Threshold feedback control for a collective flashing ratchet: threshold dependence

We study the threshold control protocol for a collective flashing ratchet. In particular, we analyze the dependence of the current on the values of the thresholds. We have found analytical expressions for the small threshold dependence both for the few and for the many particle case. For few particles the current is a decreasing function of the thresholds, thus, the maximum current is reached for zero thresholds. In contrast, for many particles the optimal thresholds have a nonzero finite value. We have numerically checked the relation that allows to obtain the optimal thresholds for an infinite number of particles from the optimal period of the periodic protocol. These optimal thresholds for an infinite number of particles give good results for many particles. In addition, they also give good results for few particles due to the smooth dependence of the current up to these threshold values.Comment: LaTeX, 10 pages, 7 figures, improved version to appear in Phys. Rev.

Injection locking of two frequency-doubled lasers with 3.2 GHz offset for driving Raman transitions with low photon scattering in 43^{43}Ca+^+

We describe the injection locking of two infrared (794 nm) laser diodes which are each part of a frequency-doubled laser system. An acousto-optic modulator (AOM) in the injection path gives an offset of 1.6 GHz between the lasers for driving Raman transitions between states in the hyperfine split (by 3.2 GHz) ground level of $^{43}$Ca$^+$. The offset can be disabled for use in $^{40}$Ca$^+$. We measure the relative linewidth of the frequency-doubled beams to be 42 mHz in an optical heterodyne measurement. The use of both injection locking and frequency doubling combines spectral purity with high optical power. Our scheme is applicable for providing Raman beams across other ion species and neutral atoms where coherent optical manipulation is required.Comment: 3 pages, 3 figure

Experimental verification of reciprocity relations in quantum thermoelectric transport

Symmetry relations are manifestations of fundamental principles and constitute cornerstones of modern physics. An example are the Onsager relations between coefficients connecting thermodynamic fluxes and forces, central to transport theory and experiments. Initially formulated for classical systems, these reciprocity relations are also fulfilled in quantum conductors. Surprisingly, novel relations have been predicted specifically for thermoelectric transport. However, whereas these thermoelectric reciprocity relations have to date not been verified, they have been predicted to be sensitive to inelastic scattering, always present at finite temperature. The question whether the relations exist in practice is important for thermoelectricity: whereas their existence may simplify the theory of complex thermoelectric materials, their absence has been shown to enable, in principle, higher thermoelectric energy conversion efficiency for a given material quality. Here we experimentally verify the thermoelectric reciprocity relations in a four-terminal mesoscopic device where each terminal can be electrically and thermally biased, individually. The linear response thermoelectric coefficients are found to be symmetric under simultaneous reversal of magnetic field and exchange of injection and emission contacts. Intriguingly, we also observe the breakdown of the reciprocity relations as a function of increasing thermal bias. Our measurements thus clearly establish the existence of the thermoelectric reciprocity relations, as well as the possibility to control their breakdown with the potential to enhance thermoelectric performanceComment: 7 pages, 5 figure

Increasing thermoelectric performance using coherent transport

We show that coherent electron transport through zero-dimensional systems can be used to tailor the shape of the system's transmission function. This quantum-engineering approach can be used to enhance the performance of quantum dots or molecules in thermal-to-electric power conversion. Specifically, we show that electron interference in a two-level system can substantially improve the maximum thermoelectric power and the efficiency at maximum power by suppressing parasitic charge flow near the Fermi energy, and by reducing electronic heat conduction. We discuss possible realizations of this approach in molecular junctions or quantum dots.Comment: 4+ pages, 4 figure

Mechanical coupling in flashing ratchets

We consider the transport of rigid objects with internal structure in a flashing ratchet potential by investigating the overdamped behavior of a rod-like chain of evenly spaced point particles. In 1D, analytical arguments show that the velocity can reverse direction multiple times in response to changing the size of the chain or the temperature of the heat bath. The physical reason is that the effective potential experienced by the mechanically coupled objects can have a different symmetry than that of individual objects. All analytical predictions are confirmed by Brownian dynamics simulations. These results may provide a route to simple, coarse-grained models of molecular motor transport that incorporate an object's size and rotational degrees of freedom into the mechanism of transport.Comment: 9 pages, 10 figure

Divergence of opinion and risk : an empirical analysis of the Ex Ante beliefs of institutional investors

Bibliography: p. [24-25

Effect of time delay on feedback control of a flashing ratchet

It was recently shown that the use of feedback control can improve the performance of a flashing ratchet. We investigate the effect of a time delay in the implementation of feedback control in a closed-loop collective flashing ratchet, using Langevin dynamics simulations. Surprisingly, for a large ensemble, a well-chosen delay time improves the ratchet performance by allowing the system to synchronize into a quasi-periodic stable mode of oscillation that reproduces the optimal average velocity for a periodically flashing ratchet. For a small ensemble, on the other hand, finite delay times significantly reduce the benefit of feedback control for the time-averaged velocity, because the relevance of information decays on a time scale set by the diffusion time of the particles. Based on these results, we establish that experimental use of feedback control is realistic.Comment: 6 pages, 6 figures, to appear in Europhysics Letter

Realization of a feedback controlled flashing ratchet

A flashing ratchet transports diffusive particles using a time-dependent, asymmetric potential. Particle speed is predicted to increase when a feedback algorithm based on particle positions is used. We have experimentally realized such a feedback ratchet using an optical line trap, and observed that use of feedback increases velocity by up to an order of magnitude. We compare two different feedback algorithms for small particle numbers, and find good agreement with simulations. We also find that existing algorithms can be improved to be more tolerant to feedback delay times

Variation of cloud horizontal sizes and cloud fraction over Europe 1985–2018 in high-resolution satellite data

Aerosol-cloud interactions are a major uncertainty in estimating the anthropogenic climate change. Adjustments of cloud properties to an aerosol perturbation concern among others the cloud fraction, and have been emphasised as particularly complex. Cloud adjustments can generate important responses on the distribution of cloud horizontal sizes. We derive the cloud-size distribution as observational constraint for the cloud-fraction response from high-resolution Landsat satellite data. The goal is to carry out long-term trends in cloud sizes and cloud fraction over Europe during 1985–2018 to investigate the impact of major aerosol reductions during that time. Landsat data with a high spatial resolution of 30m was preprocessed via the web-based platform Google Earth Engine to evade the obstacle of high computational effort and time to handle the comprehensive data archive. The observed multidecadal trends indicate a widespread increase in cloud fraction during 1985–2018. This corresponds to a decrease in the number of small clouds of several 10–100m cloud length, whereas larger clouds (1 km and more), which contribute more to the cloud fraction, became more numerous. We confirm this by showing a largescale decrease of the power-law exponent describing the relative abundance of small and large clouds in the cloud-size distribution. Even though we can interpret the observed changes in cloud properties as significant trends, we do not explicitly identify a clear aerosol signal. Untangling the pure aerosol effect from other confounding factors (e.g., the local meteorology) is therefore left as an outlook for subsequent studies.Aerosol-Wolken-Wechselwirkungen stellen eine große Unsicherheit in der Quantifizierung des anthropogenen Klimawandels dar. Die sekundären Anpassungen von Wolken an eine Veränderung atmosphärischer Aerosolkonzentrationen betreffen beispielsweise denWolken-Bedeckungsgrad und sind besonders komplex. Wolkenanpassungen können sich in der Veränderung der Wolkengrößen-Verteilung widerspiegeln. Wir präsentieren eine Methode, um mittels Beobachtungen der Wolkengrößen- Verteilung zeitliche Veränderungen in Aerosol-Wolken-Wechselwirkungen nachzuweisen. Wolkengrößen-Verteilung und Wolkenbedeckungsgrad wurden mittels hochauflösender Satellitendaten der Landsat-Serie berechnet. Das Ziel ist es, langjährige Trends im Wolkenbedeckungsgrad über Europa im Zeitraum 1985–2018 herzuleiten und ggf. den Einfluss stark rückläufiger Aerosolkonzentrationen während dieser Zeit zu identifizieren. Landsat-Daten haben eine räumliche Auflösung von bis zu 30 Metern. Um die damit verbundenen großen Datenmengen prozessieren zu können, nutzen wir dieWeb-basierte Plattform Google Earth Engine. Unsere langjährigen Trends zeigen eine großskaligen Zunahme im Wolkenbedeckungsgrad zwischen 1985 und 2018. Dies ist zurückzuführen auf einen relativen Rückgang in der Anzahl kleinerer Wolken (einige 10 bis 100 Meter Länge), während größere Wolken (mehrere Kilometer),welche mehr zum Bedeckungsgrad beitragen, häufiger wurden. Dies zeigt sich im negativen Trend des Power-Law-Exponenten der Wolkengrößen- Verteilung, welcher die relative Anzahl kleiner und großer Wolken beschreibt. Auch wenn sich diese Beobachtungen als signifikante Trends herausstellen, identifizieren wir darin kein klares Aerosol-Signal. Die Isolierung des puren Aerosoleffekts von anderen beeinflussenden Faktoren, wie der lokalen Meteorologie, bietet einen Ansatzpunkt für aufbauende Studien