Quantum mechanical evolution towards thermal equilibrium

The circumstances under which a system reaches thermal equilibrium, and how to derive this from basic dynamical laws, has been a major question from the very beginning of thermodynamics and statistical mechanics.

Despite considerable progress, it remains an open problem.

Motivated by this issue, we address the more general question of equilibration. We prove, with virtually full generality, that reaching equilibrium is a universal property of quantum systems: almost any subsystem in interaction with a large enough bath will reach an equilibrium state and remain close to it for almost all times. We also prove several general results about other aspects of thermalization besides equilibration, for example, that the equilibrium state does not depend on the detailed microstate of the bath.

How to Avoid Yourself

Every Sunday morning you go for a walk in the city, heading nowhere in particular, with just one rule to your rambling: You never retrace your steps or cross your own path. If you have already walked along a certain block or passed through an intersection, you refuse to set foot there again.

This recipe for tracing a loopless path through a grid of city streets leads into some surprisingly dark back alleys of mathematics—not to mention byways of physics, chemistry, computer science and biology. Avoiding yourself, it turns out, is a hard problem. The exact analysis of self-avoiding walks has stumped mathematicians for half a century; even counting the walks is a challenge.

My own initiation into the trials of self-avoidance came when I began experimenting with a simple model of the folding of protein molecules, a story I told in an earlier "Computing Science" column (see Hayes 1998). Protein folding is close to the historical roots of the self-avoiding walk, which was first conceived as a tool for understanding the geometry of long-chain polymer molecules. A polymer writhing and wriggling in solution forms a random tangle—random, that is, except that no two atoms can occupy the same position at the same time. This "excluded volume effect" in the polymer is modeled by the walk's insistence on avoiding itself.

Swimming in the Fermi sea

When one “immerses” a particle in a quantum many-body system, for instance an electron in a solid, the particle “swims” shrouded by a cloud of excitations of its new environment. This composite object is called a quasiparticle and possesses the same features as the initial particle, but with modified physical properties ( mass, charge, etc... ).

As with other many-body phenomena, the ab initio calculation of quasiparticle properties is highly challenging, and in most cases it can only be treated approximately. To overcome this challenge, use of the versatility of laser-cooled gases to simulate many-body condensed matter systems has recently been proposed.

Writing in Physical Review Letters, André Schirotzek, Cheng-Hsun Wu, Ariel Sommer, and Martin Zwierlein from MIT in the US present a dramatic demonstration of this scheme by studying experimentally the behavior of an atomic impurity immersed in a Fermi sea of ultracold atoms: A very simple system that constitutes the epitome of quasiparticle physics.

Steven Weinberg: From BCS to the LHC

Steven Weinberg reflects on spontaneous symmetry breaking, and the connection between condensed-matter physics and particle physics, in a talk at the University of Illinois in Urbana, celebrating the 50th anniversary of the BCS theory of superconductivity. It was a little odd for me, a physicist whose work has been mainly on the theory of elementary particles, to be invited to speak at a meeting of condensed-matter physicists celebrating a great achievement in their field. It is not only that there is a difference in the subjects that we explore. There are deep differences in our aims, in the kinds of satisfaction that we hope to get from our work. Condensed-matter physicists are often motivated to deal with phenomena because the phenomena themselves are intrinsically so interesting. Who would not be fascinated by weird things, such as superconductivity, superfluidity, or the quantum Hall effect? On the other hand, I don't think that elementary-particle physicists are generally very excited by the phenomena they study. The particles themselves are practically featureless, every electron looking tediously just like every other electron.

Ayuda de Webmasters

La finalidad de este foro es proporcionar un lugar de encuentro para que los webmasters se ayuden entre ellos y, en ocasiones, nosotros aportemos consejos, clarificaciones y os mantengamos al día de las novedades. Antes de enviar una pregunta os aconsejamos echar un vistazo a estos recursos y si quieres seguirla utiliza su feed: ...

W3C: Web Storage

This specification defines two APIs for persistent data storage in Web clients: one for accessing key-value pair data and another for accessing structured data.

HTML 5

HTML 5 ( HyperText Markup Language Version 5 ) is the next major revision of the core language of the World Wide Web, HTML. HTML 5 is said to become a game-changer in Web application development, one that might even make obsolete such plug-in-based rich Internet application (RIA) technologies as Adobe Flash, Microsoft Silverlight, and Sun JavaFX. HTML5 specifies two variants of the same language, a "classic" HTML ( text/html ) variant known as HTML 5 and an XHTML variant known as XHTML 5. This is the first time that HTML and XHTML have been developed in parallel.

Let's make the web faster

We are excited to discuss what we've learned about web performance with the Internet community. However, to optimize the speed of web applications and make browsing the web as fast as turning the pages of a magazine, we need to work together as a community, to tackle some larger challenges that keep the web slow and prevent it from delivering its full potential: ...

Fuzzy Math

Have the votes for president been properly counted in Florida ?. On the surface, that's a question of simple math. But beneath the number crunching, Republicans and Democrats are waging a war of disguised biases. When data don't turn out the way your theory predicts, should you question the theory or the data ?. When a new vote tally contradicts an old one, should you distrust the first count or the second ?. When one kind of recount is more evenhanded but another is more comprehensive, which is better ?. These dilemmas form the hidden crux of the debate over whether to recount Florida's ballots by hand, as Democrats prefer, or to rely on a machine recount, as Republicans prefer. The two parties aren't being candid about these questions. And math won't answer them.

Guilt by Calculation

This kind of statistical gumshoeing has a long history. In 1936, for example, English biologist and statistician R. A. Fisher went gunning for Gregor Mendel, whose experimental results Fisher believed had been tweaked to be more favorable to Mendel's ideas. "Fictitious data can seldom survive a careful scrutiny," Fisher wrote, "and, since most men underestimate the frequency of large deviations arising by chance, such data may be expected generally to agree more closely with expectation than genuine data would." In other words, it was precisely the beautiful agreement of experiment with theory that exposed Mendel's thumb on the scale. Only once in 15,000 times, Fisher computed, could one expect such strong conformity. ( The controversy over Mendel's research practices continues to this day, with notable scientists lining up on both men's sides ).

AWT and mechanical models of entanglement

By AWT explanation of quantum entanglement is closely related to wave function collapse. Try to imagine, you're a sailor, who is staying at night on the end of floating wharf, to which some boat is attached. Because night sea is stormy, everything ( both sailor, wharf and boat ) are wobbling up and down, but in different phases. From the perspective of sailor this boat sways randomly. The observation of quantum particle is analogous to situation, when sailor touches the boat for a moment, thus exchanging some kinetic energy with it. What will happen, after then ?

Evidence found of lake, catastrophic flood on Mars

Geologists at the Smithsonian's National Air and Space Museum have discovered a large former lake in the highlands of Mars that would cover an area the size of Texas and New Mexico combined, and which overflowed to carve one of that planet's largest valleys. The findings appear in the June 21 issue of the journal Science.

Tunable few-electron double quantum dots and Klein tunnelling in ultraclean carbon nanotubes

Quantum dots defined in carbon nanotubes are a platform for both basic scientific studies and research into new device applications. In particular, they have unique properties that make them attractive for studying the coherent properties of single-electron spins. To perform such experiments it is necessary to confine a single electron in a quantum dot with highly tunable barriers, but disorder has prevented tunable nanotube-based quantum-dot devices from reaching the single-electron regime. Here, we use local gate voltages applied to an ultraclean suspended nanotube to confine a single electron in both a single quantum dot and, for the first time, in a tunable double quantum dot. This tunability is limited by a novel type of tunnelling that is analogous to the tunnelling in the Klein paradox of relativistic quantum mechanics.

Single-electron transistors

While the electronics industry wonders what will happen when transistors become so small that quantum effects become important, researchers are building new transistors that actively exploit the quantum properties of electrons. Michel Devoret and Christian Glattli report.

THE INVENTION of the transistor by John Bardeen and William Shockley in 1948 triggered a new era in electronics. Originally designed simply to emulate the vacuum tube, scientists soon found that this solid-state device could offer much more. The great potential of the transistor for speed, miniaturization and reliability has been fully exploited since well controlled materials such as pure single-crystal silicon became available. According to the latest "road-map" for the microelectronics industry, microchips containing one billion transistors and operating with a clock cycle of a billionth of a second will be on the market just a few years into the new millennium...

Time Travel Beats Quantum Mechanics

The ability to travel back in time, though entirely hypothetical, isn't explicitly forbidden by our current understanding of space and time, embodied in the general theory of relativity. Time travel tends to play havoc with other laws of physics, however, and in the 29 May Physical Review Letters researchers report another example. They show that data encryption systems relying on quantum principles can be broken by allowing the data stream to interact with a quantum state that travels back in time. This scenario doesn't present an immediate threat to information security, the authors assert. Rather, it's an example of the kind of contradiction that any unified theory of quantum mechanics and gravity will have to resolve.

Miles V. Klein: Shining light on iron pnictides with Raman scattering

Our theoretical understanding of “classic” one-band superconductors such as simple metals is based on the BCS theory, which says that at low temperatures the minimum excitation energy of a single electron ( or the energy required to break a Cooper pair ) is twice Δ, the superconducting gap. As first shown by Abrikosov and collaborators this is seen in the electronic Raman scattering spectrum, an inelastic light scattering probe, as a continuum that sharply rises from zero at energy shift 2Δ, followed by a fall off at higher energy...

Dambisa Moyo

Dambisa Moyo was born and raised in Zambia, Southern Africa. She completed a PhD in Economics at Oxford University and holds a Masters from Harvard University. She completed a Bachelors degree in Chemistry and MBA in Finance at the American University in Washington D.C...

...This book ( How the West Was Lost, scheduled for publication with Penguin and Farrar, Straus & Giroux in 2010 ) examines the policy errors made in the US and other Western economies which culminated in the 2008 financial crisis. And discusses why financial and economic experts missed the signs of the credit crunch. It also explores the policy decisions that have placed the emerging world- China, Russia and the Middle East, in pole position to become the dominant economic players in the 21st century.

Felix Ritort: Fluctuations in open systems

Fluctuation theorems (FTs) establish fundamental identities about energy exchanges between a nonequilibrium system and its environment. Interest in such relations has been boosted by the possibility of applying perturbations to mesoscopic systems undergoing irreversible processes. While most recent research focused on classical systems, in a paper in Physical Review Letters, Michele Campisi, Peter Talkner, and Peter Hänggi ( Phys. Rev. Lett. 102, 210401 (2009) ), at the University of Augsburg in Germany, have now extended FTs to open quantum systems, paving the way for further progress in scenarios ranging from Bose-Einstein condensates to metallic nanowires.

The World Digital Library (WDL)

The World Digital Library (WDL) makes available on the Internet, free of charge and in multilingual format, significant primary materials from countries and cultures around the world. The principal objectives of the WDL are to:

Promote international and intercultural understanding;

Expand the volume and variety of cultural content on the Internet;

Provide resources for educators, scholars, and general audiences;

Build capacity in partner institutions to narrow the digital divide within and between countries.