3,191 research outputs found
Mapping for the Masses: Accessing Web 2.0 through Crowdsourcing
The authors describe how we are harnessing the power of web 2.0 technologies to create new approaches to collecting, mapping, and sharing geocoded data. The authors begin with GMapCreator that lets users fashion new maps using Google Maps as a base. The authors then describe MapTube that enables users to archive maps and demonstrate how it can be used in a variety of contexts to share map information, to put existing maps into a form that can be shared, and to create new maps from the bottom-up using a combination of crowdcasting, crowdsourcing, and traditional broadcasting. The authors conclude by arguing that such tools are helping to define a neogeography that is essentially "mapping for the masses,'' while noting that there are many issues of quality, accuracy, copyright, and trust that will influence the impact of these tools on map-based communication
Near-optimal protocols in complex nonequilibrium transformations
The development of sophisticated experimental means to control nanoscale
systems has motivated efforts to design driving protocols which minimize the
energy dissipated to the environment. Computational models are a crucial tool
in this practical challenge. We describe a general method for sampling an
ensemble of finite-time, nonequilibrium protocols biased towards a low average
dissipation. We show that this scheme can be carried out very efficiently in
several limiting cases. As an application, we sample the ensemble of
low-dissipation protocols that invert the magnetization of a 2D Ising model and
explore how the diversity of the protocols varies in response to constraints on
the average dissipation. In this example, we find that there is a large set of
protocols with average dissipation close to the optimal value, which we argue
is a general phenomenon.Comment: 6 pages and 3 figures plus 4 pages and 5 figures of supplemental
materia
Measuring thermodynamic length
Thermodynamic length is a metric distance between equilibrium thermodynamic
states. Among other interesting properties, this metric asymptotically bounds
the dissipation induced by a finite time transformation of a thermodynamic
system. It is also connected to the Jensen-Shannon divergence, Fisher
information and Rao's entropy differential metric. Therefore, thermodynamic
length is of central interest in understanding matter out-of-equilibrium. In
this paper, we will consider how to define thermodynamic length for a small
system described by equilibrium statistical mechanics and how to measure
thermodynamic length within a computer simulation. Surprisingly, Bennett's
classic acceptance ratio method for measuring free energy differences also
measures thermodynamic length.Comment: 4 pages; Typos correcte
On the Quantum Jarzynski Identity
In this note, we will discuss how to compactly express and prove the
Jarzynski identity for an open quantum system with dissipative dynamics. We
will avoid explicitly measuring the work directly, which is tantamount to
continuously monitoring the system, and instead measure the heat flow from the
environment. We represent the measurement of heat flow with Hermitian map
superoperators that act on the system density matrix. Hermitian maps provide a
convenient and compact representation of sequential measurement and correlation
functions.Comment: 4 page
Non-equilibrium Relations for Spin Glasses with Gauge Symmetry
We study the applications of non-equilibrium relations such as the Jarzynski
equality and fluctuation theorem to spin glasses with gauge symmetry. It is
shown that the exponentiated free-energy difference appearing in the Jarzynski
equality reduces to a simple analytic function written explicitly in terms of
the initial and final temperatures if the temperature satisfies a certain
condition related to gauge symmetry. This result is used to derive a lower
bound on the work done during the non-equilibrium process of temperature
change. We also prove identities relating equilibrium and non-equilibrium
quantities. These identities suggest a method to evaluate equilibrium
quantities from non-equilibrium computations, which may be useful to avoid the
problem of slow relaxation in spin glasses.Comment: 8 pages, 2 figures, submitted to JPS
Quantum Operation Time Reversal
The dynamics of an open quantum system can be described by a quantum
operation, a linear, complete positive map of operators. Here, I exhibit a
compact expression for the time reversal of a quantum operation, which is
closely analogous to the time reversal of a classical Markov transition matrix.
Since open quantum dynamics are stochastic, and not, in general, deterministic,
the time reversal is not, in general, an inversion of the dynamics. Rather, the
system relaxes towards equilibrium in both the forward and reverse time
directions. The probability of a quantum trajectory and the conjugate, time
reversed trajectory are related by the heat exchanged with the environment.Comment: 4 page
Transient fluctuation theorem in closed quantum systems
Our point of departure are the unitary dynamics of closed quantum systems as
generated from the Schr\"odinger equation. We focus on a class of quantum
models that typically exhibit roughly exponential relaxation of some observable
within this framework. Furthermore, we focus on pure state evolutions. An
entropy in accord with Jaynes principle is defined on the basis of the quantum
expectation value of the above observable. It is demonstrated that the
resulting deterministic entropy dynamics are in a sense in accord with a
transient fluctuation theorem. Moreover, we demonstrate that the dynamics of
the expectation value are describable in terms of an Ornstein-Uhlenbeck
process. These findings are demonstrated numerically and supported by
analytical considerations based on quantum typicality.Comment: 5 pages, 6 figure
Advanced pixel architectures for scientific image sensors
We present recent developments from two projects targeting advanced pixel architectures for scientific applications. Results are reported from FORTIS, a sensor demonstrating variants on a 4T pixel architecture. The variants include differences in pixel and diode size, the in-pixel source follower transistor size and the capacitance of the readout node to optimise for low noise and sensitivity to small amounts of charge. Results are also reported from TPAC, a complex pixel architecture with ~160 transistors per pixel. Both sensors were manufactured in the 0.18μm INMAPS process, which includes a special deep p-well layer and fabrication on a high resistivity epitaxial layer for improved charge collection efficiency
The geometry of thermodynamic control
A deeper understanding of nonequilibrium phenomena is needed to reveal the
principles governing natural and synthetic molecular machines. Recent work has
shown that when a thermodynamic system is driven from equilibrium then, in the
linear response regime, the space of controllable parameters has a Riemannian
geometry induced by a generalized friction tensor. We exploit this geometric
insight to construct closed-form expressions for minimal-dissipation protocols
for a particle diffusing in a one dimensional harmonic potential, where the
spring constant, inverse temperature, and trap location are adjusted
simultaneously. These optimal protocols are geodesics on the Riemannian
manifold, and reveal that this simple model has a surprisingly rich geometry.
We test these optimal protocols via a numerical implementation of the
Fokker-Planck equation and demonstrate that the friction tensor arises
naturally from a first order expansion in temporal derivatives of the control
parameters, without appealing directly to linear response theory
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