Technology Review Feed - arXiv blog

Physicists Build A Memory That Stores Entanglement

Mon, 06 Sep 2010 04:10:00 GMT

The first quantum memory that stores and releases entanglement has been built by researchers in Switzerland

Entanglement is the strange, ghostly phenomenon in which quantum particles share the same existence (actually, the same wave function). So a measurement on one instantaneously influences the other, no matter how far apart they might be.

So-called action-at-a-distance lies at the heart of many of modern physic's most dramatic new technologies: quantum cryptography, quantum teleportation and quantum computation all rely on it.

That makes entanglement important stuff.

"Stuff" is the way many physicists are beginning to think of entanglement: as a resource, rather like water or energy, to be called upon when needed in the new quantum world. These physicists want to be able to create entanglement, use it and store it whenever they need to.

The first two of these--creating and using entanglement--has been the subject of intense research for the last 30 or 40 years. But the ability to store entanglement in a useful way has eluded physicists. Until now.

Today, Christoph Clausen and buddies at the University of Geneva demonstrate not only how to store entanglement but how to release it again in fully working order.

Their device consists of a load of neodymium atoms buried in a crystal of ytterbium silicate, which when cooled, can absorb and store photons. The question that Clausen and co attempt to answer is whether this device can store entanglement too.

So they created a pair of entangled photons, sent one into the crystal and waited until it was emitted again. They were then left with this new photon and the original member of the pair. They then carried out a standard experiment, known as a Bell test, and proved that the pair were still entangled.

That's impressive for several reasons. For a start, for the entanglement to be preserved, the entire crystal has to be involved. This crystal is about a centimetre in size and the idea that entanglement can be exchanged between a photon and an object of this size is amazing.

Next is the ability to transfer entanglement form a flying qubit--the photon--to a stationary one, the crystal. And to do it with photons with a wavelength of 1338nm, the so-called telecommunications wavelength that can pass easily through fibre optic cables. Any other wavelengths are interesting but practically useless for communications.

But the most exciting aspect of all this is that the entanglement survives the process of storage and release at all. Notoriously fragile, entanglement leaks into the environment like water through a sieve. Being able to store and release it is the enabling technology that could make devices such as quantum repeaters work.

There's not shortage of uses for this kind of ability. The quantum internet, to name just one, will require the ability to store and send on entangled photons. At one time, it looked more or less impossibile to do this. Entanglement was just too fragile. Now it looks merely a matter of time before we'll have it on tap.

Ref: arxiv.org/abs/1009.0489: Quantum Storage of Photonic Entanglement in a Crystal

Chit 'n' Chat

Sat, 04 Sep 2010 04:10:00 GMT

The best of the rest from the Physics arXiv this week:

Quantum Imaging: Scattered Observations On "Copenhagen"

Applications of Machine Learning Methods to Quantifying Phenotypic Traits that Distinguish the Wild Type from the Mutant Arabidopsis Thaliana Seedlings during Root Gravitropism

Composite Materials With Uncured Epoxy Matrix Exposed In Stratosphere During NASA Stratospheric Balloon Flight

A Graphene Quantum Dot with a Single Electron Transistor as Integrated Charge Sensor

Foundations of Inference

How To Tell Who Is Influencing Whom in a Group Discussion

Fri, 03 Sep 2010 04:10:00 GMT

A computer model that detects who is influencing whom in a group discussion, can accurately predict who is likely to speak next

One fascinating question that occupies social scientists concerns groups discussions. The problem is to determine the nature of the interaction between individuals and in particular, who influences whom.

Various breakthroughs in network theory and agent-based modelling have revolutionised researchers' understanding of these processes. One approach for analysing online discussions is to look for the set of keywords that define a topic of discussion, record the various instances in which these words appear and then study the links between the sites that use them: which came first, who links to whom and so on. This data can then be used to construct a network of influence.

While this has been hugely useful, it's hard to escape the sense that it fails to capture the true dynamics of influence, the way the balance of power and influence within a group shifts from moment to moment as a discussion evolves. If you've ever participated in a face to face group discussion, you'll know what I mean. (And if you haven't, where have you been?)

Today, Wei Pan and pals at the Massachusetts Institute of Technology in Cambridge take an important step towards righting this situation. They've built a reasonable model that simulates the ebb and flow of influence between individuals during a group discussion. They do this by creating a conventional model of the network of influences between individuals and then take into account that these influences change in time.

What's impressive about this approach is that it has predictive power in the real world. Pan and co applied the model to data taken from real world discussions in which groups of four people took part in brainstorming and problem solving sessions, either face to face or in separate rooms.

The question Pan and co try to answer at each point in these discussions is: who is going to speak next. Humans listening to these discussions get this right about half the time. Presumably, they are able to use various cues such as the topic of conversation and the inferred emotional state of each speaker.

Pan and co's algorithm does significantly better than this, correctly predicting the next speaker between 55 and 67 per cent of the time. And get this: it does it using nothing but the volume of speech to determine the patterns of influence between individuals.

That's impressive but the team has even more ambitious plans. Groups of four are relatively simple. But what of groups of hundreds or thousands? "Our immediate next step is also to apply our approach to larger and longer individual human behavioral datasets which we are currently collecting," they say.

Beyond that is the question of how this data and the model they've developed can provide feedback in real time that improves the performance of the groups. Is it conceivable that a system like this could coach individuals involved in discussions in a way that improves the outcome? And if so, how long before we see the iPhone app?

Ref: http://arxiv.org/abs/1009.0240: Modeling Dynamical Influence in Human Interaction Patterns

The New Science of Network Archaeology

Thu, 02 Sep 2010 04:10:00 GMT

A new way of excavating the past structure of networks reveals important information about their evolution

The study of networks has exploded in recent years. One of the more important discoveries is that many networks share common growth patterns. So if researchers grow a model network at random using these rules, the model will have the same topological structure as the network under study

For example, the world wide web tends to grow according to a process known as preferential attachment in which new links to a page depend on the number it already has (otherwise known as the rich get richer effect).

By contrast, the growth of networks associated with protein interactions in cells is best described by another process known as "duplication-mutation with complementarity". Here new nodes become copies of old ones by connecting to all their neighbours, then a process of mutation occurs in which connections can be removed.

And social networks tend to grow according to the same model that describes the way forest fires spread.

That has given network specialists all kinds of insights into network dynamics, how they evolve, the relative importance of specific nodes, how communities change over time and how information propagates through them.

But this information is entirely generic rather than specific to a real network. So a snapshot of the Last.fm social network will tell you who the central players are now and the forest fire model will give you an idea of how this structure evolved. But ask who the central players were three years ago, given the structure that exists today, and network scientists will scratch their feet and stare at the floor.

Now that looks set to change thanks to a new approach from Saket Navlakha and Carl Kingsford at the University of Maryland at College Park. Instead of using these growth patterns to study how networks evolve, their idea is to look at the process in reverse.

"Instead of growing a random network forward according to an evolutionary model, we decompose the actual observed network backwards in time, as dictated by the model," they say. "The resulting sequence of networks constitute a model-inferred history of the present-day network." This is network archaeology.

That's significant because the result depends specifically on the network under investigation, rather than solely on the growth model used to generate it.

They go on to show the power of this idea by inferring the history of several networks. For example, they are able to accurately estimate the time at which users of last.fm joined the network simply by looking at the structure today.

Navlakha and Kingsford quite rightly point out that the possibility of inferring past behaviour from current network structures raises certain privacy issues. These will now need to be addressed by the owners this data.

Perhaps the most important application of network archaeology will be in biology. Over the last few years, molecular biologists have been painstakingly mapping the networks formed by protein interactions in cells. These give a snapshot of the way these networks have evolved.

In principle, comparing the networks of closely related species should give some insight into how they evolved. For example, a search for common structures should reveal ancestral subnetworks. That's useful but it does not show how the process of evolution.

Network archaeology should provide new insight. Navlakha and Kingsford demonstrate it by taking the network of protein-protein interactions in baker's yeast and essentially "playing the tape backwards" according to the rules of "duplication-mutation with complementarity".

They discovered they were able to determine the age of proteins, ie how long it has been since they first entered the network. They were also able to work out how the proteins were duplicated and mutated in the past, or, in other words, how they evolved.

That's a powerful tool that is likely to generate some interesting new insights into the process of evolution, not to mention the history of the networks. Expect to hear a lot more about it.

Ref: arxiv.org/abs/1008.5166: Network Archaeology: Uncovering Ancient Networks from Present-day Interactions

The Extraordinary Tale of Red Rain, Comets and Extraterrestrials

Wed, 01 Sep 2010 04:10:00 GMT

For years, claims have circulated that red rain which fell in India in 2001, contained cells unlike any found on Earth. Now new evidence that these cells can reproduce is about to set the debate alive

Panspermia is the idea that life exists throughout the universe in comets, asteroids and interstellar dust clouds and that life of Earth was seeded from one or more of these sources. Panspermia holds that we are all extraterrestrials.

While this is certainly not a mainstream idea in science, a growing body of evidence suggests that it should be carefully studied rather than casually disregarded.

For example, various bugs have been shown to survive for months or even years in the harsh conditions of space. And one of the more interesting but lesser known facts about the Mars meteorite that some scientists believe holds evidence of life on Mars, is that its interior never rose above 50 degrees centigrade, despite being blasted from the Martian surface by an meteor impact and surviving a fiery a descent through Earth's thick atmosphere.

If there is life up there, this evidence suggests that it could survive the trip to Earth.

All that seems well established. Now for the really controversial stuff.

In 2001, numerous people observed red rain falling over Kerala in the southern tip of India during a two month period. One of them was Godfrey Louis, a physicist at nearby Cochin University of Science and Technology. Intrigued by this phenomena, Louis collected numerous samples of red rain, determined to find out what was causing the contamination, perhaps sand or dust from some distant desert.

Under a microscope, however, he found no evidence of sand or dust. Instead, the rain water was filled with red cells that look remarkably like conventional bugs on Earth. What was strange was that Louis found no evidence of DNA in these cells which would rule out most kinds of known biological cells (red blood cells are one possibility but ought to be destroyed quickly by rain water).

Louis published his results in the peer-reviewed journal Astrophysics and Space in 2006, along with the tentative suggestion that the cells could be extraterrestrial, perhaps from a comet that had disintegrated in the upper atmosphere and then seeded clouds as the cells floated down to Earth. In fact, Louis says there were reports in the region of a sonic boom-type noise at the time, which could have been caused by the disintegration of an object in the upper atmosphere.

Since then, Louis has continued to study the cells with an international team including Chandra Wickramasinghe from the University of Cardiff in the UK and one of the leading proponents of the panspermia theory, which he developed in the latter half of the 20th century with the remarkable physicist Fred Hoyle.

Today Louis, Wickramasinghe and others publish some extraordinary claims about these red cells. They say that the cells clearly reproduce at a temperature of 121 degrees C. "Under these conditions daughter cells appear within the original mother cells and the number of cells in the samples increases with length of exposure to 121 degrees C," they say. By contrast, the cells are inert at room temperature.

That makes them highly unusual, to say the least. The spores of some extremophiles can survive these kinds of temperatures and then reproduce at lower temperatures but nothing behaves like this at these temperatures, as far as we know.

This is an extraordinary claim that will need to be independently verified before it will be more broadly accepted.

And of course, this behaviour does not suggest an extraterrestrial origin for these cells, by any means.

However, Wickramasinghe and co can't resist hinting at such an exotic explanation. They've examined the way these fluoresce when bombarded with light and say it is remarkably similar to various unexplained emission spectra seen in various parts of the galaxy. One such place is the Red Rectangle, a cloud of dust and gas around a young star in the Monocerous constellation.

It would be fair to say that more evidence will be required before Kerala's red rain can be satisfactorily explained. In the meantime, it looks a fascinating mystery.

Ref: arxiv.org/abs/1008.4960: Growth And Replication Of Red Rain Cells At 121oC And Their Red Fluorescence

The Sinister Link Between Infectious Agents, Bacteria and Protozoa

Tue, 31 Aug 2010 04:10:00 GMT

A new technique for studying the relationship between bacteria and protozoans could boost our understanding of how these organisms spread disease

In 1980, Tim Rowbotham, a microbiologist at the University of Bradford, made an extraordinary discovery about a tiny single-celled protozoa called Acanthamoeba. These organisms are ubiquitous, turning up almost anywhere there is liquid water. Since the 1950s they have been known to cause a number of rare diseases, mainly in humans with impaired immune systems.

What Rowbotham found was that they could be much more dangerous.

It had long been known that protozoa feed on bacteria, gradually munching through great mounds of these bugs. However, Rowbotham discovered that Legionella, the particularly nasty bacteria that causes Legionnaire's disease, could not only survive being eaten by Acanthamoeba but actually thrived on it. In fact, it turns out that there is some kind of symbiotic relationship between these organisms that even today is not yet fully understood.

Microbiologists are still coming to terms with the implications of this discovery. They have since found that Acanthamoeba can host other nasties too such as H Pylori, the bacteria responsible for stomach ulcers, various strains of the food poisoning bugs Lysteria and E coli, a type of Chlamydiae and MRSA, the superbug currently sweeping through many hospitals.

The fear is that Acanthamoeba harbours these bacterial species, providing a safe haven against attack from antibiotics and contributing to the virulence of these bugs. That could make them an important source of infectious disease that is largely ignored.

So the study of the interaction between Acanthamoeba and the bacteria it supports has become an important area of research. But it is hampered by the difficulty of studying how protozoa interact with bacteria.

Today, Giorgos Tsibidis from the Foundation for Research and Technology in Greece and a couple of mates make a contribution that could help. It takes the form of a computer vision system that can identify individual protozoa, distinguishing them from cysts by virtue of their shape, and follow them as they move. The same system is also able to monitor concentration of bacteria.

They've tested the idea by watching the behaviour of Acanthamoeba protozoa grazing on a lawn of Salmonella bacteria. The machine is able to follow the Acanthamoeba as they move and to measure the drop in concentration of the Salmonella bacteria is they are eaten.

That'll save some postdocs a huge amount of time and could dramatically improve our understanding of protozoan-bacterial interactions. it may even help save a few lives if it turns out that Acanthamoeba play a significant role in the transmission of disease.

Ref: arxiv.org/abs/1008.4662: Automated Two-Dimensional Acanthamoeba Polyphaga Tracking And Calculation Of Salmonella Typhimurium Distribution In Spatio-Temporal Images

Mathematicians Create Objective Quality of Life Index

Mon, 30 Aug 2010 04:10:00 GMT

The US comes second in a new quality of life index designed to be mathematically objective

Here's a thorny problem: to develop an objective way to rank countries according to the quality of life they offer their citizens.

There are various ways of approaching this problem. For example, the Economist Intelligence Unit compiles its quality of life index using surveys, a useful technique but one that is hard to show is objective. Another widely quoted index, the Life Quality Index is based on life expectancy at birth and the gross domestic product per person but is only able to rank countries by applying a correction factor for each country that some critics say is open to bias.

Is there another way? Andrei Zinovyev at the Institut Curie in Paris and Alexander Gorban at the University of Leicester in the UK think so, using a mathematical technique developed in the mid-90s that can cut through this kind of problem .

They chose several widely-measured and well-studied indices on which to base their index: GDP per capita, life expectancy at birth, infant mortality rate and the incidence of tuberculosis. This data from 2005 is available for 162 countries.

Zinovyev and Gorban then plot this data in four-dimensional space. To create a ranking, the important question is whether there is a linear function that reduces this four-dimensional dataset to a one-dimensional set. Unsurprisingly, the answer turns out to be no. "Any linear mapping will inevitably give strong distortions in one or other region of data space," they say. That's what makes this problem tricky.

However, in the mid-90s a group of mathematicians devised a technique for reducing the dimensionality of complex data sets. This technique is essentially equivalent to connecting various data points together with springs and allowing the system to relax; hence it's name: elastic mapping. The trick is to find an arrangement of springs that "flattens" the data set, or in other words, reduces its dimensionality.

And that's basically what Zinovyev and Gorban have done, creating what they call the Nonlinear Quality of Life Index in the process.

Here are the top and bottom 5 from 2005:

1. Luxembourg
2. USA
3. Norway
4. Ireland
5. Iceland
.
.
.
158. Zambia
159. Mozambique
160. Zimbabwe
161. Kenya
162. Swaziland

No real surprises there, although there are some interesting features of the list. For example Equatorial Guinea is ranked at 140 although its GDP per capita is more than Saudi Arabia's ranked at 37. That's because of Equatorial Guinea's appalling health statistics: 123 infant mortalities per 10,000 inhabitants, for example, compared to 21 in Saudi Arabia.

For similar reasons, Russia is ranked 71st despite having a GDP per capita that is significantly higher than other countries with a similar ranking.

Every list throws ups anomalies like this. The important point about this one is that it is done objectively and transparently.

That's important because these kinds of indices are widely used by economists and politicians as a measure of economic and social development and so used to determine spending polices and legislation.

Objectivity is hard to come by when making these kinds of decisions. If the people who matter would agree to use it, this index could help.

Ref: arxiv.org/abs/1008.4063: Nonlinear Quality of Life Index

Rings 'n' Fingers

Sat, 28 Aug 2010 04:10:00 GMT

The best of the rest from the Physics arXiv this week:

Lunar Palaeoregolith Deposits as Recorders of the Galactic Environment of the Solar System and Implications for Astrobiology

Toward A Quantitative Understanding of Gas Exchange in the Lung

Testing the No-Hair Theorem with Observations of Black Holes in the Electromagnetic Spectrum

About The Possible Role Of Hydrocarbon Lakes In The Origin Of Titan's Noble Gas Atmospheric Depletion

Distinguishing Left- And Right-Handed Molecules By Two-Step Coherent Pulses

The Mathematical Secret of Viking Jewelry

Fri, 27 Aug 2010 04:10:00 GMT

A long-standing puzzle over the craftsmanship behind Viking bracelets and necklaces has finally been solved--mathematically.

The beautiful bracelets and necklaces made by Viking artisans leave archaeologists with something of a conundrum. These objects are made from rods of gold and silver which have twisted together into double helices. The puzzle is the regularity of these helices, which are remarkably similar in jewelry found in places as diverse as Ireland, Scotland, the Orkney Islands and Scandinavia.

How could craftsmen have achieved this regularity in such disparate places?

The answer comes today thanks to the work of Kasper Olsen and Jakob Bohr at the Technical University of Denmark. They point out that two wires become maximally twisted when no more rotations can be added with deforming the double helix. They go on to demonstrate the properties of maximally twisted wires. (We looked at a similar but more detailed argument about the properties of old rope a few weeks back.)

Olsen and Bohr then measured the properties of helices in Viking jewelry are twisted. It should come as no surprise to find that Viking jewelry is maximally twisted, which neatly explains why it all looks so similar. "Maximally rotated geometry is universal and therefore independent of the skills of the craftsman," say Olsen and Bohr.

Problem solved.

Ref: arxiv.org/abs/1008.4306: Hidden Beauty in Twisted Viking Neck Rings

Fine Structure Constant Varies with Direction in Space, Says New Data

Thu, 26 Aug 2010 04:10:00 GMT

A spatial variation in the fine structure constant has profound implications for cosmology.

Over the years, many physicists have wondered whether the fundamental constants of nature might have been different when the universe was younger. If so, the evidence ought to be out there in the cosmos where we can see distant things exactly as they were in the past.

One thing that ought to be obvious is whether a number known as the fine structure constant was different. The fine structure constant determines how strongly atoms hold onto their electrons and so is an important factor in the frequencies at which atoms absorb light.

If the fine structure were different earlier in the universe, we ought to be able to see the evidence in the way distant gas clouds absorb light on its way here from even more distant objects such as quasars.

As it turns out, exactly this kind of evidence has emerged in the last ten years or so from studies of absorption spectra carried out with the Keck telescope in Hawaii. These indicate that the fine structure constant must have been smaller when the universe was younger. It's fair to say, however, that this evidence is controversial--other studies have not always corroborated the result.

That debate looks set to pale into insignificance compared to new claims being made about the fine structure constant. Today, John Webb at the University of South Wales, one of the leading proponents of the varying constant idea, and a few cobbers say they have new evidence from the Very Large Telescope in Chile that the fine structure constant was different when the universe was younger.

But get this. While data from the Keck telescope indicate the fine structure constant was once smaller, the data from the Very Large Telescope indicates the opposite, that the fine structure constant was once larger. That's significant because Keck looks out into the northern hemsiphere, while the VLT looks south

This means that in one direction, the fine structure constant was once smaller and in exactly the opposite direction, it was once bigger. And here we are in the middle, where the constant as it is (about 1/137.03599...)

That's a mind blowing result. One of the biggest conundrums that cosmologists face is explaining why the fundamental constants of nature seem fine tuned for life. If the fine structure constant were very different, stars and atoms wouldn't form and the universe as we know it couldn't exist. No theory explains why it takes the value it does which leaves scientists at a loss.

The implication from Webb and co's data is that the fine structure constant is continuously varying throughout space and is merely fine-tuned for life in this corner of the cosmos: the universe's habitable zone. Elsewhere, presumably well beyond the universe we can see, this constant is entirely different.

That's likely to put the cat among the pigeons. Webb is no stranger to controversy--he has had to fight tooth and nail to have his data and ideas accepted. But this time round, with such a radical new data on the table, the debate is likely to be fiercer still.

So sit back and enjoy the show.

Refs:

arxiv.org/abs/1008.3907: Evidence For Spatial Variation Of The fiFine Structure Constant

arxiv.org/abs/1008.3957: Manifestations Of A Spatial Variation Of Fundamental Constants On Atomic Clocks, Oklo,

Meteorites, And Cosmological Phenomena

Quantum Entanglement Can be a Measure of Free Will

Wed, 25 Aug 2010 04:10:00 GMT

The same experiments that reveal the nature of entanglement can also be interpreted as a measure of free will, say researchers.

The nature of quantum mechanics has forced researchers to reconsider their own role in the process of science. Gone is the Victorian idea that measurement is objective and absolute. Today, we know that in the quantum world, it is impossible to separate the measured from the measurer. But exactly what role measurement plays in the universe, we have yet to fathom.

One intriguing idea is that certain kinds of experiments can tease apart the nature of measurement. And one particularly important class of experiment involves quantum entanglement, the hugely puzzling phenomenon in which widely separated objects share the same existence (or in scientific terms, are described by the same wave function).

Imagine two particles that are entangled in this way. Before any measurement takes place, these particles are in a superposition of states. Then a measurement on one immediately influences the other, somehow determining the outcome of a measurement on it.

Many experiments have shown that this "influence" happens as close to instantaneously as it is possible to measure and certainly cannot be mediated by any lightspeed signal. The same experiments also rule out any hidden correlation between the particles in which the outcome of any measurement is agreed upon in advance. Imagine, for example, some unseen hand that forces experimenters to unknowingly carry out measurements that always make it look as if this spooky action at a distance was taking place.

Today, Jonathan Barrett from the University of Bristol and Nicolas Gisin from the University of Geneva provide us with an interesting new take on this problem. They assume that entanglement does occur as quantum mechanics proscribes and then ask how much free will an experimenter must have to rule out the possibility of hidden interference.

The answer is curious. Barret and Gisin prove that if there is any information shared by the experimenters and the particles they are to measure, then entanglement can be explained by some kind of hidden process that is deterministic.

In practical terms, this means that there can be no shared information between the random number generators that determine the parameters of the experiments to be made, and the particles to be measured.

But the same also holds true for the experimenters themselves. It means there can be no information shared between them and the particles to be measured either. In other words, they must have completely free will.

In fact, if an experimenter lacks even a single bit of free will then quantum mechanics can be explained in terms of hidden variables. Conversely, if we accept the veracity of quantum mechanics, then we are able to place a bound on the nature of free will.

That's an interesting way of stating the problem of entanglement and suggests a number of promising, related conundrums: what of systems that are partially entangled and others in which more than two particle become entangled.

Free will never looked so fascinating.

Ref: arxiv.org/abs/1008.3612: How Much Free Will Is Needed To Demonstrate Nonlocality?

Synaptic Behaviour Captured By New Memristor Circuit Design

Tue, 24 Aug 2010 04:10:00 GMT

When it comes to copying the real behaviour of synapses, two memristors are better than one, according to a new circuit design.

Since the 1970s, electronic engineers have known that there are four fundamental building blocks of electronic circuits: resistors, capacitors, inductors and memristors (essentially variable resistors with memory). Memristors, however, had an air of mythology about them until last year when a group of researchers at HP Labs in California announced they had discovered them for the first time.

Since then, numerous others have claimed to have played with memristance over the years (although none seem to have noticed what they were doing until now). In fact, it turns out that the synapses between neurons behave exactly like memristors. That raises the possibility that memristors can be connected together in a way that truly mimics the wiring of human brains.

One of the defining features of the connections between neurons is that they become stronger when neurons fire together; hence the phrase "neurons that fire together, wire together", a phenomenon otherwise known as Hebbian learning. Various experiments have shown that this effect is most pronounced early in the learning process, when the increase in connection strength is greatest. Later learning merely reinforces the links

That's somewhat at odds with the actual behaviour of memristors, say Farnood Merrikh-Bayat and Saeed Bagheri at the University of Tehran in Iran. They say that in a single memristor connecting two neurons, the memristance decreases when a voltage is applied which increases the current which in turn causes the memristance to drop further, in a kind of positive feedback effect.

A lower memristance allows more current to flow so this certainly increases the strength of the connection as expected but there's a problem. The positive feedback effect means that later signals have a bigger effect on the connection than earlier ones, which is the opposite way round to the way real neurons connect, where earlier signals have the strongest effect.

Merrikh-Bayat and Bagheri have a simple solution: use two memristors in series. Choosing their memristance carefully allows them to reproduce Hebbian-type synapse strengthening more or less exactly.

That may turn out to be a useful insight. The first neuromorphic chips to use memristance to mimic synapse behaviour are already being built. A small change in their design may make a significant difference.

Ref: arxiv.org/abs/1008.3450: Bottleneck Of Using Single Memristor As A Synapse And Its Solution

One Idea Solves Dark Energy and Lithium Abundance Mysteries

Mon, 23 Aug 2010 04:10:00 GMT

A simple idea explains two of cosmology's biggest problems, but introduces a conundrum of its own.

One of the great outstanding challenges of modern science is to explain the observations that point to the accelerating expansion of the universe.

These come from astronomers who say the most distant supernovas are dimmer and so further away than they should be if the universe were merely expanding. Instead, the expansion must be accelerating, they say.

The conventional explanation for this acceleration is that the universe must be filled with an unseen or dark energy that is forcing this process.

That's something that many physicists feel uncomfortable with. Conventional calculations of the universe's vacuum energy arrive at a number that is 120 orders of magnitude smaller than dark energy must have. Then there is the small problem of conservation of energy which dark energy seems to violate. It's an altogether unsatisfactory state of affairs.

There's another seemingly unconnected problem that cosmologists are wrestling with: the abundance of elements that must have been created in the Big Bang.

Our models for the Big Bang and how the universe grew in its first few minutes make very precise predictions about the abundance of elements that must have been created in this process.

For example, there must have been lots of hydrogen, deuterium and helium-4. And the measurements of this stuff more or less exactly match the predictions.

However, the theory also predicts that a certain amount of lithium must have formed too. The trouble is that, as far as we can see, the universe contains only about a third of this amount. That has caused more than a little head scratching

Now Marco Regis and Chris Clarkson from the University of Cape Town in South Africa say they can explain this shortfall in lithium. What's extraordinary, however, is that the same thinking also explains the supernova observations without any need for an accelerated expansion or dark energy.

Their new idea is that the lithium abundance can be explained by abandoning one of the fundamental assumptions of modern cosmology: the Copernican principle. This is the notion that humans have no privileged position in the universe. For cosmologists, this means that the universe must be more or less the same everywhere and on all scales.

Various cosmologists have pointed out that if we abandoned this principle, it would be straightforward to explain the supernova data. It simply means that the universe is not homogeneous on the very largest scale. Instead, we must be sitting at the centre of some kind of giant void in a much larger universe.

Now Regis and Clarkson say the same kind of thinking-that there are various irregularities in the way that stuff is distributed in the universe--can explain the lithium shortfall.

That's an interesting contribution to this debate. That the same idea seems to explain two seemingly unconnected observations is a powerful reason to look at it more carefully.

On the face of it, there seems little to lose from abandoning the Copernican Principle on this scale. After all, why should stuff in the universe be evenly distributed on this scale?

However, this introduces an uncomfortable problem. Regis and Clarkson's claim is that the Universe contains a region that is short on lithium. That's not so hard to accept. What's difficult to swallow is that if true, the observations indicate that the Earth is at the very centre of it.

That would seem to be an extraordinary coincidence, one that Regis and Clarkson say has a chance of of only 1 in 10^8 of occurring.

But they also point out that this has to be compared with the problems with the standard model of physics which is out by 120 orders of magnitude compared with the thinking behind dark energy.

Take your pick. Either way, it looks as if cosmologists will have to do some mighty hard thinking to get us out of this bind.

Ref: arxiv.org/abs/1003.1043: Do Primordial Lithium Abundances Imply There's No Dark Energy?

Buttons 'n' Levers

Sat, 21 Aug 2010 04:13:00 GMT

The best of the rest from the Physics arXiv this week:

Hylogenesis: A Unified Origin for Baryonic Visible Matter and Antibaryonic Dark Matter

Patterns of Individual Shopping Behavior

The Plateau of Gamma-ray Burst: Hint for the Solidification of Quark Matter?

Introduction To Quantum Fisher information

Realization and Properties of Biochemical-Computing Biocatalytic XOR Gate Based on Signal Change

Quantum Zeno Effect Allows "Interaction-free" Switching

Fri, 20 Aug 2010 04:10:00 GMT

Exploiting one of the quantum world's strangest effects should lead to a new generation of switches that can handle quantum information.

The quantum zeno effect is one of the stranger and more fascinating consequences of quantum mechanics. It offers a surprising and counterintuitive way of controlling quantum systems that are changing from one state to another.

That sounds useful but the ability to harness the quantum zeno effect has so far eluded physicists. Now Yu-Ping Huang and buddies at Northwestern University in Evanston Illinois, say they've worked out how to use the quantum zeno effect to make an "interaction free" switch.

First a little more about the effect itself. Imagine a photon in state 0 which has a certain probability of decaying into state 1. Now carry out a series of periodic measurements on the photon. Between the measurements, the photon evolves into a superposition 0 and 1 states and a measurement will cause it collapse into one or other of these.

However, if the time between the measurements is small, the chances of it collapsing to form a 1 are smaller than the chances of it becoming a 0. And if the periodic measurements are made rapidly enough, the probability of a measurement producing a 1 tends to zero.

In effect, the process of repeated measurement prevents the photon decaying from a 0 to a 1. That's the quantum zeno effect, sometimes also called the watched-pot-never-boils effect.

Now Huang and co have come up with a scheme that exploits this effect to create a switch. The basic idea is to take a signal wave in state 0 which will decay or evolve into a 1 when it passes it through a nonlinear waveguide.

However, Huang and pals point out that measuring the wave will prevent this evolution. They say they can perform this "measurement" by making the signal wave interact with another "control" wave.

So the presence of the control wave maintains the signal wave in a 0 state while the absence of the control waves causes the signal wave to switch to a 1. And that's it: an all optical switch that is interaction-free because it is the absence of the control wave that causes the switch.

This device's potential is interesting because it offers a number of important advantages over conventional all-optical switches. First, this type of switch should operate at extremely low power since there is no signal loss associated with the switching process. That's in stark contrast to other types of switching where optical losses are an important limitation of switching performance.

Second, the quantum state of the signal wave is preserved. That's a biggie. If this kind of switch works in practice, it could become the heart of quantum routers that will make a kind of quantum internet possible.

It's early days yet, however. Huang and co have so far investigated the properties of their device only theoretically and others are hot on their heels and possibly ahead of them.

What's for sure is that we're likely to hear a lot more abut zeno switches. This team's work is being funded by a project called the Zeno-Based Optoelectronics program set up by the Defense Advanced Projects Agency. This paper is one of the first to come out that program. Real devices shouldn't be far behind.

Ref: arxiv.org/abs/1008.2408: Interaction-Free All-Optical Switching via Quantum-Zeno Effect

The Emerging Science of Worker Productivity

Thu, 19 Aug 2010 04:10:00 GMT

Experiments with Amazon's Mechanical Turk are teasing apart the factors that determine worker productivity

There's a puzzle at the heart of our economy that has troubled economists for decades. The question is this: why do people work hard in environments where they are poorly monitored and paid a fixed wage, rather than a performance-related one.Surely any rational worker would do the bare minimum to get by.

One line of thinking focuses on the relationship between the workers and their employer, which can be influenced by contracts set out in writing and by personal relationships between workers and their managers.

That suggests that one way for an employer to improve productivity would be to perfect its employment contracts.

Another line of thinking is that peer pressure plays an important role. The people around you may affect the way you work. For example, good workers, leading by example, might raise the quality of everybody's work. On the other hand, bad apples may make the good ones rotten.

But working out which of these effects wins out is hard. Peer pressure is hard to quantify and the various results in this area are somewhat contradictory, suggesting that they may depend on the environment too.

But a new tool is emerging that can help, according to John Horton at Harvard University who says the recent development of online marketplaces, in which people can buy and sell services over the web, provides a fascinating laboratory in which to test these ideas.

Today he publishes the results of a set of experiments that reveal some of the ways in which peer pressure may influence productivity.

Horton's laboratory of choice is Amazon's Mechanical Turk service in which workers sign up to do simple repetitive tasks for a few pennies per pop. Mechanical Turk attracts workers from all over the world and provides employers with a large on-demand workforce that is available 24 hours a day.

The tasks Horton set for his workers (100 of them in each experiment) is to label a picture, for example of a breakfast table, with keywords such as fruit juice, toast, yoghurt etc and to evaluate pictures that have already been labelled by other workers.

In the first experiment he showed some workers a picture with many labels while showing others a picture with few labels. He then asked both groups to label another picture which was the same for everyone. Unsurprisingly, the workers shown more labels produced more labels themselves, probably because the example picture set expectations in the workers' minds of what the employer expected.

In another experiment, workers were shown a picture with few labels, asked to completed an image-labelling exercise and then asked to evaluate the work of another worker, which may have many or few labels. Workers knew that if they didn't approve a picture, the other worker would not get paid. This provided a measure of their willingness to punish.

What Horton found is interesting. For a start, workers who have previously produced few labels were less likely to punish others. But the effect is complex. Workers seemed willing to punish others who they perceived to have produced too few labels but not these who they perceived to have produced too many labels, where only a few were required. So workers will punish others for low productivity but not for high productivity that is not needed.

That may have important implications for management techniques that ask workers to change their own patterns of work, says Horton. "For example, it may be difficult to get workers to substitute

easy, correct procedures for difficult, inefficient procedures," he says. "Ironically, the difficulty itself might make an outdated procedure harder to replace, as workers who adopt the easier method might be perceived to be shirking."

In another experiment, Horton began with the labelling exercise, then moved to an evaluation exercise and finally asked workers to complete another labelling exercise. "On average, workers that evaluated highly productive work produced more labels in the follow-on image-labeling task than workers that evaluated less productive work," says Horton. But workers exposed to images with few labels, later produce fewer labels themselves

That leads to the possibility of destructive vicious circle in worker behaviour, says Horton. "The finding that exposure to low-output work lowers output, combined with the finding that low-productivity reduces willingness to punish, suggests the possibility of an organizational vicious cycle: after observing idiosyncratically bad work, workers may lower their own output and punish less in response, in turn reducing other workers' incentives to be highly productive."

And this, says Horton, may explain why leaders often use the language of contagion to describe morale and why management theory focuses on understanding and influencing culture within an organisation rather than trying to write perfect employment contracts.

Horton's work raises many questions, not least because it contradicts other work suggesting that it is possible to improve poor workers' output by pairing them with good workers. By contrast, Horton found that "the bad apples ruined the good apples, and the good apples did nothing for the bad."

This kind of work fascinates psychologists, economists and managers because it raises the possibility that productivity in the workplace can be manipulated by clever management rather than by expensive financial incentives.

And sure enough, the key result in Horton's work is that worker productivity is easily pliable The big question is this: if colleagues affect each other's work, should this influence be encouraged or discouraged in the workplace.

Horton's answer is that it depends; but on exactly what, he has yet to nail down. Clearly, there are interesting times ahead for workers on the Mechanical Turk.

Ref: arxiv.org/abs/1008.2437: Employer Expectations, Peer Effects and Productivity: Evidence from a Series of Field Experiments

1978 Cryptosystem Resists Quantum Attack

Wed, 18 Aug 2010 04:10:00 GMT

The search for encryption algorithms that will be safe against attacks by quantum computers has thrown up a surprise, say mathematicians

Nobody has built a quantum computer much more powerful than a pocket calculator but that hasn't stopped people worrying about the implications of the post-quantum computing world. Most worried are the people who rely on cryptographic codes to protect sensitive information. When the first decent-sized quantum computer is switched on, previously secure codes such as the commonly used RSA algorithm will become instantly breakable.

Which is why cryptographers are scurrying about looking for codes that will be secure in the post-quantum world. Today, Hang Dinh at the University of Connecticut and a couple of pals show that cryptographers have been staring at one all along. They say that a little-used code developed by the CalTech mathematician Robert McEliece in 1978 can resist all known attacks by quantum computers.

First, let's a make a distinction between symmetric and asymmetric codes. Symmetric codes use identical keys for encrypting and decrypting a message. Quantum computers can dramatically speed up an attack against these kinds of codes. However, symmetric codes have some protection. Doubling the size of the key counteracts this speed up. So it is possible for code makers to stay ahead of the breakers, at least in theory. (Although in practice, the safe money would be on the predator in this cat and mouse game. )

Asymmetric codes use different keys for encrypting and decrypting messages. In so-called public key encryption systems such as the popular RSA algorithm, a public key is available to anyone who can use it to encrypt a message. But only those with a private key can decrypt the messages and this, of course, is kept secret.

The security of these systems relies on so-called trap door functions: mathematical steps that are easy to make in one direction but hard to do in the other. The most famous example is multiplication. It is easy to multiply two numbers together to get a third but hard to start with the third number and work out which two generated it, a process called factorisation.

But in 1994, the mathematician Peter Shor dreamt up a quantum algorithm that could factorise much faster than any classical counterpart. Such an algorithm running on a decent quantum computer could break all known public key encryption systems like a 4-year old running amok in Legoland.

Here's a sense of how it works. The problem of factorisation is to find a number that divides exactly into another. Mathematicians do this using the idea of periodicity: a mathematical object with exactly the right periodicity should divide the number exactly, any others will not.

One way to study periodicity in the classical world is to use fourier analysis, which can break down a signal into its component waves. The quantum analogue to this is the quantum fourier sampling and Shor's triumph was to find a way to use this idea to find the periodicity of the mathematical object that reveals the factors.

Thanks to Shor, any code that relies on this kind of asymmetry (ie almost all popular public key encryption systems) can be cracked using a quantum fourier attack.

The McEliese cryptosystem is different. It too is asymmetric but its security is based not on factorisation but on a version of a conundrum that mathematicians call the hidden supgroup problem. What Dinh and buddies have shown is that this problem cannot be solved using quantum fourier analysis. In other words it is immune to attack by Shor's algorithm. In fact, it is immune to any attack based on quantum fourier sampling.

That's a big deal. It means that anything encoded in this way will be safe when the next generation of quantum computers start chomping away at the more conventional public key cryptosystems. One such system is Entropy, a peer-to-peer communications network designed to resist censorship based on the McEliese cryptosystem.

But Entropy is little used and there are good reasons why others have resisted the McEliese encryption system. The main problem is that both the public and private keys are somewhat unwieldy: a standard public key is a large matrix described by no fewer than 2^19 bits.

That may seem less of a problem now. It's possible that the McEleise system will suddenly become the focus of much more attention more than 30 years after its invention.

However, it's worth pointing out that while the new work guanratees safety against all known quantum attacks, it does nothing of the sort for future quantum attacks. It's perfectly possible that somebody will develop a quantum algorithm that will tear it apart as easily as Shor's can with the RSA algorithm. "Our results do not rule out other quantum (or classical) attacks," says Dinh and co.

So s more likely scenario for future research is that crytpographers will renew their efforts in one of the several other directions that are looking fruitful, such as lattice-based algorithms and multivariate cryptography.

Either way, expect to hear a lot more about post quantum cryptography--provided the powers that be allow.

Ref: arxiv.org/abs/1008.2390 : The McEliece Cryptosystem Resists Quantum Fourier Sampling Attacks

The Problem of Predicting Crowd Crush

Tue, 17 Aug 2010 04:10:00 GMT

Predicting when the crushing forces in crowds are likely to become dangerous is a task that looks beyond current techniques.

Crowd control is a significant problem for organisers of major public events. Last month, 19 people died in a crush at a dance music event called the Love Parade in Germany when a crowd was channelled through a tunnel. In 2005, 350 died in a stampede during the annual pilgrimage to Mecca, while 250 died in a similar disaster the year before.

So the ability to spot the conditions that could lead to crowd crush in real time would be a significant breakthrough. Today, Peter Harding from Manchester Metropolitan University in the UK and pals say they think they know how it can be done.

One way to determine the forces in a crowd is to calculate them using the position and velocity of every individual. But this task is computationally demanding and cannot yet be done in real time or anywhere near it. The algorithms are so slow that they are of limited use, even for crowd control planning.

What's needed is some other property of the crowd that is easier to calculate but can be directly correlated with the forces. Harding and co say they have found such a proxy. Their idea is based on the observation that in ordinary circumstances the behaviour of individuals in a crowd is ordered. However, this behaviour seems to undergo a phase transition to a more disordered state when crowd crush develops.

This suggests that a simple measure of crowd order and the way it is changing ought to give some idea of the way crowd crush is developing. And that a good understanding of these changes could provide an early warning of a potential crowd crush, like a weather forecast.

It turns out that order and disorder is easily measured by examining the position of the individuals in a crowd and their heading. If these quantities are highly correlated, the crowd is ordered. If it is not, the individuals are wandering around at random. The measure of this correlation is called mutual information and it is simple enough to calculate in real time.

Harding and co's idea is that a change in mutual information--ie a change from ordered to disordered crowd behaviour--is a good proxy for the crush forces that individuals in the crowd must be experiencing.

The team has tested this idea using a simulation of the 2003 Station Nightclub disaster in which 96 people died, many of them from crush injuries, after fire broke out in a nightclub in Rhode island.

This disaster has been well studied and the crowd behaviour simulated using data from sources such as security camera footage and the testimony of survivors about their exit strategies. It is therefore possible to calculate, using the conventional computationally intensive methods, the crush forces that must have developed in the crowd.

However, Harding and co have also calculated the correlation between the position and heading of the individuals in the crowd and say there is a good correlation to be found.

That raises the possibility that an algorithm that calculated this correlation from live footage of a crowd could give an indication of the forces at work.

Whether this measure would turn out to be useful is another matter entirely. What's at issue is whether there is a unique correspondence between Harding's chosen correlation and dangerous forces or whether harmless patterns of crowd behaviour could produce correlations that look as if dangerous forces are developing. In other words, what's the rate of false positives?

That's not a question that Harding and co address here. So it's a stretch to imagine that this method could work as an early warning sign for crowd control, as suggested by the title of their paper but barely mentioned within it. That kind of forecast would clearly need a much deeper and more complete understanding of what is going on.

Ref: arxiv.org/abs/1008.2160: An Early Warning Method For Crush

Why Physics and Not Biomechanics Determines Human Throwing Accuracy

Mon, 16 Aug 2010 04:00:00 GMT

A new mathematical model of human throwing action suggests that current thinking about the biomechanical origin of error is wrong

Here's a straightforward question. Imagine you are throwing a ball into a bin. Are you better off using an overarm or an underarm throw?

It turns out that this question has been surprisingly hard to get to grips with for physicists and biomechanicists alike. Today, however, Madhusudhan Venkadesan and Lakshminarayanan Mahadevan at Harvard University's Applied Math Lab, throw some additional light on the problem.

The difficulty is in the complexity of the problem. The arm, shoulder and wrist make up a many-jointed system that allows a large number of variations in throwing style. In addition, the parameters involved in throwing are difficult to compare. For example, a 5 per cent error in throwing angle does not easily stack up against a 5 per cent error in throwing speed because these quantities have different dimensions. It's like comparing apples and bananas.

Venkadesan and Mahadevan have a neat way round this conundrum. First, they consider only the simplest type of throwing model: an arm consisting of a lever pivoted at the "shoulder" which can throw either underarm or overarm. These throws can be described by two parameters: the angular velocity of the swing and the angle of the arm at release. Second, they introduce a natural length scale, called the arm length, and use this to make their analysis of launch angle and velocity dimensionless.

In this way, they have a simple model in which the errors in launch parameters can be easily compared.

What they find is kind of curious.

Any throw will have small errors in launch angle and velocity. The question that Venkadesan and Mahadevan ask is how different trajectories amplify these errors after launch.

The most interesting result is that there is a clear trade off between speed and accuracy: slower throws are better. That will agree with most people's experience of throwing and also with the mountain of measured data that already exists on this topic. However, the consensus till now is that slower throws are more accurate because of the limitations of our bodies. The thinking is that the performance of muscles is noisier at higher strengths and this is what reduces the accuracy of faster throws.

So the result of Venkadesan and Mahadevan will come as a surprise because the trade off between accuracy and speed comes entirely from the way the trajectory amplifies errors--muscle behaviour doesn't enter into it. The result is purely from physics rather than biology.

Venkadesan and Mahadevan have another interesting result which is that when it comes to faster throws, the overarm action is clearly more accurate. That will also chime with many people's experience but again it has little to do with biomechanics. The result is purely from the way different trajectories amplify errrors.

The work offers some potentially rewarding avenues for sports scientists to investigate the accuracy of throwing in more complex models that better reproduce the mechanics of the human throwing action. That'll be interesting to watch.

In the meantime, Venkadesan and Mahadevan suggest that human's general preference for overarm throwing may be no accident. They raise the intriguing possibility that the reason we're so good at overarm throwing compared with other other animals, is that the process of evolution has selected our body shape to best exploit the physics involved. The thinking is that better throwers make better hunters who are more likely to survive.

Good idea!

Ref: arxiv.org/abs/1008.1442: Optimal Strategies For Throwing Accurately

Point 'n' Shoot

Sat, 14 Aug 2010 04:10:00 GMT

The best of the rest from the Physics arXiv this week:

BCS as Foundation and Inspiration: The Transmutation of Symmetry

Taking "The Road Not Taken'': On the Benefits of Diversifying Your Academic Portfolio

Metropolitan All-Pass And Inter-City Quantum Communication Network

Muon Tomography of Ice-filled Cleft Systems in Steep Bedrock Permafrost: A Proposal

Born in an Infinite Universe: a Cosmological Interpretation of Quantum Mechanics