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Home » Page 1324

Source produces indistinguishable electrons

A new method to produce indistinguishable and coherent electrons has been developed by scientists in France. The researchers have created a small, electron-emitting chip and used it to produce two single electrons emitted from different sources that are in the same quantum state. This technique is a key step for developing electron-based quantum-information-processing techniques.

Fermionic rules

Electrons are fermions and so must obey Pauli’s exclusion principle that prevents identical fermions from occupying the same state, which leads to anticorrelations or “antibunching”. Although this was recognized decades ago, it has proved difficult to perform such an antibunching experiment because electron beams are not coherent – there are many electrons in any system and they all interfere with each other, as well as the environment.

This is what encouraged Erwann Bocquillon and Gwendal Fève at the Ecole Normale Supérieure in Paris, along with colleagues from the Laboratory for Photonics and Nanostructures near Paris and from the Ecole Normale Supérieure de Lyon, to see if indistinguishable electrons could be generated by independent sources, as is done in optics. “We now understand how electrons move in a system – a very fundamental issue – a lot better. Of course, it is also important to produce such electrons to encode quantum information in the future, but we were most interested in the fundamental proof of concept, in this case,” explains Fève.

Restricted movement

The researchers’ electron-emitting “chip” was built using a “very clean” micron-sized bulk-semiconductor sample in which the electrons propagate in very straight lines for several microns in 2D before getting scattered, limiting their interactions. This can be seen as the green plane in the artist’s illustration above. The team then uses a strong magnetic field to further restrict the movement of the electrons to only 1D along the edge of the plane (denoted by the arrows in the illustration), such that single electrons may be guided to each of the emitters – the two small islands of the electron gas located on each side of the picture. The gold sections next to the emitters represent metallic electrodes deposited on top of the electron gas.

By applying a voltage pulse to the metallic electrode deposited on top of the emitter, the researchers trigger the emission of a single electron to an electronic beamsplitter that is made up of two input and two output arms. Fève told physicsworld.com that their sample is capable of emitting billions of single electrons per second – one electron per nanosecond.

Perfectly synchronized

“The two sources are perfectly synchronized such that both particles arrive simultaneously on the splitter and perfect antibunching occurs, meaning the two electrons always exit in different outputs,” explains Fève. That means that if a single electron is sent in one of the input arms with the other input being empty, for example, the electron would escape randomly in one of the outputs. But in the experiment, the two electrons, generated by the two identical, synchronized but otherwise independent emitters would arrive simultaneously at the two input arms of the splitter and would always emerge in two distinct outputs, obeying Pauli’s principle.

“This electron antibunching effect can only be explained by quantum mechanics. So it is a quantum interference between two particles and that relates to their indistinguishability. This would only happen for two electrons in the same pure quantum state that has not been affected by interactions with the environment, making the electrons indistinguishable and coherent,” says Fève.

But he is also quick to point out that while the team did achieve a high degree of indistinguishability, the electrons were not completely so, meaning that some minimal environmental interaction did occur. “To be able to entangle the electrons, wherein they would violate Bell’s inequality, they need to be completely indistinguishable – so this is something we are currently investigating and working on,” says Fève. The researchers are looking at making their sample even smaller so that the electrons travel even shorter distances, while keeping in mind the effects of temperature at such sizes.

Fève says their method shows that it is indeed possible to produce well-controlled single electrons. “Our technique also provides a lot of tenability in terms of the energy and rate at which one would want to produce electron wavepackets, on demand, in the lab,” he explains, saying that the degree of control their source offers is its main advantage.

The research is published in the journal Science.

Nanosilicon produces hydrogen on demand

Silicon nanoparticles could be used to produce hydrogen almost instantly, as they react with water, according to researchers at the University at Buffalo (SUNY) in New York. The reaction does not require any heat, light or electricity and the hydrogen generated could be used to power small fuel cells. The technology could come in handy as a “just add water” approach to produce hydrogen on demand, says the team. In essence, the technique recovers some of the energy that goes into refining the silicon and producing the nanoparticles in the first place.

Splitting water to produce hydrogen is a clean and renewable way to produce energy, and traditional techniques to split water include electrolysis, thermolysis and photocatalysis. Water can also react with bulk silicon to produce hydrogen, but this route has been little studied because it is slow. In theory, silicon can release two moles of hydrogen gas per mole of silicon (or 14% of its own mass in hydrogen). Silicon is also abundant on our planet, has a high energy density and does not release any carbon dioxide when it reacts with water.

Faster reaction rates

Thanks to their high surface to volume ratio, silicon nanoparticles should naturally generate hydrogen much more quickly than bulk silicon. Now, a team led by Paras Prasad and Mark Swihart at Buffalo has shown that the increase in reaction rate is much greater than would be expected based on increased surface area alone. In fact, nanoparticles 10 nm in diameter appear to produce hydrogen in under a minute, compared with around 45 minutes for nanoparticles that are 100 nm. The 10 nm particles are also 1000 times faster at producing hydrogen than is bulk silicon.

A large group of silicon nanoparticles
A large group of silicon nanoparticles This transmission-electron-microscopy image shows spherical silicon nanoparticles about 10 nm in diameter. These particles react with water to quickly produce hydrogen. (Courtesy: Swihart Research Group, University at Buffalo)

According to the SUNY team, the difference in hydrogen production rates between the 10 and 100 nm-sized silicon particles is much greater than can be accounted for by the difference in the surface areas of the particles. To understand this difference, the researchers conducted experiments in which they stopped the reaction before all of the silicon had been fully consumed. As the reaction proceeds, the 10 nm silicon particles reduce in size but do not change shape and remain roughly spherical.

The 100 nm particles, on the other hand, do not uniformly reduce in size but form hollow shells or capsules with walls consisting of a few monolayers of silicon. These walls then slow down the water–silicon reaction because they provide an extra layer through which the reactants must diffuse. Particles that are initially larger also have less surface area per unit volume.

Ideal for powering portable devices

“With further development, this technology could be ideal for powering small portable devices and might even replace bulky gasoline or diesel generators in the future,” says Prasad.

“A typical silicon generator could comprise a small hydrogen fuel cell and some plastic cartridges of silicon nanopowder, to which water would be added when needed, to produce energy,” adds team member Folarin Erogbogbo.

Although the technique could probably not be used to generate large amounts of hydrogen, the overall efficiency of the process could be quite competitive with primary batteries and other sources of portable power, which makes it interesting for these applications, Swihart told physicsworld.com.

The researchers have already successfully tested their technique in a small fuel cell that they used to power a fan. They are now planning to study the hollow nanostructures formed by the reaction of the larger silicon particles in more detail and look at how hydrogen can be produced when silicon nanoparticles are mixed with other materials, such as alkali hydrides. “These hollow ‘nanoballoons’ may have interesting applications in other areas such as anodes for lithium-ion batteries,” explains Swihart. “Alkali-metal hydrides react with water to release hydrogen and produce alkali-metal hydroxides (such as sodium hydroxide, for example) needed to catalyse the silicon reaction with water. On their own, the metal hydrides are air reactive and unstable, but coating them with silicon nanoparticles might let us increase the hydrogen generation capacity of the system while maintaining an air-stable, easy-to-handle material.”

The research is published in Nano Letters.

Laser pulses cool semiconductor

Physicists in Singapore are the first to create a refrigerator that cools a piece of semiconductor using light, using their technique to cool a room-temperature sample of cadmium sulphide by some 40 K. Although a similar technique has previously been used to chill glasses doped with rare-earth elements, this latest work could lead to practical optical refrigeration devices for use in satellites, or even “self-cooling” lasers.

First developed in the 1980s, laser cooling has opened up the new and incredibly fruitful study of ultracold atomic gases. The technique involves firing counter-propagating laser beams at an atomic gas, with the atoms absorbing and emitting photons in such a way that the net effect is to reduce the average motion of the atoms, and thus lower the temperature of the gas.

Removing phonons

The laser cooling of solids is somewhat different because heat is stored in a solid in the form of quantized lattice vibrations called phonons, which do not interact directly with light. In the case of rare-earth-doped glasses, energy is removed from the phonons when a single atom in the glass undergoes an “anti-Stokes” transition. This involves a photon being absorbed by an atom before emitting a higher-energy photon – with the extra energy coming from phonons.

From a technological point of view, it would be much more useful to be able to laser-cool a more conventional material, such as a semiconductor, than a doped glass. Last year, Eugene Polzik and colleagues at the University of Copenhagen managed to use a laser to cool an extremely thin sheet of semiconductor that was stretched like a drumhead. Rather than being a general refrigeration method, however, the optomechanical technique focused on damping out a specific subset of drum-like phonon modes in the sheet.

Chilly nanobelts

What Qihua Xiong and colleagues at Nanyang Technological University in Singapore have now demonstrated is a more general technique that uses lasers to cool an extremely thin ribbon (or “nanobelt”) of the semiconductor cadmium sulphide (CdS). The method also relies on an anti-Stokes process, but in this case the transition involves an absorbed photon being converted into an electron–hole pair. This “exciton” annihilates and the semiconductor emits a higher-energy photon – with the extra energy coming from the annihilation of phonons. As a result, the sample loses phonons and cools.

Xiong told physicsworld.com that his team stumbled upon the effect by accident when doing laser-based Raman-spectroscopy experiments on the CdS nanobelts – materials that have a particularly strong anti-Stokes photoluminescence. The nanobelts were about 3 μm wide and about 100 nm thick, and were draped across a silicon-oxide substrate that was peppered with holes that were about 4 μm across. Measurements were made on the portions of the nanobelts that were suspended over the holes.

Strong coupling

The experiment involved firing “pump” laser pulses at the nanobelt to create excitons, with the laser energy adjusted so that the exciton energy plus the energy of several phonons equals the energy of a photon emitted in an anti-Stokes transition. Each photon emitted in this way therefore takes a significant amount of heat energy. Indeed, Xiong says that more than 100 meV of energy is removed per pump photon – the very high efficiency being because excitons and phonons in CdS nanobelts couple very strongly.

The team began the cooling process with the nanobelt at room temperature (290 K) and then reduced the temperature to about 250 K in about 40 min. This corresponds to a cooling power of 180 μW. The temperature of the sample was measured using a technique called pump-probe luminescence thermometry, which involves firing a second “probe” laser pulse at the sample.

Sensors in space

According to Xiong, the cooling technique could be used to cool tiny devices. As well as being relatively straightforward to miniaturize, laser cooling does not involve mechanical refrigeration – which can introduce unwanted vibration – or cryogenic liquids. One application that Xiong says is “particularly appealing” is the cooling of sensors used on satellites and other space missions. He also says that the technique could be used to cool a laser by using some of its own light.

While Xiong says that there are several challenges that must be overcome to make the technique work on larger samples of semiconductor, it is, in principle, possible. Polzik described this latest cooling technique as “a very interesting result” adding that, in principle, the technique could be used to remove heat from semiconductor devices.

The cooling method is described in Nature.

Einstein portrait to appear in New York show

Ink sketch of Einstein
Ink-on-paper sketch of Einstein. (Courtesy: Sigrid Freundorfer Fine Art LLC

This ink-on-paper sketch of Einstein will go on public display for the first time tomorrow as part of an art show in New York City. It is the handiwork of Josef Scharl, a German artist who produced the work in 1950 while visiting his close friend Albert Einstein at Princeton University in the US.

Born in Munich in 1896, Scharl gained recognition in his time after being part of the “New Munich Secession” artists in the 1920s. He won various awards including the Albrecht Dürer Award from the city of Nuremberg, and the Prix-de-Rome. But Scharl was a vocal critic of the Nazi Party and by 1935 he was considered a “degenerate artist” and banned from painting.

Einstein, who by this time was already working at the Institute for Advanced Study at Princeton, had met Scharl in 1927 in Berlin at the house of photographer Lotte Jacobi. Upon learning of the fate of his friend, Einstein offered to sponsor Scharl’s immigration to the US, which the artist accepted. Once in the States, Scharl used to visit Einstein regularly and when the artist passed away in 1954 Einstein wrote the eulogy that was read at the funeral.

“Scharl was an outspoken man, not shy with his opinions, often rather witty. Einstein appreciated Scharl’s candor and views on this or that, and their conversations were lively and informative for both,” says Sigrid Freundorfer, the fine-art dealer based in New York who owns the drawing. “It must have been refreshing for Einstein to have had somebody like Scharl to talk to once in a while, in German at that.”

Freundorfer bought the painting last year from someone in the field of manuscripts and rare books “Being an art dealer, I bought it first as a magnificent drawing by Josef Scharl, depicting this great man Einstein, signed by both men,” she said. The image will go on sale at the Master Drawings New York exhibition, which runs 26 January – 2 February and has a preview show on 25 January.

In your interpretation of quantum physics, do objects have their properties well defined prior to and independent of measurement?

Last week my colleague Hamish Johnston wrote about a fascinating survey carried out recently in the quantum research community. Physicists, philosophers and mathematicians were asked to give their responses to a series of questions about the foundations of quantum mechanics. Topics covered aspects of the subject from Einstein’s views on the topic to the prospects of a practical quantum computer. The survey is described and analysed in this accompanying paper posted on the arXiv preprint server.

Perhaps the most fascinating outcome of the survey was the extent of variation in responses to the questions about interpretations of quantum mechanics. This is perhaps surprising given the fact that the modern theory of quantum mechanics has been knocking around now for the best part of a century.

Perhaps it just goes to show how many of the key concepts at the heart of this strange theory are still strong sources of debate for physicists. In this week’s Facebook poll we thought it would be interesting to ask you one of the questions from this recent poll:

In your interpretation of quantum physics, do objects have their properties well defined prior to and independent of measurement?

Yes, in all cases
Yes, in some cases
No
I’m undecided

Let us know by visiting our Facebook page, and as always please feel free to post a comment to explain your answer.

In last week’s poll we asked you a question about the mechanism by which fundamental physics research is transformed into commercial products. We asked you whether you think patents are hampering the commercialization of graphene. The question was motivated by the publication of a new report from the intellectual-property consultancy CambridgeIP, which suggests that the UK might be losing out in the quest to commercialize this material. 78% of respondents said “yes” it is being hampered, while the remaining 22% said no.

Thank you to everyone for taking part and we hope to hear from you again this week.

Optical tractor beam sorts tiny particles

Schematic illustration of the tractor beam set-up
Pushing and pulling using polarized light: schematic illustration of the tractor beam set-up. The particles (red balls) are held between a mirror (lower sheet) and a transparent slide (upper sheet). Laser light (green) is focused at the region between the mirror and slide, causing the particles to arrange themselves neatly at the left. (Courtesy: O Brzobohatý et al.)

The idea that light can grab hold of objects may sound like science fiction, but optical tweezers that hold particles at a laser focus are widely used today. An even more fictional-sounding concept is the tractor beam – a beam of light or sound that can pull an object towards it. Over the past three years or so, physicists have shown that tractor beams are theoretically possible – and there has been a flurry of activity in the lab to try to build one. Now, researchers in the Czech Republic and the UK have produced a simple example using two laser beams. And as an unexpected bonus, they have also discovered a potentially powerful technique for sorting microscopic particles.

Optical tweezers can hold a particle still at the focus of a laser; but if you want to move the particle, you have to move the focus by, for example, adjusting a lens. In theory, a tractor beam is a beam of light that can reel in particles towards the source of the radiation without having to fiddle with foci. However, photons carry momentum, which can be transferred to a particle and therefore nudge it away from the source – something that seems to rule out a tractor beam.

However, in 2006 Philip Marston of Washington State University showed that there is a little flexibility. If the particle is irradiated with a specially shaped beam called a Bessel beam, consisting of wavefronts that form concentric circles centred on the object, then the recoil momentum that the particle acquires from scattering photons forward can exceed the momentum that it receives from the incoming photons. According to his calculations, the net force that the particle experiences can be directed back towards the light source.

No easy task

Actually creating a Bessel beam that can exert a pulling force over any significant distance is no easy task, however, because it would require an incredible amount of energy. As a result, tractor beams have remained a theoretical construct – although scientists such as physicist David Grier at New York University have constructed imaginative approximations.

In this latest research, physicists at the Institute of Scientific Instruments of the Academy of Sciences of the Czech Republic and the School of Medicine at the University of St Andrews have produced a simpler version of a tractor beam. It comprises two laser beams brought to a focus with a lens – a relatively simple geometry that can easily be made using a standard commercial microscope system. They found that by focusing the light inwards they could generate the same effect as if they had used a Bessel beam.

“Our geometry represents an alternative to the previously proposed optical fields based on Bessel beams” explains group leader Pavel Zemánek, “the concept allows larger extent of the tractor beam existence as well as controlling the polarization that plays a very important role”. The researchers managed to pull particles 30 μm, and could have gone further if they had used more powerful lasers.

Unexpected and useful effects

As an added bonus, the researchers found that the light had some unexpected and potentially very useful effects on the particles. For example, whether particles were pushed or pulled by the laser light depended on both the size of the particles and the polarization of the light. Light that was s-polarized, so that the electric field lay in the plane of the incident and reflected rays, was often found to pull particles of a particular size. On the other hand, light of the same wavelength that was p-polarized – where the electric field was perpendicular to the plane – would push it.

This allowed the researchers to devise a way of separating mixtures containing particles of two different sizes simply by switching the polarization of the light. The researchers also observed “optical binding”, whereby illuminated particles could be induced to stick together and self-arrange into various structures.

Polarization has potential

Optical physicist Miles Padgett of the University of Glasgow believes that the discovery of the polarization dependence is where the real interest of the work lies. “You would always expect some difference with polarization,” he says, “but the fact that the difference is big enough to do something with is the surprising thing.”

David Grier agrees. “This really is a clean demonstration of Marston’s principle in action,” he says. “It had to work, but it was really disappointing that it had not been made to work, given how many people had been working on it for so many years. But probably, going forward, the big long-term applications are using polarization to control self-organization and creating an extended light field that achieves sorting in the way that this does. And there it is not so much as a tractor beam but as the driving force for a lab-on-a-chip system.”

The research is published in Nature Photonics.

Philosophical about space–time

Conceptual illustration showing a clock suspended in space, surrounded by stars and nebulae
Ways of thinking: Tim Maudlin’s new book looks at space and time from a philosophical, as well as a technical, point of view. (Courtesy: Mike Agliolo/Science Photo Library)

Canonical works in the philosophy of physics tend to begin with the Greeks. This practice stems from the philosopher’s need to flag the intellectual tradition from which current physics and its philosophy are derived – albeit with plenty of ruptures, fractures and slippages through the centuries. Even with these bumps in the road, there appears to be a more distinct connection between Greek science, early modern science and the discipline we know today than there is between modern science and the scientific schools of, say, the ancient Indus Valley or Babylon.

The first source of this connection is a set of logical and mathematical formalisms. The second source consists of specific concepts – space, time and matter – that interest modern physicists as much as they did ancient philosophers. The offspring of these two sources is geometry, the flexible mathematical tool that allowed some of humanity’s earliest preoccupations to be translated into tangible scrawls and shapes, ready for thought experiments and demonstrations.

It makes sense, then, for the philosopher Tim Maudlin to begin his latest book, Philosophy of Physics: Space and Time, with geometry – and, more specifically, with that classical Greek-inspired version called Euclidean geometry, which describes the 3D space of our mundane world. Maudlin is also careful to note the physical heritage that accompanied geometry’s introduction, via a brief foray into Aristotle’s Physike Akroasis. There, Plato’s most famous student laid the foundation for thinking about the nature of an object’s motion – something Maudlin returns to later in the book with his discussion of Newton’s laws. By providing such background, he ensures that the reader knows the prehistory of many terms used throughout the book.

Although Maudlin mentions matter in his discussion of the Greek philosophers’ atomistic postulate, and then later, in his account of the philosophical debate between Newton’s supporter, Samuel Clarke, and his great rival, Gottfried Leibniz, he ultimately decided to reserve much of this weighty topic for the forthcoming second volume of the book. In the current volume, he concentrates instead on giving a comprehensive historical overview of the development of the concept of space and time, beginning with an expostulation of Newton’s three laws of motion and continuing as far as Einstein’s special and general theories of relativity.

One of the book’s underlying themes is the way certain aspects of physics have been subject to misconceptions over the years. For example, the book explains how the theory of relativity became a potent source of confusion for those physicists (including, at one point, Richard Feynman) who attempted to frame the theory in terms of the symmetry between the actions of two objects interacting with one another, and external forces that interact with the motion of the objects. The confusion, in Maudlin’s view, comes from a common misunderstanding, even among latter-day scientists, over the question of absoluteness in motion and space, and of the inertial reference frames pertaining to the coordinate system used.

Such confusion has a long history, and in the book, Maudlin describes an interesting early conflict between the claim that all motions are relative and the possibility that absolute motion could exist. This is Newton’s famous “bucket argument”, in which he described a thought experiment featuring a bucket filled with water and hung from the ceiling by a strongly twisted rope. When the rope is released, the water will slosh in the bucket – evidence, in Newton’s view, that a force is acting on the bucket in relation to absolute space. In another thought experiment, Newton imagined two globes rotating about their centre of gravity and connected by a cord. In his view, the tension in the cord is an effect of the absolute rotation of the spheres, since their position in relation to each other is constant. Hence, the globes and the bucket experiments were seen as confirmation of Newton’s theory on absolute space and motion. Newton’s beliefs have not persisted to this day as scientists have stopped believing in the idea of absolute motion.

Modern physics texts re-interpreted Newton’s early geometrical conception of his three laws through the rules of arithmetic, hence stripping them of the form familiar to their inventor. Since the theory of relativity takes Newtonian mechanics as its starting point, Maudlin argues that much confusion would be avoided by using geometrical spatial coordinates, rather than the more often used inertial frame of reference, to explain relativistic effects such as the famous “twin paradox”.

Throughout the book, Maudlin takes much pain to correct what he considers to be misconceptions and confusions about space and time. He may have adopted this strategy because his book is targeted at the non-physicist, and therefore at an audience with no specific investment in other explanations. However, even though the book is meant for the non-specialist, its subject matter is highly technical, and reading it requires constant switching between a philosophical and technical way of thinking. A non-expert reader would have to work patiently through some non-intuitive and potentially confusing points that are not so clearly set out by the author.

Although Maudlin concentrates on pre-quantum physics in the book, he connects some of the questions arising in relativity, such as observability and a discussion of the light-cone, as a way of visualizing events within temporal space to certain experimental conditions in particle physics. This seems like a smart move, given the recent media interest in accelerator physics and experiments at CERN, and the corresponding greater familiarity with these topics among a lay audience. He also touches on the problem of interatomic forces raised in Bell’s Theorem, which builds a discursive bridge between classical determinism and some types of quantum “hidden variables” known as non-locally observable properties. (Maudlin discusses this in more detail in his book Quantum Non-Locality and Relativity: Metaphysical Intimations of Modern Physics.) Such extensions allow the reader a peek at why physical ideas hundreds of years old are still relevant to today’s most cutting-edge problems.

  • 2012 Princeton University Press $29.95pb 200pp

Digital files stored and retrieved using DNA memory

Forget hard disks or DVDs. If you want to store vast amounts of information look instead to DNA, the molecule of which genes are made. Scientists in the UK have stored about a megabyte’s worth of text, images and speech into a speck of DNA and then retrieved that data back almost faultlessly. They say that a larger-scale version of the technology could provide an extremely dense and long-lived form of digital storage that is particularly well suited to data archiving.

As ever-greater quantities of electronic data are produced, the problem of how to store that data becomes more acute. There are many options for archiving data but all have their drawbacks. For example, hard disks used in data centres are expensive and need a constant source of electricity, and magnetic tape, while requiring no power, starts to degrade after a few years.

Neanderthal bones

In the latest research, Nick Goldman and colleagues at the European Bioinformatics Institute near Cambridge have stored digital information by encoding it in the four different bases that make up DNA. While the storage technique does not offer the convenience of random access or being rewriteable, it does have a couple of major advantages. One is its extremely high density – as a result of the information being stored at the molecular level – and the other is its durability. As Goldman points out, intact DNA has been extracted from Neanderthal bones tens of thousands of years old. “Nature has discovered that this molecule is very stable,” he says. “And we are piggy-backing on nature.”

The group used DNA that was produced in the lab rather than from inside living organisms, since the latter is vulnerable to mutation and hence data loss. But in choosing this approach the researchers had to overcome a couple of significant hurdles. One was the fact that using current technology it is only possible to make, or “synthesize”, DNA in short strings – and the shorter a string the lower is its information-carrying capacity. To get round this problem, Goldman and colleagues devised a coding scheme in which a fraction of each string is reserved for indexing purposes, specifying which file the string belongs to and at what point in the file it is located, so allowing a single file to be made up of many strings.

Encoding trits

The second challenge was how to avoid errors that occur during both writing and reading, a particular problem when neighbouring bases are of the same variety. The solution was simply to encode data in trits – digits with the values 0, 1 or 2 – and stipulate that a given trit is represented by one of the three bases not used to code the trit immediately preceding it. An additional measure was to copy the final 75% of each string into the start of the successive string.

The team tested the scheme by encoding five data files into single DNA sequences and then split those sequences up into roughly 150,000 individual strings, all 117 bases long. Fittingly, one of the files was a PDF of Watson and Crick’s famous double-helix paper – successfully encoded into double helices. The text of Shakespeare’s sonnets and an audio recording of 30 s of Martin Luther King’s “I have a dream” speech were also stored in MP3 format. The team then uploaded the encoded files to a private webpage to enable Agilent Technologies in California to synthesize the DNA. This involved using a sophisticated kind of inkjet printer to fire chemical reagents onto a microscope slide in such a way as to add one molecule at a time to a growing string of DNA, and then repeating the process to produce the thousands of strings required.

Sent as a tiny quantity of powder at room temperature and without specialized packaging, the DNA arrived in Heidelberg, Germany, at the main site of the European Molecular Biology Laboratory, of which the European Bioinformatics Institute is a part. After being put into solution the DNA was read, or “sequenced”, using a now fairly standard laboratory machine, and the resulting series of bases was then decoded on a computer to reproduce the five files. Four of the files were identical copies of the originals, while the fifth required some minor adjustment to recover its full set of data.

Video in a teacup

Goldman and colleagues claim to have achieved a density of 2 petabytes (1015 bytes) per gram of DNA which, they calculate, would allow at least 100 million hours of high-definition video to be stored in a teacup. Their DNA sample was therefore very small. “In our test tube the DNA looks like a speck of dust,” says Goldman. “In fact the sample is so small that when it arrived it looked like the test tube was empty.”

Currently the technology is too expensive to be competitive for all but the most long-term archiving. But Goldman is confident that prices will come down, given the continuing interest in DNA research. If the cost of synthesizing DNA falls by a factor of 100 over the next decade, which he says is possible, he says the technique will be as cheap as magnetic tapes for archives extending over at least 50 years. This is because unlike tapes, which need to be periodically rewritten, DNA remains unchanged as long as it is stored somewhere that is cold, dry and dark.

The current work follows similar research done last year by a team that included Sriram Kosuri of Harvard Medical School. His group used an encoding scheme that involved bits rather than trits and which included relatively little redundancy. However, he says that the two techniques are nevertheless “similar approaches to the same concept,” adding that both sets of research show DNA storage to be “approaching scales that should be of interest to investors”.

The latest research is published in Nature.

All together now

How do you tackle the world’s biggest problems such as making sure everyone has enough food, clean water, a secure energy supply and access to proper medicine and healthcare?

According to Shirley Ann Jackson, president of Rensselaer Polytechnic Institute in the US, the answer lies in a novel kind of research endeavour, which she dubs “the new polytechnic”.

Speaking at the 2013 ERA Foundation international lecture at the Royal Academy of Engineering in London last night, Jackson spelled out the principles of such an endeavour, which would essentially involve bringing researchers from different subjects, countries, cultures and sectors together to work on important multidisciplinary problems.

Exploiting computer technology, the Web and big data sets would be the key to tackling such challenges, she reckons.

Jackson, who trained as a particle theorist and is also a member of Barack Obama’s science advisory council discussed three interesting fields that, she thinks, could – indeed, already do – benefit from such an approach. They are using tissue-regeneration techniques to heal injured patients, incorporating solar panels and other forms of energy-saving devices into buildings, and exploiting the “data trails” we leave when we use social media.

I found Jackson a polished speaker, no doubt honed by her years in top positions in the US. Apart from being one of Obama’s science-policy wonks, she was boss of the US Nuclear Regulatory Commission between 1995 and 1999 and is a former president of the American Association for the Advancement of Science. Before that Jackson was part of the faculty at Rutgers University, spent 16 years at Bell Labs and had spells as a postdoc at Fermilab and CERN. Her impressive CV also includes a string of directorships at the likes of IBM and FedEx.

Jackson was less detailed on the nitty-gritty of setting up her “new polytechnic” or explaining who would fund such an enterprise. Last night was perhaps not the forum for those questions. But to me the elephant in the room was the whole concept of multidisciplinarity itself, which surely can only work if you have strong, vigorous disciplines in the first place. Jackson pointed to the likes of Cardinal Newman as advocates of the need for a broad education as long ago as the mid-19th century, but there are probably very valid reasons why so many of us prefer to hone our talents in a particular discipline and why the goal of multidisciplinarity can be so hard to put into practice.

Jackson hinted that leadership is the key in her new vision, highlighting Nelson Mandela as an example of the kind of bold, visionary thinker who is needed to get her blueprint off the ground. I found Mandela on odd choice given that he is not the first person you would associate with revolutions in higher education, but Jackson was right that managing, leading and encouraging multidisciplinary teams – particularly if they are spread over different continents and different time zones – is crucial.

Next stop for Jackson on her European tour is the World Economic Forum in Davos, Switzerland, this week. Whether she’s got time to pop in on her old haunt – CERN – I’m not sure.

Take photos for our 25 year anniversary

Camera phones at the ready! 2013 marks the 25th anniversary of Physics World and we want you to be a part of the celebrations this year. To kick things off we would like you to submit photos containing the text “PW25” to our Flickr group, a selection of which we will then publish later in the year.

To give you an idea of the sort of photos we are looking for, we created the scene above using copies of Physics World magazine. If you’re looking for inspiration, think about your working environment. You might consider using laser writing, empty drinks cans, lines in the sand, basically use anything you can find in your vicinity. Happy snapping!

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