2,178 research outputs found
Soft Theorems For Shift-Symmetric Cosmologies
We derive soft theorems for single-clock cosmologies that enjoy a shift
symmetry. These so-called consistency conditions arise from a combination of a
large diffeomorphism and the internal shift-symmetry and fix the squeezed limit
of all correlators with a soft scalar mode. As an application, we show that our
results reproduce the squeezed bispectrum for Ultra-slow-roll inflation, a
particular shift-symmetric, non-attractor model which is known to violate
Maldacena's consistency relation. Similar results have been previously obtained
by Mooij and Palma using background-wave methods. Our results shed new light on
the infrared structure of single-clock cosmological spacetimes.Comment: 4 pages, v2: citation added, v3: citations added and edited in
accordance with published versio
Quantum resonant activation
Quantum resonant activation is investigated for the archetype setup of an
externally driven two-state (spin-boson) system subjected to strong dissipation
by means of both analytical and extensive numerical calculations. The
phenomenon of resonant activation emerges in the presence of either randomly
fluctuating or deterministic periodically varying driving fields. Addressing
the incoherent regime, a characteristic minimum emerges in the mean first
passage time to reach an absorbing neighboring state whenever the intrinsic
time scale of the modulation matches the characteristic time scale of the
system dynamics. For the case of deterministic periodic driving, the first
passage time probability density function (pdf) displays a complex,
multi-peaked behavior, which depends crucially on the details of initial phase,
frequency, and strength of the driving. As an interesting feature we find that
the mean first passage time enters the resonant activation regime at a critical
frequency which depends very weakly on the strength of the driving.
Moreover, we provide the relation between the first passage time pdf and the
statistics of residence times.Comment: 14 pages, 13 figure
Quantum dissipative dynamics of a bistable system in the sub-Ohmic to super-Ohmic regime
We investigate the quantum dynamics of a multilevel bistable system coupled
to a bosonic heat bath beyond the perturbative regime. We consider different
spectral densities of the bath, in the transition from sub-Ohmic to super-Ohmic
dissipation, and different cutoff frequencies. The study is carried out by
using the real-time path integral approach of the Feynman-Vernon influence
functional. We find that, in the crossover dynamical regime characterized by
damped \emph{intrawell} oscillations and incoherent tunneling, the short time
behavior and the time scales of the relaxation starting from a nonequilibrium
initial condition depend nontrivially on the spectral properties of the heat
bath.Comment: 16 pages, 7 figure
An Analytical Solution for Probabilistic Guarantees of Reservation Based Soft Real-Time Systems
We show a methodology for the computation of the probability of deadline miss
for a periodic real-time task scheduled by a resource reservation algorithm. We
propose a modelling technique for the system that reduces the computation of
such a probability to that of the steady state probability of an infinite state
Discrete Time Markov Chain with a periodic structure. This structure is
exploited to develop an efficient numeric solution where different
accuracy/computation time trade-offs can be obtained by operating on the
granularity of the model. More importantly we offer a closed form conservative
bound for the probability of a deadline miss. Our experiments reveal that the
bound remains reasonably close to the experimental probability in one real-time
application of practical interest. When this bound is used for the optimisation
of the overall Quality of Service for a set of tasks sharing the CPU, it
produces a good sub-optimal solution in a small amount of time.Comment: IEEE Transactions on Parallel and Distributed Systems, Volume:27,
Issue: 3, March 201
High-Performance Passive Macromodeling Algorithms for Parallel Computing Platforms
This paper presents a comprehensive strategy for fast generation of passive macromodels of linear devices and interconnects on parallel computing hardware. Starting from a raw characterization of the structure in terms of frequency-domain tabulated scattering responses, we perform a rational curve fitting and a postprocessing passivity enforcement. Both algorithms are parallelized and cast in a form that is suitable for deployment on shared-memory multicore platforms. Particular emphasis is placed on the passivity characterization step, which is performed using two complementary strategies. The first uses an iterative restarted and deflated rational Arnoldi process to extract the imaginary Hamiltonian eigenvalues associated with the model. The second is based on an accuracy-controlled adaptive sampling. Various parallelization strategies are discussed for both schemes, with particular care on load balancing between different computing threads and memory occupation. The resulting parallel macromodeling flow is demonstrated on a number of medium- and large-scale structures, showing good scalability up to 16 computational core
Finite-temperature geometric properties of the Kitaev honeycomb model
We study finite-temperature topological properties of the Kitaevâs spin-honeycomb model in the vortex-free sector with the use of the recently introduced mean Uhlmann curvature. We employ an appropriate fermionization procedure to study the system as a two-band p-wave superconductor described by a Bogoliubovâde Gennes Hamiltonian. This allows us to study relevant quantities such as Berry and mean Uhlmann curvatures in a simple setting. More specifically, we consider the spin honeycomb in the presence of an external magnetic field breaking time-reversal symmetry. The introduction of such an external perturbation opens up a gap in the phase of the system characterized by non-Abelian statistics. The resulting model belongs to a symmetry-protected class, so that the Uhlmann number can be analyzed. We first consider the Berry curvature on a particular evolution line over the phase diagram. The mean Uhlmann curvature and the Uhlmann number are then analyzed by assuming a thermal state. The mean Uhlmann curvature describes a crossover effect as temperature rises. In the trivial phase, a nonmonotonic dependence of the Uhlmann number, as temperature increases, is reported and explained
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