139 research outputs found
Spin-boson dynamics: A unified approach from weak to strong coupling
We present a novel approximation scheme to describe the influence of a
harmonic bath on the dynamics of a two-level particle over almost the whole
regime of temperatures and coupling to the environment, for a wide class of
bath spectral densities. Starting from the exact path-integral solution for the
two-level system density matrix, effective intra-blip correlations are fully
included, while inter-blip and blip-sojourn interactions are considered up to
first order. In the proper regimes, an excellent agreement with conventional
perturbative approaches and ab-initio path-integral results is found.Comment: 6 pages, 4 figures (enlarged figures and minor changes to the text
Unimodal Thompson Sampling for Graph-Structured Arms
We study, to the best of our knowledge, the first Bayesian algorithm for
unimodal Multi-Armed Bandit (MAB) problems with graph structure. In this
setting, each arm corresponds to a node of a graph and each edge provides a
relationship, unknown to the learner, between two nodes in terms of expected
reward. Furthermore, for any node of the graph there is a path leading to the
unique node providing the maximum expected reward, along which the expected
reward is monotonically increasing. Previous results on this setting describe
the behavior of frequentist MAB algorithms. In our paper, we design a Thompson
Sampling-based algorithm whose asymptotic pseudo-regret matches the lower bound
for the considered setting. We show that -as it happens in a wide number of
scenarios- Bayesian MAB algorithms dramatically outperform frequentist ones. In
particular, we provide a thorough experimental evaluation of the performance of
our and state-of-the-art algorithms as the properties of the graph vary
Spin-boson dynamics beyond conventional perturbation theories
A novel approximation scheme is proposed to describe the dynamics of the
spin-boson problem. Being nonperturbative in the coupling strength nor in the
tunneling frequency, it gives reliable results over a wide regime of
temperatures and coupling strength to the thermal environment for a large class
of bath spectral densities.
We use a path-integral approach and start from the exact solution for the
two-level system population difference in the form of a generalized master
equation (GME). Then, we approximate inter-blip and blip-sojourns interactions
up to linear order, while retaining all intra-blip correlations to find the
kernels entering the GME in analytical form. Our approximation scheme, which we
call Weakly-Interacting Blip Approximation (WIBA), fully agrees with
conventional perturbative approximations in the tunneling matrix element
(Non-Interacting Blip Approximation) or in the system-bath coupling strength.Comment: 15 pages, 14 figure
Second spectrum of charge carrier density fluctuations in graphene due to trapping/detrapping processes
We investigate the second spectrum of charge carrier density fluctuations in
graphene within the McWorther model, where noise is induced by electron traps
in the substrate. Within this simple picture, we obtain a closed-form
expression including both Gaussian and non-Gaussian fluctuations. We show that
a very extended distribution of switching rates of the electron traps in the
substrate leads to a carrier density power spectrum with a non-trivial
structure on the scale of the measurement bandwidth. This explains the
appearance of a component in the Gaussian part of the second spectrum,
which adds up to the expected frequency-independent term. Finally, we find that
the non-Gaussian part of the second spectrum can become quantitatively relevant
by approaching extremely low temperatures.Comment: 10 pages, 2 figure
Does Intestine Morphology Still Have Secrets to Reveal? A Proposal about the “Ghost” Layer of the Bowel
In this brief Opinion paper, the term “muco-microbiotic layer” is introduced to describe the innermost layer of the intestinal wall. This layer may contribute not only to the overall health of the bowel, but also to that of extraintestinal organs. Its constituents, in terms of soluble molecules and nanovesicles, need to be studied further. Moreover, one can hypothesize the existence of an analogous layer in other organs, such as the airways or some parts of the genital tracts. Further studies on it are neede
Multi-Criteria Optimization of Real-Time DAGs on Heterogeneous Platforms under P-EDF
This paper tackles the problem of optimal placement of complex real-time embedded applications on heterogeneous platforms. Applications are composed of directed acyclic graphs of tasks, with each DAG having a minimum inter-arrival period for its activation requests, and an end-to-end deadline within which all of the computations need to terminate since each activation. The platforms of interest are heterogeneous power-aware multi-core platforms with DVFS capabilities, including big.LITTLE Arm architectures, and platforms with GPU or FPGA hardware accelerators with Dynamic Partial Reconfiguration capabilities. Tasks can be deployed on CPUs using partitioned EDF-based scheduling. Additionally, some of the tasks may have an alternate implementation available for one of the accelerators on the target platform, which are assumed to serve requests in non-preemptive FIFO order. The system can be optimized by: minimizing power consumption, respecting precise timing constraints; maximizing the applications’ slack, respecting given power consumption constraints; or even a combination of these, in a multi-objective formulation.
We propose an off-line optimization of the mentioned problem based on mixed-integer quadratic constraint programming (MIQCP). The optimization provides the DVFS configuration of all the CPUs (or accelerators) capable of frequency switching and the placement to be followed by each task in the DAGs, including the software-vs-hardware implementation choice for tasks that can be hardware-accelerated. For relatively big problems, we developed heuristic solvers capable of providing suboptimal solutions in a significantly reduced time compared to the MIQCP strategy, thus widening the applicability of the proposed framework.
We validate the approach by running a set of randomly generated DAGs on Linux under SCHED_DEADLINE, deployed onto two real boards, one with Arm big.LITTLE architecture, the other with FPGA acceleration, verifying that the experimental runs meet the theoretical expectations in terms of timing and power optimization goals
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