1,199 research outputs found
New higher-order transition in causal dynamical triangulations
We reinvestigate the recently discovered bifurcation phase transition in
Causal Dynamical Triangulations (CDT) and provide further evidence that it is a
higher order transition. We also investigate the impact of introducing matter
in the form of massless scalar fields to CDT. We discuss the impact of scalar
fields on the measured spatial volumes and fluctuation profiles in addition to
analysing how the scalar fields influence the position of the bifurcation
transition.Comment: 15 pages, 11 figures. Conforms with version accepted for publication
in Phys. Rev.
Simulation of some quantum gates, with decoherence
Methods and results for numerical simulations of one and two interacting
rf-Squid systems suitable for adiabatic quantum gates are presented. These are
based on high accuracy numerical solutions to the static and time dependent
Schroedinger equation for the full Squid Hamiltonian in one and two variables.
Among the points examined in the static analysis is the range of validity of
the effective two-state or ``spin 1/2'' picture. A range of parameters is
determined where the picture holds to good accuracy as the energy levels
undergo gate manipulations. Some general points are presented concerning the
relations between device parameters and ``good'' quantum mechanical state
spaces.
The time dependent simulations allow the examination of suitable conditions
for adiabatic behavior, and permits the introduction of a random noise to
simulate the effects of decoherence. A formula is derived and tested relating
the random noise to the decoherence rate. Sensitivity to device and operating
parameters for the logical gates NOT and CNOT are examined, with particular
attention to values of the tunnel parameter beta slightly above one. It appears
that with values of beta close to one, a quantum CNOT gate is possible even
with rather short decoherence times.
Many of the methods and results will apply to coupled double-potential well
systems in general.Comment: 26 pages, 15 figures, Some clarification added on decoherence
treatment, many small errors corrected, symbols on some figures enlarged,
refs added. No change in conten
Planckian Birth of the Quantum de Sitter Universe
We show that the quantum universe emerging from a nonperturbative, Lorentzian
sum-over-geometries can be described with high accuracy by a four-dimensional
de Sitter spacetime. By a scaling analysis involving Newton's constant, we
establish that the linear size of the quantum universes under study is in
between 17 and 28 Planck lengths. Somewhat surprisingly, the measured quantum
fluctuations around the de Sitter universe in this regime are to good
approximation still describable semiclassically. The numerical evidence
presented comes from a regularization of quantum gravity in terms of causal
dynamical triangulations.Comment: Article unchanged. Line added in acknowledgmen
Toward Realistic Intersecting D-Brane Models
We provide a pedagogical introduction to a recently studied class of
phenomenologically interesting string models, known as Intersecting D-Brane
Models. The gauge fields of the Standard-Model are localized on D-branes
wrapping certain compact cycles on an underlying geometry, whose intersections
can give rise to chiral fermions. We address the basic issues and also provide
an overview of the recent activity in this field. This article is intended to
serve non-experts with explanations of the fundamental aspects, and also to
provide some orientation for both experts and non-experts in this active field
of string phenomenology.Comment: 85 pages, 8 figures, Latex, Bibtex, v2: refs added, typos correcte
The Cleo Rich Detector
We describe the design, construction and performance of a Ring Imaging
Cherenkov Detector (RICH) constructed to identify charged particles in the CLEO
experiment. Cherenkov radiation occurs in LiF crystals, both planar and ones
with a novel ``sawtooth''-shaped exit surface. Photons in the wavelength
interval 135--165 nm are detected using multi-wire chambers filled with a
mixture of methane gas and triethylamine vapor. Excellent pion/kaon separation
is demonstrated.Comment: 75 pages, 57 figures, (updated July 26, 2005 to reflect reviewers
comments), to be published in NIM
Coupling of hard dimers to dynamical lattices via random tensors
We study hard dimers on dynamical lattices in arbitrary dimensions using a
random tensor model. The set of lattices corresponds to triangulations of the
d-sphere and is selected by the large N limit. For small enough dimer
activities, the critical behavior of the continuum limit is the one of pure
random lattices. We find a negative critical activity where the universality
class is changed as dimers become critical, in a very similar way hard dimers
exhibit a Yang-Lee singularity on planar dynamical graphs. Critical exponents
are calculated exactly. An alternative description as a system of
`color-sensitive hard-core dimers' on random branched polymers is provided.Comment: 12 page
On the top eigenvalue of heavy-tailed random matrices
We study the statistics of the largest eigenvalue lambda_max of N x N random
matrices with unit variance, but power-law distributed entries, P(M_{ij})~
|M_{ij}|^{-1-mu}. When mu > 4, lambda_max converges to 2 with Tracy-Widom
fluctuations of order N^{-2/3}. When mu < 4, lambda_max is of order
N^{2/mu-1/2} and is governed by Fr\'echet statistics. The marginal case mu=4
provides a new class of limiting distribution that we compute explicitely. We
extend these results to sample covariance matrices, and show that extreme
events may cause the largest eigenvalue to significantly exceed the
Marcenko-Pastur edge. Connections with Directed Polymers are briefly discussed.Comment: 4 pages, 2 figure
Potential effects of optical solar sail degredation on trajectory design
The optical properties of the thin metalized polymer films that are projected for solar sails are assumed to be affected by the erosive effects of the space environment. Their degradation behavior in the real space environment, however, is to a considerable degree indefinite, because initial ground test results are controversial and relevant inspace tests have not been made so far. The standard optical solar sail models that are currently used for trajectory design do not take optical degradation into account, hence its potential effects on trajectory design have not been investigated so far. Nevertheless, optical degradation is important for high-fidelity solar sail mission design, because it decreases both the magnitude of the solar radiation pressure force acting on the sail and also the sail control authority. Therefore, we propose a simple parametric optical solar sail degradation model that describes the variation of the sail film's optical coefficients with time, depending on the sail film's environmental history, i.e., the radiation dose. The primary intention of our model is not to describe the exact behavior of specific film-coating combinations in the real space environment, but to provide a more general parametric framework for describing the general optical degradation behavior of solar sails. Using our model, the effects of different optical degradation behaviors on trajectory design are investigated for various exemplary missions
- …