## Archive for the ‘Mean machines’ Category

### Rule breakers make traffic jams less likely

Monday, January 26th, 2009

Rules are a good thing when it comes to road traffic: drive on the wrong side of the highway and you’ll cause chaos, if you live.  If that seems forehead-smackingly obvious, then an analysis by Seung Ki Baek at Umea University in Sweden and pals my come as a surprise.

They say that a small proportion of lunatics driving on the wrong side of the road actually reduces the chances of a jam rather than increasing it and they have an interesting model to prove it.

Their model is a 100 lane highway in which cars can drive in either direction in any lane.  When two cars collide, that lane becomes blocked and other vehicles have to move to one side or the other to get round them.

In theory, it’s easy to imagine that the best strategy is for everyone to agree to move to their left (or right, the model is symmetrical)  when they meet.

The question is what happens when there are two kinds of drivers: rule-followers and rule-breakers who move either way.

Ki Baek and co considered the two obvious extremes.  When everybody is a rule-breaker, the result is chaos and the road jams up quickly as collisions ensue. Equally, when everybody is a rule-follower, the likelihood of jam is much lower and road users travelling in the same direction tend to end up driving on the left (or right), just as they do on real roads.

But here’s the strange thing: the probability of a jam reaches a minimum somewhere in between, when the number of rule-breakers is between 10 and 40 per cent.

That’s kinda counterintuitive but Ki Baek and co say several factors explain what is going on.

First, a small number of collisions disperses the rule-followers to their respective side of the roads more quickly,  making jamming less  likely.

And second, rule-followers tend to form convoys which can lead to pile ups that jam the road. A few collisions here and there helps to break up these convoys into smaller groups, making large pile ups and the jams they cause, less likely .

“Our result suggests that there are situations when abiding too strictly by a traffic rule could lead to a jamming disaster which would be avoided if some people just ignored the traffic rule altogether,” say the team.

Might be fun to try it on the San Diego Freeway one of these days. Dare ya!

Ref:  arxiv.org/abs/0901.3513: Flow Improvement Caused by Traffic-Rule Ignorers

### Electronic nose spots anthrax bacteria by smell alone

Thursday, January 22nd, 2009

In the last ten years or so electronic noses have become commercially available, based on a detection device known as a Taguchi sensor. These are heated semiconductor oxide films that change their resistance when they absorb gases. The gases break down inside the film and the various molecular species gather at grain boundaries within the film changing its resistance.

Electronic noses consist of an array of Taguchi sensors designed to spot different molecular species. These are connected to a computer that analyses the pattern of signals they produce to identify the gases present in a mixture. These can work with fairly complex volatiles and some electronic noses are capable of evaluating various foods .

Now Hung-Chih Chang at Texas A&M University and a few pals say it is possible to use the tools to identify bacteria by their smell alone. (more…)

### Trick of the light boosts atom interferometer sensitivity

Thursday, January 15th, 2009

While preparing for the job of US Secretary of Energy in the incoming Obama administration (and being  director of one the top labs in the US and Nobel Prize winner to boot), Steven Chu has somehow found time to post the results of his latest experiment on the arXiv. And it’s an impressive piece of work too.

Chu, who is director of the Lawrence Berkeley National Laboratory, and his colleagues have built an atom interferometer with a sensitivity that is dramatically higher than previous models. To prove its worth, they’ve measured the fine structure constant to an accuracy of 3.4 parts per billion, which is within an order of magnitude of the best measurements.

But the real benefit of the new device is that, among other things, it will allow a new generation of tests of the equivalence principle. That is, it will test whether  the m in F=ma and the m’s in F = Gm1.m2/r^2 refer to the same thing.

In physics-speak, the question is whether gravitational and inertial mass are the same. It’s something we always assume but have never proven and there are a number of ongoing programs to study the question.

Here’s how Chu’s work will change the game…

### Harvesting energy from the airwaves

Monday, January 12th, 2009

Antennae are the most fundamental energy harvesting devices that we know, says Sung Nae Cho at the Samsung Advanced Institute of Technology in south Korea. So why aren’t they more widely used?

Turns out that helical antennae are already used to harvest energy and most of us probably own one already in the form of a transformers. These contain a helical winding that rectifies  AC into DC.

Cho points out that it has recently become possible to build nanohelices and that these might also be used for rectification. He’s designed a device that rectifies, not current, but electromagnetic waves. It consists of a nanohelix layer, a diode  layer and a capacitor layer, all the components of a standard rectifying circuit.

The nanohelix layer consists of an array of  100 million “pixels” which each contain a single nanohelix. That makes the array no bigger than the imaging chips in digital cameras . Cho calculates that if only 10 per cent of the nanohelices harvest energy from ambient electromagnetic waves to the tune of 130 nA, then the device would produce 1.3A.

If he’s right, that’s a handy amount by any standards. Anybody volunteer to prove him right.

Ref: arxiv.org/abs/0901.0769: Energy Harvesting by Utilization of Nanohelices

### Graphene transistors clocked at 26GHz

Thursday, December 11th, 2008

IBM has seen the future of computing and it may not involve silicon. Instead the company has been looking at graphene, the single atom-thick sheets of carbon that has materials scientists entranced by its dazzling array of amazing properties.

If graphene ever becomes the material of choice for a new generation of superfast chips, then the work of Yu-Ming Lin and buddies at the IBM T. J. Watson Research Center in upstate New York may well turn out to be one of the foundations of that revolution.

Today, they say they’ve built the high quality graphene transistors and clocked them running at 26 GHz.

That doesn’t quite knock silicon off its perch–the fastest silicon transistors are an order of magnitude faster than that but the record is held by indium phosphide transistors which have topped 1000 GHz.

Still, 26 GHz isn’t bad for the new kid on the block. It took silicon 40 years to get this far. By contrast, the first graphene transistor was built only last year.

As the team puts it: “The work represents a significant step towards the realization of graphene-based electronics.”

Ref: arxiv.org/abs/0812.1586: Operation of Graphene Transistors at GHz Frequencies

### How to decelerate a molecule

Wednesday, December 10th, 2008

When it comes to shuttling individual atoms about, physicists have made giant strides in cooling, trapping and even collimating them into matter wave beams. These kinds of tricks are already being used for matter-wave interferometry on chips.

But if you want to do the same kinds of things with molecules, you’re out of luck. There are two problems. First, molecules are much harder to slow down and trap in decent quantities. And second, they are much more difficult to ID. Atoms are usually identified by the light emitted by electronic transitions, which are usually in the visible part of the spectrum. In most molecules, however, these transition are in the UV and so much harder to access.

Now Samuel Meek and friends from the Fritz-Haber Institute, a Max Plank Institute in Berlin, have tackled one of these problems by building a molecular decelerator on a chip. The device consists of an array of electrodes that create an electric field with a local minimum, or well, that polar molecules tend to fall into. The well can be moved along the array.

Decelerating molecules is then a matter of matching the velocity of the well to that of the incoming molecules and then rapidly slowing it down. Meek and co say that in this way they have halved the kinetic energy of carbon monoxide molecules by slowing them from 360m/s to 240m/s.

That’s impressive and the team reckons that with a little tweaking, the chip will be able to bring the CO molecules to a standstill.

Strangely, nobody has given much thought to what you can do with stationary CO molecules. One option is to use them to store qubits for quantum computing but there seem to be few other ideas.

Which means there’s a good opportunity here for a creative thinker to make a mark.

Ref: arxiv.org/abs/0812.1487: A Stark Decelerator on a Chip

### Silicon ribbons pave the way for graphene-like sheets

Wednesday, November 19th, 2008

Graphene is the hottest property in materials science these days. Its extraordinary electronic, thermal and physical properties make it the most heavily studied substance on the plant right now.

But there is one thing that graphene can’t do and that is to fit easily into the silicon-based electronics industry. And while graphene based chips hold much promise, it’s hard to see chip makers re-tooling to use carbon instead of silicon in the near future.

That’s why a number of groups have become to look at the possibility of making silicon versions of grahene, a material called silicene. Silicon nanowires made their first appearance in 2005. And now Christelle Leandri at the Center for Interdisciplinary Nanoscience in Marseille, France, and a few buddies have made silicene for the first time, albeit in the form of stripes or nanoribbons.

What the team has done is create parallel stripes of silicene, just one atom thick on a silver substrate. The team says the physical and chemical properties of these nanoribbons is striking.

For a start silicon nanoribbons seem to be more chemically stable than their graphene cousins. In particular, graphene is highly reactive around its edges where carbon bonds dangle freely. This can make graphene hard to handle. The edges of silicene on the other hand seems to be naturally inert.

Leandri et amis have high hopes for silicene, saying that it could be incorporated into current manufacturing processes and thereby “help prolong the life of Moore’s law.”

Tanatalising work.

Ref: arxiv.org/abs/0811.2611: Physics of Silicene Stripes

### Quantum cloaking makes molecules invisible

Friday, November 14th, 2008

Cloaking is surely the zeitgeist topic of the moment and for proof, you need look no further than the work of Jessica Fransson from the University of Upssala in Sweden and colleagues. This is a group who have who have applied the ideas of cloaking to the quantum world and come up trumps. the result is a design for a molecular cloak that could turn out to be extremely useful.

First what does it mean to see or not see a quantum object? Fransson and co say that seeing is equivalent to detecting quantum objects and in the case of molecules that means looking for the terahertz radiation they produced when they vibrate.

“We propose a method for detecting and manipulating quantum invisibility based on THz cloaking of molecular identity in coherent nanostructures,” says Fransson and buddies.

In practice, this means designing quantum corals, elliptical nanostructures, that absorb terahertz waves of specific frequencies. When a molecule that emits this frequency is placed at the focus, it cannot be spotted. It is essentially invisible.

Useful? You bet. Such a quantum coral would be ideally suited to detecting molecules of specific species while ignoring others. For example, if you have a particular molecular species that poisons your measurements, then what you need is a cloak that will make it invisible to your detectors

It’s ideas like this that are going to make cloaking mighty useful one of these days.

Ref: arxiv.org/abs/0811.1782: Quantum Detection and Invisibility in Coherent Nanostructures

### Cloaking objects at a distance

Wednesday, November 5th, 2008

One of the disadvantages of invisibility cloaks is that anything placed inside one is automatically blinded, since no light can get in.

Now Yun Lai and colleagues from The Hong Kong University of Science and Technology have come up with a way round this using the remarkable idea of cloaking at a distance. This involves using a “complementary material” to hide an object outside it.

Here’s the idea: complementary materials are designed to have a permittivity and permeability that are complementary to the values in a nearby region of space. “Complementary” means that the values cancel out the effect that that this region of space has on a plane lightwave passing through. To an observer, that region of space simply vanishes.

Cloaking a region of space is relatively straightforward but cloaking an object in that space is another matter. Lai and co say the trick is to work out the optical properties of the object and then embed the “complementary image” within the cloaking material. So a plane wave would be bent by the object but then bent back into a plane as it passes through the cloaking material.
Et voila: cloaking at a distance. And in a way that doesn’t leave the cloaked object blind.

Of course , creating the complementary materials necessary to do this trick is another matter. And the usual caveats apply: it works only at a single frequency in 2D. But cloaking, in theory at least, is looking more interesting by the day.

Ref: arxiv.org/abs/0811.0458: A Complementary Media Invisibility Cloak that can Cloak Objects at a Distance Outside the Cloaking Shell

### Here come the quantum robots

Wednesday, October 22nd, 2008

Quantum robots were first investigated in the late 1990s by Paul Benioff, a remarkably original thinker at Argonne National Laboratory in Illinois.

Benioff is currently occupied in holding a candle for a theory of everything based on quantum numbers (more or less single handedly).

So a team of Chinese physicists led by Daoyi Dong at the University of Science and Technology of China in Hefei , China, has taken up the challenge to develop our ideas about quantum robotics a little further.

Benioff’s work explored the way in which a quantum robot might explore a 2D or 3D space using the laws of quantum mechanics to speed up the search. If memory serves, there is a decent speed up in two dimensions but not in three (which has interesting implications for molecular building machines). But he gave no thought to the internal structure of his robots or how they might be constructed.

The Chinese team have  now given form to this structure. Quantum robots, they say, will consist of three parts:

i. an information processor consisting of one or several quantum computers

ii. some kind of quantum actuator that interacts with the environment to carry out a task

iii. a quantum sensor which monitors the environment, such as a SQUID  (superconducting  quantum interference device)  which detects magnetic fields.

The team has mysteriously omitted a quantum communication module to send and receive data from their classical masters.

So what can a quantum robot do that, say, a classical robot attached to quantum sensors cannot?

That’s not entirely clear. Daoyi and co  say that most planning and control problems in robotics can be posed as search problems. So Grover’s search algorithm gives a significant speed up in the time it takes to solve these problems. But presumably the same would be true of a classical robot controlled by a quantum computer.

Where they might prosper is in size. Presumably quantum robots will operate at a scale that is not accessible to classical robots. And this raises the prospect of a world beneath our own populated by quantum machines operating on entirely different principles to ours. All this needs some fleshing out.

The China team’s vision is far more sanguine, having dreamt up the following predictable  applications. They say:
“Quantum robots have many potentially important applications in military affairs, national defense, aviation and spaceflight, biomedicine, scientific research, safety engineering and other daily life tasks.”

(It must have taken them weeks to think up that list.)

Now all they have to do is build one of these guys.

Ref: arxiv.org/abs/0810.3283: Quantum robot: structure, algorithms and applications