8,260 research outputs found
Problems with Fitting to the Power-Law Distribution
This short communication uses a simple experiment to show that fitting to a
power law distribution by using graphical methods based on linear fit on the
log-log scale is biased and inaccurate. It shows that using maximum likelihood
estimation (MLE) is far more robust. Finally, it presents a new table for
performing the Kolmogorov-Smirnof test for goodness-of-fit tailored to
power-law distributions in which the power-law exponent is estimated using MLE.
The techniques presented here will advance the application of complex network
theory by allowing reliable estimation of power-law models from data and
further allowing quantitative assessment of goodness-of-fit of proposed
power-law models to empirical data.Comment: 4 pages, 1 figure, 2 table
Symmetric M-ary phase discrimination using quantum-optical probe states
We present a theoretical study of minimum error probability discrimination,
using quantum- optical probe states, of M optical phase shifts situated
symmetrically on the unit circle. We assume ideal lossless conditions and full
freedom for implementing quantum measurements and for probe state selection,
subject only to a constraint on the average energy, i.e., photon number. In
particular, the probe state is allowed to have any number of signal and
ancillary modes, and to be pure or mixed. Our results are based on a simple
criterion that partitions the set of pure probe states into equivalence classes
with the same error probability performance. Under an energy constraint, we
find the explicit form of the state that minimizes the error probability. This
state is an unentangled but nonclassical single-mode state. The error
performance of the optimal state is compared with several standard states in
quantum optics. We also show that discrimination with zero error is possible
only beyond a threshold energy of (M - 1)/2. For the M = 2 case, we show that
the optimum performance is readily demonstrable with current technology. While
transmission loss and detector inefficiencies lead to a nonzero erasure
probability, the error rate conditional on no erasure is shown to remain the
same as the optimal lossless error rate.Comment: 13 pages, 10 figure
The spontaneous emergence of ordered phases in crumpled sheets
X-ray tomography is performed to acquire 3D images of crumpled aluminum
foils. We develop an algorithm to trace out the labyrinthian paths in the three
perpendicular cross sections of the data matrices. The tangent-tangent
correlation function along each path is found to decay exponentially with an
effective persistence length that shortens as the crumpled ball becomes more
compact. In the mean time, we observed ordered domains near the crust, similar
to the lamellae phase mixed by the amorphous portion in lyotropic liquid
crystals. The size and density of these domains grow with further compaction,
and their orientation favors either perpendicular or parallel to the radial
direction. Ordering is also identified near the core with an arbitrary
orientation, exemplary of the spontaneous symmetry breaking
Extending Feynman's Formalisms for Modelling Human Joint Action Coordination
The recently developed Life-Space-Foam approach to goal-directed human action
deals with individual actor dynamics. This paper applies the model to
characterize the dynamics of co-action by two or more actors. This dynamics is
modelled by: (i) a two-term joint action (including cognitive/motivatonal
potential and kinetic energy), and (ii) its associated adaptive path integral,
representing an infinite--dimensional neural network. Its feedback adaptation
loop has been derived from Bernstein's concepts of sensory corrections loop in
human motor control and Brooks' subsumption architectures in robotics.
Potential applications of the proposed model in human--robot interaction
research are discussed.
Keywords: Psycho--physics, human joint action, path integralsComment: 6 pages, Late
Correlations in Bipartite Collaboration Networks
Collaboration networks are studied as an example of growing bipartite
networks. These have been previously observed to have structure such as
positive correlations between nearest-neighbour degrees. However, a detailed
understanding of the origin of this phenomenon and the growth dynamics is
lacking. Both of these are analyzed empirically and simulated using various
models. A new one is presented, incorporating empirically necessary ingredients
such as bipartiteness and sublinear preferential attachment. This, and a
recently proposed model of team assembly both agree roughly with some empirical
observations and fail in several others.Comment: 13 pages, 17 figures, 2 table, submitted to JSTAT; manuscript
reorganized, figures and a table adde
Electron-lattice kinetics of metals heated by ultrashort laser pulses
We propose a kinetic model of transient nonequilibrium phenomena in metals
exposed to ultrashort laser pulses when heated electrons affect the lattice
through direct electron-phonon interaction. This model describes the
destruction of a metal under intense laser pumping. We derive the system of
equations for the metal, which consists of hot electrons and a cold lattice.
Hot electrons are described with the help of the Boltzmann equation and
equation of thermoconductivity. We use the equations of motion for lattice
displacements with the electron force included. The lattice deformation is
estimated immediately after the laser pulse up to the time of electron
temperature relaxation. An estimate shows that the ablation regime can be
achieved.Comment: 7 pages; Revtex. to appear in JETP 88, #1 (1999
Quantum SDP Solvers: Large Speed-ups, Optimality, and Applications to Quantum Learning
We give two quantum algorithms for solving semidefinite programs (SDPs) providing quantum speed-ups. We consider SDP instances with m constraint matrices, each of dimension n, rank at most r, and sparsity s. The first algorithm assumes access to an oracle to the matrices at unit cost. We show that it has run time OÌ(s^2(â((mÏ”)^(â10)) + â((nÏ”)^(â12))), with Ï” the error of the solution. This gives an optimal dependence in terms of m, n and quadratic improvement over previous quantum algorithms when m â n. The second algorithm assumes a fully quantum input model in which the matrices are given as quantum states. We show that its run time is OÌ (âm + poly(r))â
poly(log m,log n,B,Ï”^(â1)), with B an upper bound on the trace-norm of all input matrices. In particular the complexity depends only poly-logarithmically in n and polynomially in r.
We apply the second SDP solver to learn a good description of a quantum state with respect to a set of measurements: Given m measurements and a supply of copies of an unknown state Ï with rank at most r, we show we can find in time âmâ
poly(log m,log n,r,Ï”^(â1)) a description of the state as a quantum circuit preparing a density matrix which has the same expectation values as Ï on the m measurements, up to error Ï”. The density matrix obtained is an approximation to the maximum entropy state consistent with the measurement data considered in Jaynes' principle from statistical mechanics.
As in previous work, we obtain our algorithm by "quantizing" classical SDP solvers based on the matrix multiplicative weight method. One of our main technical contributions is a quantum Gibbs state sampler for low-rank Hamiltonians with a poly-logarithmic dependence on its dimension, which could be of independent interest
The Harmonic Measure for critical Potts clusters
We present a technique, which we call "etching," which we use to study the
harmonic measure of Fortuin-Kasteleyn clusters in the Q-state Potts model for
Q=1-4. The harmonic measure is the probability distribution of random walkers
diffusing onto the perimeter of a cluster. We use etching to study regions of
clusters which are extremely unlikely to be hit by random walkers, having
hitting probabilities down to 10^(-4600). We find good agreement between the
theoretical predictions of Duplantier and our numerical results for the
generalized dimension D(q), including regions of small and negative q.Comment: 20 pages, 10 figure
Equation of state of resonance-rich matter in the central cell in heavy-ion collisions at =200 AGeV
The equilibration of hot and dense nuclear matter produced in the central
cell of central Au+Au collisions at RHIC ( AGeV) energies is
studied within a microscopic transport model. The pressure in the cell becomes
isotropic at fm/ after beginning of the collision. Within the
next 15 fm/ the expansion of matter in the cell proceeds almost
isentropically with the entropy per baryon ratio , and the
equation of state in the plane has a very simple form,
. Comparison with the statistical model of an ideal hadron gas
indicates that the time fm/c may be too short to reach the fully
equilibrated state. Particularly, the creation of long-lived resonance-rich
matter in the cell decelerates the relaxation to chemical equilibrium. This
resonance-abundant state can be detected experimentally after the thermal
freeze-out of particles.Comment: LATEX, 21 pages incl. 7 figure
- âŠ