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Visible improvements

We are living in a golden age for scientific communication, a time of increasing appreciation for all things quantitative, when particle colliders find themselves at the heart of novels and scientists and statisticians can write bestsellers. In this golden age, the likes of Nate Silver, political statistician for the New York Times, and Hans Rosling, a medical doctor, public health statistician and frequent TED speaker, have attained near rock-star status thanks to their heady combination of stunning results and ability to bring mathematics to the public sphere with seemingly effortless grace.

The growing popularity of science, however, has made the effective communication of complex scientific results more important than ever: with a bigger audience comes a bigger responsibility to get things right. The popularity of Silver, Rosling and their intellectual kin has also highlighted the gap between researchers who communicate well, and those who struggle to present their results even to specialist audiences. Visual Strategies is a book that should appeal to people in both categories. Billed as a “practical guide for scientists and engineers”, it lies somewhere between a coffee-table conversation piece and a reference manual for wizened data masters, and its goal is to educate researchers about the foundations of good design, in the hopes of improving the way they communicate about science.

The thing that struck me immediately when I picked up Visual Strategies was that it is unlike any other textbook or reference manual on my shelf. The book’s jacket is covered with a colourful pattern of arrows, and it employs a technique called lenticular printing to make the arrows change from warm red to yellow depending on which way you look at them. The effect is instantly engaging, compelling you to pick up the book and flip through the richly coloured pages.

The authors, Felice Frankel and Angela DePace, have chosen a tabbed layout for the book, reminiscent of a cookbook or children’s story, which not-so-subtly indicates that Visual Strategies is not meant to be read front to back. Instead, its structure encourages nonlinear browsing, and each subsection is colour-coded for quick reference. This is very unusual for a publication aimed at scientists and engineers, since most journals or textbooks are simply black on white, with sparing colour and apparently little thought given to the design of the physical article itself. One of the book’s best features is the “visual index” at the end, which contains small thumbnail versions of the major graphics featured within each section. This is an absolute treat, and in my view it ought to become a standard feature for all textbooks and scientific journals.

Readers who do start from the front of the book will find that it begins with a conversation between the authors (who are both research scientists) and the book’s graphic designer about choices made in construction and layout. This conversation sets the tone for the entire book, which is ultimately about the decisions and changes that go into making scientific figures better and more easily understood by colleagues or members of the public. This same decision-making process carried through to the choices the authors made in creating the book itself. The interior of Visual Strategies is filled with graphs, photos and figures (both hand drawn and computer generated) from a dozen different scientific disciplines, creating an anthology of visual communication. The graphics chosen for Visual Strategies also include many hand-drawn sketches and early rough drafts, which highlight the way some figures were developed and conceptualized. Being able to see these early versions, and the changes made to them, gives readers much to consider in their own visualizations.

The effective communication of complex scientific results is more important than ever now

The book’s visual strategies are presented in two ways: as the authors’ own suggestions for improving various published graphics, or as case studies on the evolution of graphics written by a handful of expert contributors. Differences between the “before” and “after” images are sometimes subtle, and a few may even seem arbitrary. Other changes, however, are dramatic and once shown they seem obvious and almost inevitable. These can sometimes be as simple as adding a bit of colour, or removing labels and lines to declutter the image. The authors note early on that, while you may not agree with a particular change in font or colour, the mere process of considering these factors will improve and enrich your own visualizations, as well as your ability to convey their underlying scientific meaning.

For all the innovation and charming creativity of Visual Strategies, however, its lack of broad structure may limit its utility. The use of coloured tabs, chapter headings and a very dynamic layout is confusing at times, and it lacks a treatment of more traditional graphs, such as scatter plots, bar charts and the growing varieties of density maps. Error bars, plotting symbols and dotted versus dashed lines in plain figures are not always the most thrilling things to discuss, and perhaps they are better left to works such as Edward Tufte’s classic The Visual Display of Quantitative Information (Graphics Press, 1983). Still, because they are the most common forms of data visualization for many researchers, their absence is felt. Along with the many decisions about detail and design that went in to creating the book, the authors also made a conscious choice to aim it at more experienced scientists and researchers. It would be difficult for novices or young students to gain a foundation in data visualization using this text alone.

The book’s supplemental website (http://visual-strategies.org) is full of good-quality content and links, and may yet provide an interesting forum for data-visualization discussions and resources. Anyone interested in visualization should spend at least a little time browsing through it. It is fitting, too, that the book discusses interactive graphics, since researchers can now convey complicated structures or hierarchies within data to anyone with a web browser – making interactivity a hallmark of this new era in science communication.

The tips and discussions throughout this book give scientists a foundation for making their visualizations more honest and more obvious. It lacks the structure needed to be a teaching tool for novices, and is not quite encyclopedic enough to truly be a reference guide for professionals. But Visual Strategies is effective and entertaining at sparking discussion and thought, particularly for those in the trenches of discovery.

  • 2012 Yale University Press £25.00/$35.00pb 160pp

Teaching an old blog new tricks

By Hamish Johnston

(iStockphoto/webphotographeer)

When the Physics World editorial team started blogging in earnest early in 2008, it was our first chance to interact much more directly and informally with the physics community – prior to that we had been mainly restricted to conventional news stories and features. Since then, of course, the social-media world has grown out of all recognition and Physics World is now on Twitter, Facebook, Flickr, Google+ and YouTube, with these sites giving us new ways of communicating with you (and vice versa).

But we think the Physics World blog still has a big role to play in what we do and today marks a major upgrade to it. While most of the improvements are behind the scenes, I thought I’d mention a few new features we hope you’ll enjoy.

(more…)

The quantum coin toss

All unpredictability in the world around us, be it the outcome of a coin flip or the weather conditions a month from now, is a fundamentally quantum rather than classical phenomenon. This is the conclusion of two physicists in the US, who have worked out that molecular interactions in gases and liquids can amplify tiny quantum fluctuations, to the point where the fluctuations are large enough to account for the uncertainties we experience at the macroscopic scale. This insight, they argue, could prove important in cosmology, as it might rule out some theories of the multiverse that rely on classical as opposed to quantum probabilities.

Classical to quantum predictions

In classical theories of probability the chances we attribute to a flipped coin landing either heads-up or tails-up simply reflect how much or how little we know about the coin flipping. To say that there is a 50:50 chance means we have no idea how the coin will land. In principle, however, if we understood exactly which physical processes determine the outcome of the flip and also know with enough precision all of the relevant parameters – such as the force imparted to the coin, the height at which it lands and the air resistance – we could predict the outcome with certainty.

According to the latest research, this view is not correct. Andreas Albrecht and Daniel Phillips of the University of California at Davis argue that the probabilities we use in our everyday lives and in science do not “quantify our ignorance” but instead reflect the inherently random nature of the physical world as described by quantum mechanics. They maintain that quantum fluctuations can be amplified sufficiently by known physical processes to the point where they can entirely account for the outcome of these everyday macroscopic events. In fact, they claim that all practically useful probabilities can be accounted for in this way. In other words, all classical probabilities can be reduced to quantum ones.

To back up their case Albrecht and Phillips consider an idealized fluid of billiard-ball-like molecules that continually collide with one another. The Heisenberg uncertainty principle dictates that the trajectory of a billiard ball will have an inherent uncertainty, resulting from the uncertainties in its position and momentum. The researchers worked out – by inputting suitable values of radius, mean free path, average speed and mass of the billiard balls into a couple of simple equations – how much this uncertainty grows with each collision between the balls. They show that in water and air (nitrogen) the uncertainty becomes so large in the space of one collision that every single fluctuation in the properties of these fluids has a fully quantum-mechanical origin.

Quantum outcomes

The researchers then show that the quantum fluctuations manifest in the water can wholly determine the outcome of a coin flip. They calculate that a typical flipped coin can spin through half a revolution in about 1 ms. This is also the temporal uncertainty in the neuronal process governing the coin flipping, a process that a group of neuroscientists in 2008 argued is caused by fluctuations in the number of open neuron ion channels. Since these fluctuations are, in turn, caused by the Brownian motion of molecules called polypeptides in a fluid that is largely water, quantum uncertainty (which drives the Brownian motion) can completely randomize the coin flipping.

Cat’s tail

As such, the researchers say that anyone tossing a coin is, in fact, performing a Schrödinger’s cat style experiment. But rather than a cat that is both alive and dead, the quantum object in this case is a coin, the final state of which is simultaneously heads and tails. The outcome of the flip therefore remains genuinely open until the upwards face of the coin is looked at, at which point the system takes on a definite value of either heads or tails.

The researchers admit that their example is very simplified and that they would have a hard job tracing the amplification of quantum uncertainties in all familiar contexts, be it rolling dice or picking out a card at random. They also point out that it would only take one counterexample to falsify their idea – a use of classical probabilities that is clearly isolated from the physical, quantum world.

David Papineau, a philosopher at King’s College London, believes that Albrecht and Phillips are likely to be correct but he doesn’t think their conclusion is terribly surprising. “It is very likely that all serious probabilities, be it a coin landing heads-up or a child being female, are manifestations of quantum chanciness,” he says. “Indeed we have devices, such as Geiger counters, that show how big results are often caused by chancy micro-events.”

Albrecht replies that he and Phillips are perhaps the first physicists to have tackled the relationship of quantum and classical probabilities head-on, and he argues that the latest research might also rule out some theories in which physical processes (such as “eternal inflation”) produce multiple copies of pocket universes like the one we observe around us. Such theories of the “multiverse”, he says, need to import purely classical probabilities because a quantum wave function on its own cannot determine in which universe a particular measurement would be made. But Albrecht points out that such a move would not be possible if classical probabilities are, at root, quantum.

A preprint of the research is available on arXiv.

Tungstenite triangles emit light

Researchers in the US have succeeded in growing single atomic layers of the naturally occurring mineral tungstenite for the first time. The sheets appear to have unusual photoluminescence properties that might be exploited in optics devices like lasers and light-emitting diodes.

2D materials have dramatically different electronic and mechanical properties from their 3D counterparts and so may find use in a host of novel device applications. Until now, however, most research in this field has focused on the most famous of 2D materials, graphene, but the fact that this material lacks a direct electronic band gap means that scientists are now starting to look at other 2D candidates too.

A team led by Mauricio Terrones and Vincent Crespi of Penn State university in the US grew monolayers of tungstenite (WS2) by depositing tiny crystals of tungsten oxide less than a nanometre tall and then passing these crystals though sulphur vapour at high temperatures of 850 °C. The result – monolayers of tungsten disulphide arranged in a honeycombed pattern of triangles comprising tungsten atoms bonded to sulphur atoms.

“We were astonished that we could grow such perfect, atomically thick triangle shapes using a chemical vapour deposition method,” Terrones told physicsworld.com. “Moreover, and again to our surprise, we observed that these triangles glow quite strongly at their edges rather than at their centres – a peripheral photoluminescence effect that we never expected and which has not been reported on before.”

Photoluminescence occurs when charge carriers (electrons and holes) recombine in a structure to emit light of a different wavelength from that used to initially excite the material. Normally, light emission is a delicate thing, explained Crespi, and structural defects – like edges – prevent light emission as they tend to give excited electrons and holes ways of recombining without emitting light. “We saw just the opposite effect,” he said, “in that the structural defects created close to the edges of a triangle seem to be the favoured place for emitting light.”

Direct band gap

2D systems are intrinsically different from their bulk 3D counterparts, and WS2 is no exception. While the bulk material is an indirect band gap semiconductor, the single-layer material boasts a direct band gap. Direct band gaps are important in semiconductors because they allow devices made from these materials to emit light efficiently – as in this case.

According to the Penn State team, the WS2 triangles might find applications in optoelectronics. “They might even come in handy as biomarkers or in drug delivery, but much more research still needs to be carried out before we can say this with any certainty,” added Terrones. “They could also be useful in a new generation of planar, 2D optoelectronic devices, such as light-emitting diodes – where we control the propagation of light in thin film layers of material – and even in laser technology.”

The researchers now plan to grow other 2D materials that have different optical and electronic properties. Some examples in the pipeline include MoSe2, NbS2 and WSe2, revealed Crespi. “We would also like to better understand and control the light emission from 2D materials in general, and try our hand at sculpting the triangles into multicomponent devices.”

The work is detailed in Nano Letters.

Extremely bright supernovae may break the Chandrasekhar limit

White dwarfs forming in extreme magnetic fields could be stabilized, allowing them to get bigger before they explode, leading to a brighter bang when they finally do, according to a team of researchers in India. Type Ia supernovae, caused by exploding white dwarfs, are often used by astronomers as “standard candles” to calculate the distance to a point in space because they are extremely bright and usually have similar luminosity. But some anomalously bright type Ia supernovae that scientists could not explain have been observed recently and the new work could provide an explanation for them.

A white dwarf is a star that has used all its hydrogen and helium and is too cool to burn carbon. It has therefore collapsed into a highly dense state. With no source of energy, it glows only because of residual heat, and over billions of years it will cool down and become a black dwarf, if it is left undisturbed.

Limits on stellar evolution

In 1935 the Indian astrophysicist Subrahmanyan Chandrasekhar famously showed that a star would not form a white dwarf if its mass was greater than 1.44 solar masses because the core temperature would be sufficient to ignite carbon fusion. If a star’s mass increased beyond this “Chandrasekhar limit” of 1.44 solar masses after it has collapsed to form a white dwarf, the star shrinks still further. The loss in gravitational potential energy causes an increase in temperature, and a runaway fusion process begins, creating a massive thermonuclear explosion that obliterates the star in seconds.

Because type Ia supernovae are almost always formed by the thermonuclear explosion of an object with about the same mass, they almost always have about the same brightness. Observations of distant type Ia supernovae proved the expansion of the universe was accelerating, a discovery rewarded with the 2011 Nobel Prize for Physics. However, there have been a small number of troubling observations recently of nearby type Ia supernovae that are abnormally bright, and which appear to have been formed by the detonation of a white dwarf well above the Chandrasekhar limit. The absence of a satisfactory model for how these could be produced has placed a question mark over the use of type Ia supernovae as standard candles for observing distant galaxies.

Super-sizing

In the new research, Upasana Das and Banibrata Mukhopadhyay of the Indian Institute of Science in Bangalore suggest that these “super-Chandrasekhar” white dwarfs might occur in very high magnetic fields. Such fields, they reason, could stabilize a white dwarf of mass up to 2.58 solar masses by a process known as Landau quantization. This would increase the stellar remnant’s resistance to gravitational collapse, allowing it to continue accreting mass until it reached a higher limit. At this point it would detonate with an even bigger bang than would otherwise have been possible.

But how might such a field be generated? Das and colleagues point out that magnetic fields of 107–108 G can be detected in about 25% of accreting white dwarfs. If such a star collapses, the magnetic flux is conserved, while the radius is reduced dramatically. The magnetic fields therefore become orders of magnitude stronger.

Mukhodpadhyay now believes that the team needs to focus on observing a larger sample of highly magnetized white dwarfs in the hope of observing this spike in field as one collapses. “You start with an observed white dwarf of field 109 G,” says Mukhopadhyay, “Later on, it should pass through an intermediate phase when the field increases to 1011 G – those we have not seen yet.” He cautions, however, that an increase in field might not be detectable if the matter accreting onto the white dwarf caused magnetic shielding.

Impressive increase

Mukhopadhyay believes that it is too early to say whether the model has any direct implications for the expansion rate of the universe. He does, however, tell physicsworld.com that “the existence of super-Chandrasekhar white dwarfs is a major paradigm shift in our understanding of white dwarfs and several of the related results may have to be examined in this light”.

Jeffrey Silverman, an astrophysicist at the University of Texas at Austin, says the paper presents “a pretty impressive increase in the maximum white dwarf mass which, as they point out, matches some of these recent observations – or at least gets closer to them”. He is more sceptical, however, about the researchers’ claims of a paradigm shift: “We’ve seen very few of these super-Chandrasekhar objects. They’re very rare, they’re pretty distinguishable from more typical type Ia supernovae that we use for cosmology. It’s highly unlikely that our calculations of the history of the universe would have many – if any – of these objects contaminating them.”

The research in published in Physical Review Letters.

Searching for magnetic monopoles in polar rocks

The first search for magnetic monopoles in mantle-derived polar igneous rocks – thought to be likely to contain a higher ratio of monopoles to matter – has been conducted by researchers in Switzerland. The team analysed 23.4 kg of samples from Arctic and Antarctic regions. While no monopoles were found, the monopole to nucleon ratio in the search samples was constrained, with a 90% confidence level, to an upper limit of 1.6 monopoles per 1028 nucleons. The team claims that its study, which adds to existing matter-bound monopole searches, has comparable or better sensitivity than the most extensive meteorite search to date.

Magnetic monopoles were famously predicted by Paul Dirac in 1931 as a way of explaining electric charge quantization. Their existence is also predicted by a number of grand unification theories, but the much-anticipated particle has so far remained elusive.

“The magnetic monopole is a truly fascinating hypothetical object – it explains electric charge quantization and it is needed in theories that unify the fundamental interactions,” says lead researcher Philippe Mermod, from the University of Geneva. “This makes us wonder why it has never been found in nature, as its non-existence would be a complete mystery.”

Cosmic origins?

It is believed that monopoles might be created within high-energy particle accelerators, such as the Large Hadron Collider. However, if monopoles are very heavy, the beam energy could be insufficient to create them in particle collisions. In this case it could be that the only events energetic enough to produce monopoles happen in the cosmos, or occurred just after the Big Bang.

“In such a case, monopoles should be around us: either travelling freely through space or bound to matter,” explains Mermod, “but [past] experiments were unable to observe any in hundreds of kilograms of terrestrial matter, and even in asteroids and rocks from the Moon, showing that if they are there, they are extremely rare.”

Mermod and his team believe that the key to finding monopoles might lie not with more powerful accelerators or the analysis of larger amounts of source material, but with novel search designs.

During its formation, the Earth was molten, and has since differentiated into a number of chemically distinct layers – the crust, the mantle, and the inner and outer core. During this stage, any monopoles bound to the matter that formed the Earth would likely have sunk towards the core. The crust, therefore, is expected to be depleted in such stellar monopoles – monopoles already trapped in stardust before the formation of our solar system.

In-depth look

Monopoles in the solid mantle, however, would be restricted in their movement and, regardless of their polarity and the magnetic-field direction, would be slowly moved along in line with mantle convection. On reaching the liquid core, their mass would pull them towards the Earth’s centre, before being attracted pole-ward by their magnetic charge.

Mermod and colleagues therefore predict that monopoles might be distributed throughout the mantle, up to a distance of 3400 km – the core radius – from the Earth’s magnetic axis. Rocks from polar mantle-derived sources are therefore potential candidates in the search for monopole-bearing material. Samples selected for this study were mainly confined to mantle-derived rocks from high (greater than 63 degrees) latitudes.

The team searched for monopoles in the samples by looking for the signature of a persistent current in a loop of superconducting material – a SQUID-based rock magnetometer – that is housed in a shielded room at the Laboratory of Natural Magnetism in Zurich.

Probing polar rocks

“Polar volcanic rocks were never probed for monopoles before,” says Mermod. He goes on to explain that since his calculations showed that the gravitational/electromagnetic force balance was advantageous for a range of monopole masses and charges, and he had access to a magnetometer that was able to detect the signature of a monopole in rock samples, he decided “that this needed to be done. There was truly a chance that monopoles would be spectacularly discovered”.

In addition, the team also analysed rocks with chemical compositions that hint at deep mantle origins. These included: basaltic rocks extruded above hotspots where local heat increases in the mantle cause an upwelling plume; samples from large igneous provinces – areas of massive flood basalt deposits that have been connected to continental break up and mantle plume activity; lava containing lherzolite nodules that have been transported, unaltered, up from the mantle; and basaltic lava from Coleman Nunatak, a ridge of rock located at the head of the Berry glacier in Antarctica, the high 206Pb/204Pb ratios of which indicate a low extent of melting and deep origin.

Control samples for the study were taken from crust-derived lava from an Antarctic subduction zone and from low-latitude hotspots (Hawaii) and mid-ocean ridges (the Mid-Atlantic Ridge and the East Pacific Rise), as these should be depleted of monopoles.

Continuing the hunt

The monopole hunt in matter, however, will not end here. “The future of searches for monopoles trapped in matter depends on possible access to better instruments based on new technologies, or to exotic material samples,” Mermod says. “As soon as new material becomes available – for instance, returned samples from asteroid or comet survey missions – it will be important to probe it for monopoles.”

A pre-print of the research is available on the arXiv server.

Of physics and famine

Physics and medieval history don’t overlap that often. I should know: I got an undergraduate minor in medieval and renaissance studies in part because I wanted a break from doing physics. So the fact that this arXiv paper and this documentary have both come out in the past 10 days is about as unusual as – well, finding a medieval king buried in a car park.

Fascinating as the discovery of Richard III’s skeleton is, though, I’m going to write instead about the arXiv paper, which proposes something even more remarkable: a possible link between space weather and episodes of famine in late medieval Europe.

The paper focuses on the years 1590–1702, a period during which Europe’s population suffered repeatedly from famine. Over the same period, the Sun was experiencing a decades-long lull in activity, known as the Maunder minimum. Might there be a connection?

To answer this question, the paper’s authors – physicist Lev Pustilnik and economist Gregory Yom Din – begin by summarizing the evidence for a connection between space weather and local weather. Overall, this appears fairly convincing, if a bit circumstantial. For example, a 1997 study found a link between cosmic rays and cloud cover, while a 2004 paper demonstrated a similar correlation between global atmospheric circulation and level of activity in the Earth’s magnetosphere.

With the principle of a connection thus established, Pustilnik and Yom Din go on to suggest three conditions under which space weather could lead to famine:

• Local weather has to be in a “threshold state” such that it is sensitive to space weather. For example, if there is no water vapour present, clouds won’t form even if space weather is “seeding” the Earth’s atmosphere with lots of extra ions.

• Harvests must be sensitive to weather anomalies. This is more likely in areas of so-called “risk farming”, where conditions are marginal enough that a few days of bad weather can completely wipe out a crop.

• The area has to be economically isolated, such that local shortages cannot be ameliorated by buying grain from elsewhere.

To test these hypotheses, Pustilnik and Yom Din begin by comparing levels of solar activity with grain prices in 17th century England. Between 1590 and 1700, the price of grain in England and the abundance of 10Be isotopes (a proxy for solar activity) in Greenland ice cores exhibit an almost exactly inverse relationship. High prices correspond to periods of low solar activity and vice versa. Several other European markets that the authors studied also showed strong correlations between grain prices and solar activity, but in southern Europe, where crops are more likely to suffer from drought than from excess rain, prices tended to spike during solar maxima rather than minima.

Things get a bit shakier when the authors turn their attention to 19th century Iceland. In this case, famines seem to correlate with both minima and maxima in solar activity. Pustilnik and Yom Din claim this is what they expected to see, but don’t really say why; in particular, they don’t explain why the Icelandic pattern should differ so markedly from the English one.

Still, it’s an interesting study, and reading it stirred up some memories from my brief foray into medieval studies. In particular, I thought of a book called Lost Worlds whose author, a Swiss historian called Arthur Imhof, makes unusually good use of hard data in analysing what life was like for an ordinary person in early modern Europe. Might his book have something to add to the famine/space weather debate?

I skimmed my copy of Lost Worlds a couple of times before I located the bit where Imhof writes about famine. Tree-ring data and written sources from the 16th and 17th centuries, he notes, indicate a long series of harsh winters and summers with too much rain, resulting in exceptionally bad growing conditions. As a result, he adds, “our ancestors had more reason to beg for their daily bread between 1550 and 1700” than they did at almost any point before or since.

This is, of course, almost exactly the same period that Pustilnik and Yom Din studied, and it’s nice to see that Imhof’s sources corroborate their grain-price data. But Imhof wasn’t interested in climate for climate’s sake. Instead, he was trying to demonstrate that populations in areas prone to famine, plague and war became traumatized by their repeated misfortunes. You’d have to read the book to appreciate Imhof’s argument in full, but among other things, he suggests that people in these “unlucky” areas developed fatalistic attitudes to life, death and birth. These attitudes show up not only in religious beliefs, but also in data on infant and maternal mortality. For example, even in peaceful, plague-free years, more than one-third of babies born in the plague-prone and war-torn German village of Gabelbach died in infancy. In “luckier” villages, the comparable figure was one in eight.

Where does this leave us regarding space weather? Well, if we add Imhof’s conclusions to Pustilnik and Yom Din’s, it seems that the behaviour of heavenly bodies could have influenced not only the viability of medieval grain crops, but also the habits and attitudes of the people who tended them – perhaps even to the extent of determining whether their children were likely to live or die. That might not be very surprising to the peasants of 17th century Gabelbach, who lived in a more religious age (and, according to Imhof, believed fervently in astrology). But to me, it’s absolutely mind-blowing – and a whole lot more interesting than England’s “Tricky Dick” turning up in a car park.

Physicists extract photons from diamond ring

Physicists in the US are the first to make an integrated device that extracts photons from a tiny piece of diamond before the light is sent through a waveguide to the outside world. The photons all have the same frequency and originate in a nitrogen vacancy (NV), which is a defect that occurs in diamond when two neighbouring carbon atoms are replaced by a nitrogen atom and an empty lattice site. According to the researchers, the chip could be used to create quantum-information technology such as quantum repeaters.

For anyone trying to build a quantum computer NVs are useful because they have an electronic spin that is extremely well isolated from the surrounding lattice – so if an NV is placed in a certain spin state then it will remain in that state for ages, even at room temperature. An NV can also emit just a single photon if excited by a laser of the right wavelength. Taken together, these properties mean that NVs allow data to be stored for long times in a defect, before being read out as a single photon.

Researchers are particularly interested in extracting photons that do not interact with the surrounding lattice because these “zero phonon line” (ZPL) photons have a well defined frequency. Unfortunately, one challenge in building NV-based quantum systems is how to reliably get ZPL photons out of the diamond and into an integrated optical system, where it can be processed further. What Andrei Faraon and colleagues from Caltech, Hewlett Packard and the University of Washington have managed to do is to create an integrated optical system that does just that.

Matching frequencies

At the heart of their device is a ring of diamond that is just 4.5 μm in diameter and contains NV centres. The ring sits next to a waveguide that is about 10 μm long (see figure). The device is cooled to below 10K and the ring is scanned with a green laser until a NV centre with a resonant frequency close to that of the ring is located. The team then introduces a noble gas into the cryostat and some of it condenses on the ring – changing its resonant frequency. More gas is added until the frequencies of the NV centre and the ring exactly match.

The ZPL photons are created by firing the green laser at the NV centre. The photons first circulate around the ring before jumping into the wave guide. They then travel to either end of the waveguide, where a diffraction grating scatters them out of the device, where they can be observed with a microscope connected to a spectrometer and a photodetector.

The researchers found that they collected about 25 times more ZPL photons from these devices than were collected from NV centres in similar samples of diamond that were not part of integrated devices.

Faraon sees this work as an important step towards creating integrated circuits in which ZPL photons carry quantum information from one NV centre to another. “We demonstrate that photons – the information carriers – from a single NV centre can be coupled to an optical resonator and then further coupled to a photonic waveguide,” he says. “We hope that multiple devices of this kind will be interconnected in a photonic network on a chip.”

What Faraon and colleagues want to do now is to develop devices that include more than one NV centre and show that photons emitted by two NVs can be made to interfere – a pre-requisite for entangling NV centres. Once entanglement has been achieved, the devices could then be used as quantum repeaters, which absorb and re-emit entangled photons without disturbing the entangled state – something that is necessary if quantum information is to be transmitted over large distances. Faraon told physicsworld.com that his colleagues at Hewlett Packard are now working on entangling NV centres on the same chip.

The device is described in the New Journal of Physics.

Condensing matters drastically at Imperial College

Will the universe go on expanding forever? Why should we care about climate change? Can we make objects invisible?

These are Big questions with a capital B, which individually could occupy the mind of a scientist for an entire academic career. In fact I am sure they have.

But yesterday at Imperial College in London we asked a bunch of physicists to tackle questions of this size and stature and to answer them in 100 seconds or less – using nothing more than a white board and a few marker pens. It was a seriously tough challenge in the overlapping arts of brevity and clear communication.

The presentations were filmed as part of our 100 Second Science video series and they will be joining the existing batch of these “mini lectures”. The picture above shows the PhD student and radio DJ Martin Archer preparing for his moment in the spotlight during which he tackled several questions on the fundamentals of quantum mechanics.

One of the questions Martin addressed related to one of the seemingly paradoxical implications of quantum mechanics: “Is Schrödinger’s cat dead or alive?”. I won’t spoil Martin’s 100 Second Science video on this famous thought experiment, but let us know your thoughts on this question by visiting our Facebook page and taking part in our poll where we ask:

Is Schrödinger’s cat dead or alive?

It must be one or the other at any given time
It exists in a superposition of dead and alive
It could be dead and alive in separate universes
Another outcome is possible

We look forward to your responses. Look out for more in this series of films over the coming months.

Voltage boosts bubble fluid flow

A team of researchers in France has found that applying a voltage across a 100-nm-thick cylindrical soap film causes the fluid inside it to flow upwards. If the voltage is increased, the film thickens and the flow rate increases significantly. The researchers say this could be useful in microfluidic systems and to stabilize liquid foams.

The field of microfluidics involves the manipulation of minute amounts of liquids – generally picolitre (10–12) quantities – within micron-width channels. This allows scientists to work with substances that are expensive to produce in large quantities. Also, it allows many different liquids to be used simultaneously, creating a “lab on a chip” platform for the study of many chemical processes at once.

Soft channels

Anne-Laure Biance, Oriane Bonhomme and colleagues at the University of Lyon in France have been studying nanochannels in their lab, and became interested in producing soft nanochannels, as compared with hard ones, because they felt that such soft channels would be easier to make and use, and would cost less. So the researchers looked at soap films – a soft deformable channel that consists of two sheets of ionic surfactant molecules that enclose a layer of water. Biance also feels that soap films are convenient as their thickness is usually at the nanometre scale and they are easy to make.

The team used two platinum-covered plates (the electrodes), separated by about 0.5 cm, facing each other. The researchers then created a bubble out of a soapy liquid made of water, a surfactant and potassium chloride that produces the free ions, and trapped this bubble between the plates.

“We then applied different voltages to the plates, and observed that the liquid inside the bubble flows from the bottom to the top,” explains Biance. The walls of the bubble act as the channel for the flow. As the surfactant molecules are positively charged and the chloride ions are negatively charged, the molecules attract and this causes the drag. That is, the surface ions are pulled by the electric field and they drag the fluid inside the channel along with them.

Drag and flow

This behaviour, known as an “electro-osmotic” flow, can be described as the motion of liquid induced by an applied potential across a porous material, capillary tube, membrane, microchannel or any other fluid conduit. The effect of this type of flow becomes more pronounced at the micro- and nanoscale, where there is a high surface-to-volume ratio. While this has been studied with solid nanochannels, Biance says that not enough is known about the behaviour in soft channels, where the ratio can change.

When Biance and her colleagues measured the magnitude of the flow in their experiment, they also found that the soap film itself had thickened. The researchers could distinguish between the current caused by surface ions and the current caused by the flow of the liquid being dragged (bulk flow). They found that as they increased the voltage, the bulk flow increased at a faster rate than expected. “This was because our film was actually thickening. We could see a change of colour of the film too, and this is only seen when there is a thickness modification,” explains Biance. “We measured the growth of the film’s upper meniscus, where it touches the plate, and we measured the growth rate,” she says, which proved that a wider channel allows the rate of flow to increase quickly.

Thickening effects

Biance told physicsworld.com that this thickening effect is comparable to another effect that explains why a solid plate lifted out of a liquid bath is never dry. Instead, it lifts up a film of fluid thanks to viscous forces that come into play because the plate is being pulled and is known as the Landau–Levich flow. She also points out that the flow and the applied voltage have a nonlinear relationship. “This is interesting because it almost has diode-like properties,” says Biance. The team is also interested in seeing whether any of these effects would help in stabilizing liquid foams that collapse because of the effects of gravity on the liquid. “We might be able to use a voltage to stabilize them,” says Biance.

Currently, Biance and her colleagues are looking at how their experiment would change if they used two bubbles, placed side by side or on top of each other. “Other factors come into play, such as the border between the bubbles and the flow there. In the end, we would like to do this with many bubbles,” says Biance.

The research is published in Physical Review Letters.

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