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Home » Page 1336

Spooky action with twisted beams

Physicists in Austria have devised a new technique for entangling photons using the property of “orbital angular momentum”. The researchers say that the large amount of orbital momentum they have imparted to the photons paves the way for the entanglement of macroscopic objects and could also find applications in remote sensing and quantum computing.

Entanglement is a connectedness between two (or more) particles that does not exist in classical physics. It means that determining the quantum state of one of the particles automatically and instantaneously fixes the quantum state of the second particle, no matter how far apart those particles are – a phenomenon that Einstein famously called “spooky action at a distance”. Often this is achieved using the polarization of photons – the direction of vibration of a light wave’s electric field – such that pairs of entangled photons are constrained to vibrate at right angles to one another even though each of the photons is randomly polarized.

Entangled pairs

In the latest research, Anton Zeilinger, Robert Fickler and colleagues at the University of Vienna entangled photons in orbital angular momentum (OAM). Giving photons OAM means twisting a beam’s wavefront so that as the beam travels forward its wavefront rotates around the propagation axis. This property has been well studied using laser beams and is exploited in so-called optical spanners, which use lasers to trap and rotate small objects. But Zeilinger’s group was interested specifically in entangling twisted photons; in other words, producing pairs of photons with opposite directions of twistedness. That twistedness is represented by the quantum number l – the number of times the wavefront rotates around the propagation axis in the space of one wavelength. “The goal of our experiment was to see how high we could get this number,” says team member Radek Lapkiewicz.

Other groups have previously entangled photons with OAM by firing laser beams into “nonlinear” crystals and then siphoning off the very small fraction of photons that spontaneously split inside the crystal to produce two lower-energy entangled photons. Those entangled photons carry a broad spectrum of OAM. But, says Lapkiewicz, this approach, being “limited by what nature gives”, yields l values only up to about 20.

Twisting around

The Austrian group also used a nonlinear crystal to generate photons. In this case, however, the photons were entangled in polarization and this entanglement served only as a first step. The next step was to send the photons within each pair down separate optical fibres and then impart them with OAM. The researchers did this by bouncing the photons off a tiny screen known as a spatial light modulator, which is a device that alters the phase of the reflected light from point to point, so changing the shape of the beam’s wavefront. This wavefront deformation depended on the photons’ polarization, so that photons polarized in one direction received a kick of positive OAM whereas those polarized at right angles got a negative kick. The net result was to change the photons’ entanglement from one of polarization to one of OAM.

Using this technique, Zeilinger and co-workers found they could obtain differences in quantum number as high as 600 (in other words l = +300 on one photon and l = –300 on the other). Lapkiewicz points out that there is, in theory, no upper limit to a photon’s l value, which suggests that a photon – a quantum object – could acquire as much OAM as a macroscopic object, leading to what he calls a “tension between the quantum and classical worlds”. But he cautions that the current result is still “many orders of magnitude” too small to rotate even tiny objects. He speculates that such manipulation might one day be possible by combining the momentum of many photons entangled together.

Remote senses and quantum information

Zeilinger’s group also says that its technique could be useful for carrying out remote sensing, particularly in low-light biological-imaging experiments. The idea would be to measure tiny rotations by attaching the rotating object to a circular mask with regularly spaced radial slits. One photon in each entangled pair would be given a high OAM value, while the other would keep its polarization. With the mask placed in the path of the OAM photons and rotated very slightly, the rate of simultaneous detection of the two sets of photons would change. The trick is that a polarizer placed in the path of the polarized photons would need to rotate through l times as large an angle to register the same change, so multiplying the sensitivity of the measurement by l times.

According to Lapkiewicz, the work might also be applied to quantum information. For example, he says, it might allow quantum processors that rely on polarization entanglement to be connected to those that instead exploit OAM.

Hans Bachor of the Australian National University believes that OAM entanglement “will have profound implications for both the communication of quantum information as well as quantum-logic protocols”, and he says that Zeilinger’s group has taken “an important step ahead” in this field. But he argues that it will be crucial to demonstrate entanglement with many modes. “The work here shows entanglement between two modes with widely different quantum numbers,” he adds. “That is important, but it is still only the entanglement between two modes.”

The work is published in Science.

Which scientist has been the best political leader?

By James Dacey

Facebook poll
With the US presidential election now just a few days away, it got us talking here at the Physics World offices about politics and the qualities required for effective political leadership. We were all in agreement that the decision-making process in politics is very different from that in science: in politics decisions often need to be made quickly before all the facts are known. Solutions in politics also tend to be more controversial, with opposing interest groups vying for different outcomes. (Though that’s not to say that science is devoid of politics!) These differences are discussed in a much more nuanced way in this recent article by the philosopher and historian Robert P Crease.

We were also in agreement on two other points. First, that there are not enough top politicians with science backgrounds, and second, that a government made up entirely of scientists would more than likely be a disastrous one. There have, however, been several notable exceptions of scientists who have been political leaders of countries. In this week’s Facebook poll we want you to let us know how effective you think these leaders were.

Which of these scientists has been the best political leader?

Angela Merkel (Germany, physics)
Margaret Thatcher (UK, chemistry)
Yukio Hatoyama (Japan, engineering)
Abdul Kalam (India, physics)
Lucas Papademos (Greece, physics)

To cast your vote please visit our Facebook page, and please feel free to post a comment to explain your decision.

In last week’s poll we looked at the issue of gender bias in the job market. It followed a recent psychological study that found that a set of researchers assessing the employability of early-career scientists subconsciously favoured male students over female candidates. We asked if you believe that physics employers have a subconscious bias towards male job applicants. 67% of respondents replied “yes”.

Thank you for taking part and we hope to hear from you again in this week’s poll.

Between the lines

Artwork of nuclear bomb test
Atomic history: The story behind the Trinity nuclear test in New Mexico is told in Jonathan Fetter-Vorm’s graphic novel. (Jonathan Fetter-Vorm/Hill and Wang)

From desert to devastation

The past few years have witnessed a boom in physics-themed graphic novels, with the lives of Marie Curie, Richard Feynman and Yuri Gagarin all getting the high-concept cartoon treatment. The latest example in this genre is Trinity: a Graphic History of the First Atomic Bomb. The book begins with J Robert Oppenheimer explaining the Greek myth of Prometheus to a young soldier at the Trinity test site in the New Mexico desert, and ends with American schoolchildren learning how to “duck and cover” in the event of a nuclear attack. In-between, the book’s New York-based author and artist, Jonathan Fetter-Vorm, mixes physics and history to create an eye-catching account of how the atomic bomb was transformed from a theoretical possibility into a very real – and very world-threatening – device. The text has a spare, elegiac quality that suits its subject matter, and despite its brevity, it manages to cover the most important aspects of the bomb’s scientific, political and moral implications. As for the illustrations, they are done in stark greyscale, and they do not shy away from depicting the horrors of the atomic bombing of Hiroshima and Nagasaki. While Trinity may look like a comic book on the outside, it is certainly not written for children.

  • 2012 Hill and Wang £15.99/$22.00hb 160pp

War and peace

The life of the physicist Joseph Rotblat was long and eventful. Born in Poland to Jewish parents in 1908, he escaped the Holocaust thanks to a timely appointment at the University of Liverpool, and during the Second World War, he joined hundreds of other émigré scientists in contributing to the Anglo-American atomic bomb projects. Unusually, however, Rotblat recognized the peculiar horrors of nuclear warfare even before the first bombs were dropped, and after leaving the US-led Manhattan Project early in 1945, he dedicated the remaining six decades of his life to advocating the elimination of nuclear weapons. Andrew Brown’s new biography of Rotblat, Keeper of the Nuclear Conscience, covers the entire span of its subject’s life, with a special focus on Rotblat’s work with the anti-nuclear Pugwash Conferences on Science and World Affairs – an organization he founded, and with which he shared the 1995 Nobel Peace Prize. The book’s early chapters are full of perceptive details about Rotblat’s character and the forces that shaped it. One particularly good example is Brown’s observation that Rotblat, in his later life, refused to eat potatoes; apparently, they reminded him of the bitter-tasting tubers that he and his family consumed in Poland during the First World War, under near-starvation conditions. It is also interesting to see familiar stories of physics in the 1930s refracted through a Polish lens. As Brown makes clear, during the lean inter-war years, the level of physics talent in this newly reborn country far outstripped the available research funds. As a result, Rotblat and his Warsaw-based colleagues did their nuclear research on a shoestring: while the likes of Enrico Fermi could afford radioactive samples weighing whole grams, Rotblat had to make do with a few tens of milligrams. The chapters on Rotblat’s participation in (and eventual departure from) the Manhattan Project are similarly insightful, and much enhanced by an account of the early atom-bomb work done by Britain’s Maud Committee and Tube Alloys programme. Somewhere in the middle, though, the book loses focus. The back-and-forth politico-scientific discussions on nuclear test bans that took place during the 1950s and 1960s make slow reading, and long passages contain little, if any, mention of Rotblat himself. Things do, however, liven up a little towards the end, as Brown describes how Rotblat became “an old man in a hurry”, anxious to keep the spirit of Pugwash alive into the new millennium and as unconcerned as ever about irritating government officials.

  • 2012 Oxford University Press £18.99/$29.95hb 368pp

Share your photos of animal physics

By James Dacey

mosquito


(Courtesy: Nowack, Dickerson, Hu/Georgia Tech)

All animals obey the laws of physics but some creatures do so with more panache than others. The November issue of Physics World reveals the extraordinary physics behind animal activities from the everyday – such as how cats and dogs drink – to the otherworldly, such as the super shrimp that can fracture aquarium glass with its clubs.

For the latest Physics World photo challenge we want you to share your photos of animal physics. As always we encourage you to be creative in the way you interpret the theme. But if you are looking for inspiration you might want to think about some of the animal behaviour that has dazzled and intrigued scientists over the years. How the peacock’s feathers have structures that produce beautiful shimmering colours to attract female mates, or how pond skaters can skip so effortlessly across water, for example.

To take part please upload your images to our Flickr page by Friday 4 January, and after this date we will showcase a selection of the best animal physics photos on physicsworld.com. Happy snapping!

Members of the Institute of Physics can access the digital version of the November animal physics issue of Physics World via this link. It’s packed with a series of fascinating photos, videos and features on a selection of animals, all of which have some interesting physics involved in their daily lives.

The November 2012 issue of Physics World is out now

By Matin Durrani

PWNov12cover-200.jpg
If you’re a member of the Institute of Physics, it’s time to tuck into the November 2012 issue of Physics World, which is a special issue devoted to the facinating field of “animal physics”. It’s packed with a series of fascinating photos, videos and features on a selection of animals all of which have some interesting physics involved in their daily lives.

Read – and watch – how mosquitoes survive collisions with raindrops, find out why a certain species of hornet has a in-built solar cell, and listen to why lions – strange as it may seem – roar like babies. We also examine the age-old question of why zebras have stripes and ask whether cats and dogs drink in the same way.

Plus, we have a series of seven fabulous images each devoted to a particular animal with some amazing physics powers.

Members of the Institute of Physics (IOP) can access the entire new issue online free of charge through the digital edition of the magazine by following this link or by downloading the Physics World app onto an iPhone or iPad or Android device, available from the App Store and Google Play, respectively.

For the record, here’s a rundown of highlights of the issue.

• The industrial academy – IBM’s Zurich Research Centre opened its doors 50 years ago, quickly becoming one of the world’s top research institutions. But does it still live up to its illustrious past? Philip Ball reports

• The benefits of reaching out – Publicizing research is becoming more important as part of a physicist’s job. Pablo Jensen argues that rather than just take time away from research, outreach can actually foster it

• Primate physics – Having recently discussed in this column whether skateboarders and other athletes really “know” physics, here Robert P Crease wonders if primates do as well

• How the zebra got its stripes – Biophysicists are offering new clues to this age-old mystery, as Jon Cartwright reports

• Lapping it up – Cats are slow and elegant, dogs are quick and messy – but is the physics of their drinking all that different? Jon Cartwright reports

• Vespan voltage – Tushna Commissariat explains why Oriental hornets are masters of solar power

• Fly away home – Far from being “bird brained”, members of the avian family have an amazing array of techniques to help them navigate their way across vast oceans and continents. Mark Denny examines the physics of bird navigation

• Riding raindriops – Mosquitoes regularly collide with raindrops up to 50 times their own body mass and yet, remarkably, they live on to bite another victim. Stephen Ornes explains how scientists have figured out how these insects survive such a violent impact

• Walking on water – Why can pond skaters skip so effortlessly across water? Stephen Ornes explains how these creatures’ secrets were revealed using dyed water and a high-speed video camera

• Why lions roar like babies cry – When an angry lion roars, the sounds it emits can terrify anyone within earshot. But, as Ingo Titze explains, the properties of a lion’s roar have some surprising similarities with those of a crying baby

• A strange cat in Dublin’ – Cormac O’Raifeartaigh reviews Erwin Schrödinger and the Quantum Revolution by John Gribbin

• Soft matter’s charismatic pioneer – Tom McLeish reviews Pierre-Gilles de Gennes: a Life in Science by Laurence Plévert

• Starting from scratch – Mehdi Yazdanpanah describes how he turned his PhD research into a successful small business, despite starting off with just $500 in his bank account

• Consider a spherical cow – In this month’s Lateral Thoughts column, Margaret Harris wonders just what a spherical bovine animal would really be like

If you’re not yet a member, you can join the IOP as an imember for just £15, €20 or $25 a year via this link. Being an iMember gives you a full year’s access to Physics World both online and through the apps.

Walking on water

Gods in Ancient Egypt could do it; so allegedly could Buddha and Jesus. But for the rest of us lowly human beings – at least, those of us without divine parentage or supernatural abilities – the closest we can get to walking on water is to strap on a pair of pontoon shoes and hope for the best (an idea first envisaged in sketches by Leonardo da Vinci). But in the animal kingdom, the ability is nothing new. The basilisk lizard uses its specially shaped feet to slap the surface hard enough to keep from sinking as it runs across water. Dolphins use the same technique with their tails, as do some birds.

Most insects take a different approach. The vast majority of the million or so identified species of insects live either in the air or on land, but a tiny fraction – about 0.1% – live on water, at least part of the time. One of those water dwellers, the pond skater (also known as the water strider), is particularly adept at staying afloat. It can sit still atop the water, or scurry across at speeds of 150 cm/s – about three miles per hour.

The reason why pond skaters can stay afloat is that the surface tension of water acts like a skin. Water molecules have cohesive forces between them, and the pond skater’s weight is too small to overcome those forces. That is fine for standing still, but if the insect wants to move, Newton’s third law of motion dictates that it has to push on something – and the only thing available is the water.

Our latest knowledge of these little animals is largely thanks to the efforts of David Hu, who as a mathematics graduate student at the Massachusetts Institute of Technology (MIT) spent four years studying them, analysing their sizes and shapes and trying to understand the physics that keeps these bugs afloat. He worked on the project with his PhD supervisor, John Bush, who focuses on tackling real-world fluid-dynamics problems.

“They have to row without breaking the surface of the water,” says Hu, now running his own biology-meets-mechanical-engineering lab at the Georgia Institute of Technology in Atlanta. “If they move too quickly, they will break the surface. That’s a non-intuitive idea for people to grasp, but you can see it if you push on the surface of water gently with a paperclip. You see these ripples of waves shoot out, and the pond skaters basically have to row on them very gently.”

Hidden vortices

Before Hu’s work, experiments and observations had suggested that pond skaters propel themselves forwards by making tiny ripples in the water with their legs. This idea was intuitively appealing because these “capillary waves” are readily visible in the wake of a pond skater skipping across a wet surface. But it led to a problem called “Denny’s paradox”, first articulated by biologist Mark Denny in 1993. He pointed out that infant pond skaters cannot move their legs faster than the phase speed of the capillary waves – a feat necessary to create them.

Hu and Bush decided to investigate using pond skaters gathered from local ponds. These creatures reproduced in the laboratory and so gave the researchers plenty of infants with which to investigate Denny’s paradox. And to see what was really happening in the water, Hu and Bush filmed the insects using a high-speed camera.

They found that pond skaters use the middle of their three pairs of legs like a rower uses oars in a rowboat. When an oar slices the water, it creates swirling vortices just beneath the surface that twist away from the boat, imparting forward momentum to the boat. Similarly, the pond skater’s legs leave behind the same vortices under the water’s surface. In Bush’s lab at MIT, the vortices became visible when the scientists sent the pond skaters scampering across water filled with colourful floating particles.

Birds-eye view of a pattern left in dyed-blue water by a pond skater that has just moved across it
Water dance Researchers have used dyed water to reveal the swirling vortices created by pond skaters as they move across water. (Courtesy: David Hu, Georgia Tech/John Bush, MIT)

Bush and Hu noted that the action also created capillary waves – those tiny ripples observed by Denny and other biologists – but calculated that those waves’ contributions to the bug’s forward motion were much smaller than they had anticipated, and not strong enough to move the bug.

Coveted coating

What is also interesting about pond skaters is that specialized hairs coated in a wax-like substance cover their legs, and bubbles on these hairs keep the water out. There are, according to Hu, thousands of hairs per square millimetre and the waxy substance on the hairs is coveted by human designers and materials scientists because synthetic materials usually rely on waterproofing chemicals that wash away. “No-one knows how to make a permanent water-repellent material,” he says.

It is these hairs that make the pond skater’s rowing action possible. Only the very tips of the hairs penetrate the surface of the water, creating those vortices and, in turn, transferring momentum past the air-water barrier. The particular arrangement of those hairs resembles those on a butterfly’s wings that shuttle water droplets towards the wingtips. On pond skaters, those hairs point in a particular direction, giving the insect a preferred direction and ensuring that the pond skater does not veer off course – whether alive or dead.

“If you have a dead water strider and blow on it, it will still go forward,” Hu says.

Hu has gone on to look at how other insects, including land dwellers, survive around water. In 2011 he and a team from his lab described how colonies of Brazilian fire ants, normally abysmal swimmers, weave themselves together to build a waterproof raft. On its own, a single ant is not very water repellent, but when they link together arm in arm they can create “waterproof surfaces similar to how we create Gore-Tex”, Hu says. The principal idea is the same: create a fabric that is highly textured and uses tiny pockets of air to keep water out. But the ants, like the pond skaters – and the gods – are much better at it than human inventors.

“We understand all these great things in nature,” Hu says, “but we still can’t build an equivalent in our everyday lives.”

Unexpected ‘ridge’ seen in CMS collision data again

The first data from proton–lead collisions at the Compact Muon Solenoid (CMS) experiment at the Large Hadron Collider (LHC) at CERN include a “ridge” structure in correlations between newly generated particles. According to theorists in the US, the ridge may represent a new form of matter known as a “colour glass condensate”.

This is not the first time such correlations have been seen in collision remnants – in 2005, physicists working on the Relativistic Heavy-Ion Collider (RHIC) at Brookhaven National Laboratory in New York found that the particles generated in collisions of gold nuclei had a tendency to spread transversely from the beam at very small relative angles, close to zero. A similar correlation was seen in 2010 at CMS in proton–proton collisions and then later that year in lead–lead collisions. (See image below, parts a and b.)

Observing ridges

When a graph is plotted of the fraction of particles versus the relative transverse emission angle and the relative angle to the beam axis, the correlation appears as a distinct ridge. Now, this ridge has been seen in proton–lead collisions for the first time – within a week of data collection at CMS (see image below, part c) (arXiv:1210.5482).

Three plots of CMS data showing the emerging ridge in p-p, Pb-Pb and p-Pb collisions
Mysterious ridge emerges once more

Although observations of ridges in different experiments would suggest a single cause, theorists believe there may be more than one explanation. When pairs of nuclei (such as gold or lead) collide, they can produce a hot, dense medium similar to quark–gluon plasma, a type of matter thought to have existed very soon after the Big Bang. The motion of this plasma probably correlates the underlying particles into the ridge structure.

Proton–proton collisions, on the other hand, are not expected to form a quark–gluon plasma, so theorists have come up with other explanations. One idea, presented by Raju Venugopalan at Brookhaven National Laboratory in the US and Kevin Dusling at North Carolina State University in Raleigh, US, is that the ridge correlation is an unusual type of quantum entanglement in which generated particles carry information about the state of protons before those protons collided.

Particle swarms

At very high energies, protons can fluctuate into quantum states that incorporate not just three quarks – their normal constituents – but a swarm of accompanying gluons, the carriers of the strong force. Venugopalan and Dusling think this swarm could have been so dense in proton–proton collisions at CMS that it reached “maximum occupancy” – saturation, in other words – and thereby turned into a colour glass condensate – a hypothetical and controversial form of matter that could explain certain problems in high-energy physics, such as how particles are generated in collisions.

The colour-glass-condensate interpretation of CMS’s 2010 proton–proton ridge was not widely accepted. However, shortly before the latest CMS results were published, Venugopalan and Dusling predicted that, if it existed in the proton–proton collisions, the condensate should also exist in proton–lead collisions. In other words, the theorists predicted that the ridge in proton–lead collisions should bear a greater resemblance to that in proton–proton collisions than that in lead–lead collisions, which are a consequence of a quark–gluon plasma (arXiv:1210.3890).

As predicted

Venugopalan and Dusling say that the new CMS data match their prediction, and that they are preparing a follow-up paper to describe their conclusions. “A more detailed analysis will cast more light on our theories, and hence [on] the fascinating collective behaviour of gluon states that make up the structure of matter at high energies,” says Venugopalan.

Still, other theorists are likely to have their own interpretations of the data. Writing for the collaboration’s public website, CMS experimentalists Wei Li and Gunther Roland refer to the ridge structure as an unexplained phenomenon, and look forward to a longer proton–lead run at the LHC next year that will increase the data sample a thousand fold. “Combined with the surprisingly large magnitude of the ridge seen by CMS, this will enable detailed correlation studies and open a new testing ground for basic questions in the physics of strongly interacting systems and the nature of the initial state of nuclear collisions,” they write.

The work has been submitted to Physics Letters B. A preprint is available on arXiv.

Zombies in the machine

By James Dacey

It’s a brilliant conceit – to film a zombie film at the Large Hadron Collider. That’s precisely what a group of PhD students at CERN have done, producing a feature length film called Decay.

The film follows a group of students – played by real physicists – who are desperately trying to escape from underground maintenance tunnels at the LHC. They are being pursued by a bunch of maintenance workers who have been turned into blood-thirsty zombies after exposure to the newly discovered Higgs boson.

Writer and director Luke Thompson, a PhD student at Manchester University in the UK, came up with the idea back in 2010 after joking that the tunnels at the LHC would make a cracking place to shoot a zombie film. Unlike most such ideas – often dreamt up late at night in a bar – Thomson actually set about recording the film. Armed with a budget of roughly £2000 and a regular cast and crew of 20, Thompson has spent the past two years filming and producing the 75 minute film.

The film is set to premiere in Manchester at the end of November, after which time it will be released free online under a Creative Commons licence. For updates keep an eye on the film’s website.

In good scientific fashion, Thompson accompanies promotion of the film with a strong caveat. “There is absolutely no evidence that [the Higgs boson] is harmful in any way,” he says.

Physicists detect malaria using light and magnets

A technique that identifies malaria infections in blood using cheap magnets and run-of-the-mill pocket lasers has been developed by scientists in Hungary. It exploits the unique magnetic and optical properties of crystalline waste produced by malaria parasites in the bloodstream and offers an inexpensive, sensitive and reliable alternative to existing diagnostic tools.

Malaria is the world’s number-one vector-borne infectious disease. It is contracted by 200 million people each year and proves fatal for one million of those; yet it is easy to treat, making many of these deaths avoidable. To date, medical science has come up with sensitive, equipment-heavy diagnostic tests as well as cheaper portable tests with lower sensitivity and accuracy – but none meet all the needs to efficiently combat the disease.

It was a 2008 paper, by Dave Newman and colleagues at the University of Exeter in the UK, describing a way to exploit the magneto-optical behaviour of “haemozoin” – a crystalline substance excreted by malaria parasites – that first caught the attention of István Kézsmárki, of the Budapest University of Technology and Economics and the Hungarian Academy of Sciences. When the parasites digest haemoglobin, they are left with a substance known as “haem” that is highly toxic to them, until they convert it into insoluble haemozoin microcrystals – also known as malaria pigment.

Unique properties

“The crystals are quite unusual…when the parasites turn haem into malaria pigment, it becomes magnetic,” explains Kézsmárki. “There is no other material in human blood that would have the same properties and produce the same effects.”

This is down, in part, to the dimensions of the crystals and how highly anisotropic they are at a molecular level. A crystal’s orientation governs the absorption or scattering intensity of incident polarized light on it.

So, placing an infected blood sample in a strong magnetic field forces all the crystals, which are normally thermally buffeted and jostled by surrounding molecules, to point in the same direction. Their collective effect on polarized light clearly points towards any malaria infection.

New twist

While these characteristics had already marked out haemozoin as ideal for use in malaria diagnosis, Kézsmárki and colleagues “made another twist to make it really feasible for cheap daily diagnosis”.

Instead of using research-grade instruments such as superconducting magnets and highly stable lasers, the researchers generated a uniform magnetic field by arranging a ring of standard €1 permanent magnets around the sample. By spinning the ring, they got the crystals to spin, with their moment of inertia coupled with the viscosity of the fluid, causing them to align in the field.

When the researchers shone a simple laser through the sample, the crystals acted as secondary polarizers, alternately transmitting and scattering light as they spun. A polarizing beamsplitter was used to separate the exiting light into its horizontal and vertical components. For uninfected blood, the intensities of the two components were the same, independent of the orientation of the magnetic field. For infected blood, the two oscillated inversely with one another as the magnetic field rotated.

Schematics showing the magnetic orientation of hemozoin crystals in magnetic fields
Magnetic orientation of the hemozoin crystals

Sensitive solution

Today’s best lab-based malaria test can identify parasite concentrations as low as 5/μL of blood, but it is too costly and impractical for large-scale diagnosis in rural areas where malaria is endemic. Rapid diagnostic tests, which involve no more than a drop of blood on a strip of antigen-coated plastic, are fast, portable, cheap and uncomplicated, but they have a sensitivity threshold of about 100 μL – too high to catch early-stage infection.

Kézsmárki’s team found that it could spot parasite concentrations as low as 25/μL of blood, and when the researchers ran the test on plasma instead of whole blood, their sensitivities jumped to unprecedented levels – one parasite/μL. Their method could potentially be applicable at the very earliest stage of the disease – the first symptom-free few weeks when the parasites have invaded the liver and are producing haemozoin but have not yet been dispatched into the bloodstream.

Also, haemozoin is extremely stable – the exact same form of the chemical is seen in fossilized remains of ancient malaria-infected creatures – and is common to all mutations of malaria. This means that the test will be viable in all locations and will never become obsolete, unlike the strain-specific rapid-detection tests, which face a constant struggle to keep up with the swiftly mutating malaria genome.

The real test

“The new method is exciting and may have the capability to be used as a rapid screening tool [once it is developed further],” says Stephen Karl of the Walter and Eliza Hall Institute in Australia, who was not involved in the research. He says the technique shows promise and anticipates that “the cost-per-test with this instrument will be very low since almost no disposable materials are required”.

David Bell, a malaria expert at the Foundation for Innovative New Diagnostics in Geneva, is a little more reserved. “It is difficult to tell at this stage,” he cautions, describing existing rapid-diagnostic tests as “adequate for case management”. To make an impact, he feels the new test will need to be “equally cheap, robust, without lots of moving parts, and not need batteries. That is a difficult ask”.

“The other question is whether you can determine one species from another by this method, which is important because different species of parasite need different treatments,” he adds.

For now, Kézsmárki and colleagues are looking to collaborate with engineers on reducing the size of the apparatus from its current “laptop” size to about 20 cm across, and are keenly pursuing the optimum way to separate red blood cells and plasma while keeping the malaria pigment intact in the plasma. “This is a crucial point to resolve,” Kézsmárki says. “Our target is to find the method that is the simplest, which requires no special biolab.”

The preprint of the research is available on arXiv.

  • The upcoming November special issue of Physics World is devoted to “animal physics”. You can download a free PDF of the issue from physicsworld.com from Wednesday 7 November 2012
  • David Hu from Georgia Institute of Technology’s laboratory for biolocomotion presents a special online lecture at 3.00 p.m. GMT on Thursday 8 November 2012, which you can view by registering here

New formula explains the dynamics of fractal growth over time

A team of researchers in Spain says that it has developed an equation that describes how intricate surface patterns, resembling a cauliflower-like motif, evolve and develop over time. The researchers also show that their theory can be applied to everything from actual cauliflower plants to combustion fronts, all of which obey the same scaling laws. The team says that it is the first time that a theoretical explanation has been provided for the growth of surfaces in systems that are extremely dissimilar, be it their physical nature or the scale at which they grow.

The researchers based their study, published in New Journal of Physics, on two central tenets: fractals and universality. A fractal is an object or a quantity that is self-similar, or almost so, on all scales. The object need not exhibit exactly the same structure at all scales, but the same “type” of structures must appear on all scales. With a cauliflower, for example, it is impossible to tell if a close-up image of it is the entire head of the cauliflower or just a single floret. Simply put, a fractal is a system where any one part is similar to the whole.

Similar systems

Universality, on the other hand, refers to any physical systems that “look” extremely similar despite their specific details or the scale at which their effects are felt being very different. And fractals are a great example of universality – everything from a single fern leaf that resembles the entire plant, to clouds, snowflakes, blood vessels and cauliflowers has a similar fractal pattern. But despite the properties of fractals, such as their shapes and sizes, having been studied extensively since the 1970s, the physical mechanics of their formation have remained elusive.

Nanocauliflowers

The new work has been carried out by Mario Castro and colleagues from Comillas Pontifical University, Universidad Carlos III de Madrid, Instituto de Ciencia de Materiales-CSIC, Ecole Polytechnique and Katholieke Universiteit Leuven. Initially, the researchers did not start out looking at fractals, instead they were studying a widely used technique to grow thin films known as chemical vapour deposition (CVD), which allows the thickness and composition of layers to be accurately controlled. The team was looking at the evolution of various films grown in the lab and found that one of the films – an amorphous hydrogenated carbon film – had an extremely recognizable if peculiar pattern, namely that of a cauliflower.

Inside joke

The team initially thought that its cauliflower motif was just that – a random pattern that had caught the eye. “It used to be our private joke almost – we used to call them our nanocauliflowers,” says Castro, who adds that although the films resemble cauliflowers, they are much smaller, being just several hundreds of nanometres. “But then we realized that our nanocauliflowers also had the same self-similar fractal features as those of the plant,” he adds.

Through experiments on its CVD carbon films, the team studied the cauliflower-like fronts, and developed a statistical formula that explains how these fronts grow over time. It turns out that this equation can also be used to successfully predict the growth of an actual cauliflower plant or even a combustion front – or how a flame grows over time – both of which occur at larger and different scales but obey the same scaling laws. “This proves our theory over seven orders of magnitude in length scales,” says Castro.

Ingredient’s list

The key finding in this work, according to the team, is the identification of the four ingredients needed for the formation of this kind of random growth. The first is a system that grows in time. This is crucial to the second part, which is “non-locality” or competition – a growing system, such as a plant trying to grow taller to receive more sunlight or even part of a flame expanding in time and trying to reach oxygen to combust. The non-locality plays a part here, as what happens in one part of the system remotely affects other distant parts of the same system. The third ingredient is randomness, which, according to the team, nature provides in abundance. The final ingredient is self-similarity – a fractal pattern, where the parts are similar to the whole.

“In spite of the widespread success of fractal geometry to describe natural and artificial fractal shapes, purely geometrical descriptions do not provide insight into the laws that govern the emergence of the shapes in time,” Castro told physicsworld.com. “We believe that by knowing the general laws that dictate how these patterns form and grow, it will help to identify the biological and physical mechanisms that are at play.” The team hopes its work will inspire other researchers to look into the real-world dynamics of fractals, instead of only their morphology.

The work is published in the New Journal of Physics.

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