate fuzzy k-means algorithm. Growing and prediction of the cluster centroids of foreground pixels ensure that each cluster centroid is associated with a moving object in the scene. In the algorithm for learning motion patterns, trajectories are clustered hierarchically using spatial and temporal information and then each motion pattern is represented with a chain of Gaussian distributions. Based on the learned statistical motion patterns, statistical methods are used to detect anomalies and predict behaviors. Our system is tested using image sequences acquired, respectively, from a crowded real traffic scene and a model traffic scene. Experimental results show the robustness of the tracking algorithm, the efficiency of the algorithm for learning motion patterns, and the encouraging performance of algorithms for anomaly detection and behavior prediction
A system for learning statistical motion patterns
Analysis of motion patterns is an effective approach for anomaly detection and behavior prediction. Current approaches for the analysis of motion patterns depend on known scenes, where objects move in predefined ways. It is highly desirable to automatically construct object motion patterns which reflect the knowledge of the scene. In this paper, we present a system for automatically learning motion patterns for anomaly detection and behavior prediction based on a proposed algorithm for robustly tracking multiple objects. In the tracking algorithm, foreground pixels are clustered using a fast accurate fuzzy k-means algorithm. Growing and prediction of the cluster centroids of foreground pixels ensure that each cluster centroid is associated with a moving object in the scene. In the algorithm for learning motion patterns, trajectories are clustered hierarchically using spatial and temporal information and then each motion pattern is represented with a chain of Gaussian distributions. Based on the learned statistical motion patterns, statistical methods are used to detect anomalies and predict behaviors. Our system is tested using image sequences acquired, respectively, from a crowded real traffic scene and a model traffic scene. Experimental results show the robustness of the tracking algorithm, the efficiency of the algorithm for learning motion patterns, and the encouraging performance of algorithms for anomaly detection and behavior prediction
Quantum orders in an exact soluble model
We find all the exact eigenstates and eigenvalues of a spin-1/2 model on
square lattice: . We show
that the ground states for have different quantum orders
described by Z2A and Z2B projective symmetry groups. The phase transition at
represents a new kind of phase transitions that changes quantum orders
but not symmetry. Both the Z2A and Z2B states are described by lattice
gauge theories at low energies. They have robust topologically degenerate
ground states and gapless edge excitations.Comment: 4 pages, RevTeX4, More materials on topological/quantum orders and
quantum computing can be found in http://dao.mit.edu/~we
Effects of land incremental value allocation on rural operational construction land (ROCL) under market mechanism: case study in China
The use of the market mechanism to convert the rural operational construction land (ROCL) into urban construction land without ownership changes is currently being introduced into reform pilot projects in China, changing the only form of governmental expropriation in the past. The new system allows rural collective economic organizations and members of the rural collective economy to directly participate in the allocation of land incremental value increases due to changes in land use. This replaces the previous way of allocating only the original use compensation from the government. This paper investigates the collectively owned new system, to establish the positive effects and shortcoming of the new model. Three cases are applied for the analysis using inductive-deductive reasoning methodology based on the property right and landrent theories. We have found that local government land adjustment charges on the transactions of rural construction land are suggested to be from 16 to 20 percent. The share ownership quantification model (SOQM) of land incremental value allocation between the collective economic organizations and members is effective and beneficial to the development of the rural collective economy and its members
Dynamical behavior of interacting dark energy in loop quantum cosmology
The dynamical behaviors of interacting dark energy in loop quantum cosmology
are discussed in this paper. Based on defining three dimensionless variables,
we simplify the equations of the fixed points. The fixed points for interacting
dark energy can be determined by the Friedmann equation coupled with the
dynamical equations {in Einstein cosmology}. But in loop quantum cosmology,
besides the Friedmann equation, the conversation equation also give a constrain
on the fixed points. The difference of stability properties for the fixed
points in loop quantum cosmology and the ones in Einstein cosmology also have
been discussed.Comment: 7 pages, 5 figure
Translation-symmetry protected topological orders on lattice
In this paper we systematically study a simple class of translation-symmetry
protected topological orders in quantum spin systems using slave-particle
approach. The spin systems on square lattice are translation invariant, but may
break any other symmetries. We consider topologically ordered ground states
that do not spontaneously break any symmetry. Those states can be described by
Z2A or Z2B projective symmetry group. We find that the Z2A translation
symmetric topological orders can still be divided into 16 sub-classes
corresponding to 16 new translation-symmetry protected topological orders. We
introduced four topological indices at , , , to characterize those 16 new
topological orders. We calculated the topological degeneracies and crystal
momenta for those 16 topological phases on even-by-even, even-by-odd,
odd-by-even, and odd-by-odd lattices, which allows us to physically measure
such topological orders. We predict the appearance of gapless fermionic
excitations at the quantum phase transitions between those symmetry protected
topological orders. Our result can be generalized to any dimensions. We find
256 translation-symmetry protected Z2A topological orders for a system on 3D
lattice
String and Membrane condensation on 3D lattices
In this paper, we investigate the general properties of lattice spin models
that have string and/or membrane condensed ground states. We discuss the
properties needed to define a string or membrane operator. We study three 3D
spin models which lead to Z_2 gauge theory at low energies. All the three
models are exactly soluble and produce topologically ordered ground states. The
first model contains both closed-string and closed-membrane condensations. The
second model contains closed-string condensation only. The ends of open-strings
behave like fermionic particles. The third model also has condensations of
closed membranes and closed strings. The ends of open strings are bosonic while
the edges of open membranes are fermionic. The third model contains a new type
of topological order.Comment: 10 pages, RevTeX
Exact solution of gyration radius of individual's trajectory for a simplified human mobility model
Gyration radius of individual's trajectory plays a key role in quantifying
human mobility patterns. Of particular interests, empirical analyses suggest
that the growth of gyration radius is slow versus time except the very early
stage and may eventually arrive to a steady value. However, up to now, the
underlying mechanism leading to such a possibly steady value has not been well
understood. In this Letter, we propose a simplified human mobility model to
simulate individual's daily travel with three sequential activities: commuting
to workplace, going to do leisure activities and returning home. With the
assumption that individual has constant travel speed and inferior limit of time
at home and work, we prove that the daily moving area of an individual is an
ellipse, and finally get an exact solution of the gyration radius. The
analytical solution well captures the empirical observation reported in [M. C.
Gonz`alez et al., Nature, 453 (2008) 779]. We also find that, in spite of the
heterogeneous displacement distribution in the population level, individuals in
our model have characteristic displacements, indicating a completely different
mechanism to the one proposed by Song et al. [Nat. Phys. 6 (2010) 818].Comment: 4 pages, 4 figure
Equation of motion for multiqubit entanglement in multiple independent noisy channels
We investigate the possibility and conditions to factorize the entanglement
evolution of a multiqubit system passing through multi-sided noisy channels. By
means of a lower bound of concurrence (LBC) as entanglement measure, we derive
an explicit formula of LBC evolution of the N-qubit generalized
Greenberger-Horne-Zeilinger (GGHZ) state under some typical noisy channels,
based on which two kinds of factorizing conditions for the LBC evolution are
presented. In this case, the time-dependent LBC can be determined by a product
of initial LBC of the system and the LBC evolution of a maximally entangled
GGHZ state under the same multi-sided noisy channels. We analyze the realistic
situations where these two kinds of factorizing conditions can be satisfied. In
addition, we also discuss the dependence of entanglement robustness on the
number of the qubits and that of the noisy channels.Comment: 14 page
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