25,395 research outputs found
Identifying Functional Thermodynamics in Autonomous Maxwellian Ratchets
We introduce a family of Maxwellian Demons for which correlations among
information bearing degrees of freedom can be calculated exactly and in compact
analytical form. This allows one to precisely determine Demon functional
thermodynamic operating regimes, when previous methods either misclassify or
simply fail due to approximations they invoke. This reveals that these Demons
are more functional than previous candidates. They too behave either as
engines, lifting a mass against gravity by extracting energy from a single heat
reservoir, or as Landauer erasers, consuming external work to remove
information from a sequence of binary symbols by decreasing their individual
uncertainty. Going beyond these, our Demon exhibits a new functionality that
erases bits not by simply decreasing individual-symbol uncertainty, but by
increasing inter-bit correlations (that is, by adding temporal order) while
increasing single-symbol uncertainty. In all cases, but especially in the new
erasure regime, exactly accounting for informational correlations leads to
tight bounds on Demon performance, expressed as a refined Second Law of
Thermodynamics that relies on the Kolmogorov-Sinai entropy for dynamical
processes and not on changes purely in system configurational entropy, as
previously employed. We rigorously derive the refined Second Law under minimal
assumptions and so it applies quite broadly---for Demons with and without
memory and input sequences that are correlated or not. We note that general
Maxwellian Demons readily violate previously proposed, alternative such bounds,
while the current bound still holds.Comment: 13 pages, 9 figures,
http://csc.ucdavis.edu/~cmg/compmech/pubs/mrd.ht
How to identify the youngest protostars
We study the transition from a prestellar core to a Class 0 protostar, using
SPH to simulate the dynamical evolution, and a Monte Carlo radiative transfer
code to generate the SED and isophotal maps. For a prestellar core illuminated
by the standard interstellar radiation field, the luminosity is low and the SED
peaks at ~190 micron. Once a protostar has formed, the luminosity rises (due to
a growing contribution from accretion onto the protostar) and the peak of the
SED shifts to shorter wavelengths (~80-100 micron). However, by the end of the
Class 0 phase, the accretion rate is falling, the luminosity has decreased, and
the peak of the SED shifts back towards longer wavelengths (90-150 micron). In
our simulations, the density of material around the protostar remains
sufficiently high well into the Class 0 phase that the protostar only becomes
visible in the NIR if it is displaced from the centre dynamically. Raw submm/mm
maps of Class 0 protostars tend to be much more centrally condensed than those
of prestellar cores. However, when convolved with a typical telescope beam, the
difference in central concentration is less marked, although the Class 0
protostars appear more circular. Our results suggest that, if a core is deemed
to be prestellar on the basis of having no associated IRAS source, no cm radio
emission, and no outflow, but it has a circular appearance and an SED which
peaks at wavelengths below ~170 micron, it may well contain a very young Class
0 protostar.Comment: Accepted by A&A (avaliable with high-res images at
http://carina.astro.cf.ac.uk/pub/Dimitrios.Stamatellos/publications
Shortcuts to Thermodynamic Computing: The Cost of Fast and Faithful Erasure
Landauer's Principle states that the energy cost of information processing
must exceed the product of the temperature and the change in Shannon entropy of
the information-bearing degrees of freedom. However, this lower bound is
achievable only for quasistatic, near-equilibrium computations -- that is, only
over infinite time. In practice, information processing takes place in finite
time, resulting in dissipation and potentially unreliable logical outcomes. For
overdamped Langevin dynamics, we show that counterdiabatic potentials can be
crafted to guide systems rapidly and accurately along desired computational
paths, providing shortcuts that allows for the precise design of finite-time
computations. Such shortcuts require additional work, beyond Landauer's bound,
that is irretrievably dissipated into the environment. We show that this
dissipated work is proportional to the computation rate as well as the square
of the information-storing system's length scale. As a paradigmatic example, we
design shortcuts to erase a bit of information metastably stored in a
double-well potential. Though dissipated work generally increases with erasure
fidelity, we show that it is possible perform perfect erasure in finite time
with finite work. We also show that the robustness of information storage
affects the energetic cost of erasure---specifically, the dissipated work
scales as the information lifetime of the bistable system. Our analysis exposes
a rich and nuanced relationship between work, speed, size of the
information-bearing degrees of freedom, storage robustness, and the difference
between initial and final informational statistics.Comment: 19 pages, 7 figures;
http://csc.ucdavis.edu/~cmg/compmech/pubs/scte.ht
Stability of Magneto-optical Traps with Large Field Gradients: Limits on the Tight Confinement of Single Atoms
We report measurements of the stability of magneto-optical traps (MOTs) for neutral atoms in the limit of tight confinement of a single atom. For quadrupole magnetic field gradients at the trap center greater than ∼1 kG/cm, we find that stochastic diffusion of atoms out of the trapping volume becomes the dominant particle loss mechanism, ultimately limiting the MOT size to greater than ∼5 μm. We measured and modeled the diffusive loss rate as a function of laser power, detuning, and field gradient for trapped cesium atoms. In addition, for as few as two atoms, the collisional loss rates become very high for tightly confined traps, allowing the direct observation of isolated two-body atomic collisions in a MOT
Revealing Network Connectivity From Dynamics
We present a method to infer network connectivity from collective dynamics in
networks of synchronizing phase oscillators. We study the long-term stationary
response to temporally constant driving. For a given driving condition,
measuring the phase differences and the collective frequency reveals
information about how the oscillators are interconnected. Sufficiently many
repetitions for different driving conditions yield the entire network
connectivity from measuring the dynamics only. For sparsely connected networks
we obtain good predictions of the actual connectivity even for formally
under-determined problems.Comment: 10 pages, 4 figure
Polarization squeezing of light by single passage through an atomic vapor
We have studied relative-intensity fluctuations for a variable set of
orthogonal elliptic polarization components of a linearly polarized laser beam
traversing a resonant Rb vapor cell. Significant polarization squeezing
at the threshold level (-3dB) required for the implementation of several
continuous variables quantum protocols was observed. The extreme simplicity of
the setup, based on standard polarization components, makes it particularly
convenient for quantum information applications.Comment: Revised version. Minor changes. four pages, three figure
Religious leaders\u27 perceptions of advance care planning: a secondary analysis of interviews with Buddhist, Christian, Hindu, Islamic, Jewish, Sikh and Bahai leaders
Background: International guidance for advance care planning (ACP) supports the integration of spiritual and religious aspects of care within the planning process. Religious leaders’ perspectives could improve how ACP programs respect patients’ faith backgrounds. This study aimed to examine: (i) how religious leaders understand and consider ACP and its implications, including (ii) how religion affects followers’ approaches to end-of-life care and ACP, and (iii) their implications for healthcare.
Methods: Interview transcripts from a primary qualitative study conducted with religious leaders to inform an ACP website, ACPTalk, were used as data in this study. ACPTalk aims to assist health professionals conduct sensitive conversations with people from different religious backgrounds. A qualitative secondary analysis conducted on the interview transcripts focussed on religious leaders’ statements related to this study’s aims. Interview transcripts were thematically analysed using an inductive, comparative, and cyclical procedure informed by grounded theory.
Results: Thirty-five religious leaders (26 male; mean 58.6-years-old), from eight Christian and six non-Christian (Jewish, Buddhist, Islamic, Hindu, Sikh, Bahá’Ã) backgrounds were included. Three themes emerged which focussed on: religious leaders’ ACP understanding and experiences; explanations for religious followers’ approaches towards end-of-life care; and health professionals’ need to enquire about how religion matters. Most leaders had some understanding of ACP and, once fully comprehended, most held ACP in positive regard. Religious followers’ preferences for end-of-life care reflected family and geographical origins, cultural traditions, personal attitudes, and religiosity and faith interpretations. Implications for healthcare included the importance of avoiding generalisations and openness to individualised and/ or standardised religious expressions of one’s religion.
Conclusions: Knowledge of religious beliefs and values around death and dying could be useful in preparing health professionals for ACP with patients from different religions but equally important is avoidance of assumptions. Community-based initiatives, programs and faith settin
Instability and spatiotemporal rheochaos in a shear-thickening fluid model
We model a shear-thickening fluid that combines a tendency to form
inhomogeneous, shear-banded flows with a slow relaxational dynamics for fluid
microstructure. The interplay between these factors gives rich dynamics, with
periodic regimes (oscillating bands, travelling bands, and more complex
oscillations) and spatiotemporal rheochaos. These phenomena, arising from
constitutive nonlinearity not inertia, can occur even when the steady-state
flow curve is monotonic. Our model also shows rheochaos in a low-dimensional
truncation where sharply defined shear bands cannot form
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