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Home » Page 1871

Atoms perform a quantum flip

We often think of quantum tunnelling in terms of the ‘marble and the mountain’. If you flicked a marble towards a mountain, you would not expect it to roll over the top. But a quantum particle confined to a similar region of low potential energy can ‘tunnel’ through the mountain – or potential barrier – that confines it. This feature of quantum mechanics arises because the wave-like nature of the particle means that it is smeared out in space.

The wavefunction of the particle describes the probability of finding it in a certain region of space. This probability is non-zero beyond the potential barrier, which gives the particle a chance of appearing on the other side of it – that is, ‘tunnelling’ through it. However, in dynamical tunnelling – a phenomenon that was predicted to exist in the 1980s – there is no potential barrier, although classical mechanics forbids particles from changing from one state to the other.

Mark Raizen led the team at of the University of Texas at Austin, while Bill Phillips of the National Institute of Standards and Technology collaborated with researchers at the University of Queensland in Australia. Both groups used a similar optical technique to demonstrate dynamical tunnelling. Around a million atoms are trapped in an ‘optical lattice’, which consists of criss-crossed laser beams. The electric component of the laser light induces a dipole moment in the atoms, which draws them to the regions of maximum intensity in the lattice. The atoms are also chilled to within a few nanoKelvin of absolute zero to eliminate thermal vibrations that would obscure quantum effects.

Once the atoms are gathered into these pockets and placed into the same initial quantum state, the intensity of the laser light is made to oscillate at 250 kilohertz. The momentum of the atoms is measured at intervals during this modulation, using an absorption imaging technique. Both teams found that the momentum of the atoms changed back and forth as the intensity fluctuated – but did not pass through an intermediate zero-momentum phase.

The fact that the atoms bypassed the zero-momentum state means that the atoms were effectively in both momentum states at the same time during the transition from one state to another. This is an example of ‘quantum superposition’, the phenomenon that could form the basis of a quantum computer.

The experiments also hint at a new effect dubbed chaos-assisted dynamical tunnelling. Regions of chaos are present in the optical lattice, and both groups found evidence that this effect – which is common in the macroscopic world – can help atoms to flip between the opposite states of momentum.

Burst bangers and brain busters

Why is it that you can warm your hands by blowing gently against them and cool them by blowing hard? The answer requires a clear understanding of physics and a rudimentary knowledge of the workings of the human body. And how is the vanishing elephant trick carried out? This feat, which probably had its heyday in Victorian music halls, involved a magician showing the audience an elephant inside a large and well protected cage. On cue, the magician would make the elephant disappear. The trick presumably involved some basic physics using mirrors, but how are the mirrors arranged and what physics is involved?

These questions all appear in this fascinating new book, Mad About Physics, by Christopher Jargodzki and Franklin Potter. As the subtitle puts it, it is a compilation of some 400 “braintwisters, paradoxes and curiosities”, each of which comes with a full explanation. Divided into 12 chapters, the book covers the full range of mechanics and classical physics. I had a pretty good idea of the type of problem to expect in the chapters entitled “Fly like an eagle” (aerodynamics), “Good vibrations” (sound), “Opposites attract” (electrostatics and magnetism) and “Bodies in motion” (Newtonian mechanics).

I was, however, less certain of what to expect from “Stairway to heaven”, which turned out to be an interesting selection of problems on structures and the strength of materials. It includes the “pork-sausage mystery”: in which direction is the skin of a pork sausage more likely to burst as the internal pressure builds up during frying? Will it break longitudinally down the length of the sausage or round its circumference? To answer this question, one has to be aware that the applied stress needed to rupture a cylindrical object of uniform thickness is nearly twice as much round its circumference as along its length.

The final chapter, which has the ambitious title “Across the universe” but limits itself to our solar system, includes some interesting problems involving the motion of rockets and satellites as well as some about the Moon and the planets. Why, for example, are the tallest mountains on Mars more than twice as high as those on Earth, despite the fact that Mars is only about half the size?

Many of the explanations of the 400 questions include references for further information, generally taken from well known journals such as Physics Teacher and the American Journal of Physics. Scattered throughout the book is also an interesting collection of quotes and useless, off-beat scientific facts with which to amaze and impress your friends. For example, did you know that February 1866 is the only month to have ever elapsed without a full Moon and that this event will not be repeated for another 2.5 million years? Or that Los Angeles is moving north towards San Francisco at the rate at which your fingernails grow?

As for the quotes, I particularly liked the remark made by Lord Bowden about Lord Rutherford: “No-one else I ever knew could copy a dozen numbers down wrongly, add them up wrongly, and come up with the right answer.” More thoughtful is Einstein’s famous remark: “Great spirits have always encountered violent opposition from mediocre minds.”

Older readers may recall a book from the 1970s by Jearl Walker with the inspired title The Flying Circus of Physics. It contained more than 1000 questions, problems and brainteasers about the physics behind many common and not-so-common happenings in the world around us. Both it and Mad About Physics have a similar format, and indeed many of the same problems are covered in both books. However, while The Flying Circus of Physics had some delightful cartoons, Mad About Physics is more up to date. It also has an eye-catching cover and a design that gives it a more modern and zany appearance. Nevertheless, I can highly recommend both books.

I was interested to note in Mad About Physics that one of the authors dedicates the book to his late father, while the other dedicates it to his school science teachers who, he says, “first challenged me in chemistry and physics to bring out the joy in doing science”. In my view, good parenting and excellent science teaching are two vital prerequisites for a modern and progressive society. Unfortunately, physics teaching in schools in the UK and elsewhere appears to be going through something of a lean period at the moment, with nothing like enough able physicists entering the teaching profession.

Fewer students are reading the subject at university and some physics departments have already closed down, while others are under threat of closure. The consequences for the future are pretty dire. The remedy is inspirational teaching from qualified people who are enthusiastic and knowledgeable, and who can communicate the excitement of the subject. The profile of physics must be raised and a substantial injection of funding and resources is needed to attract more people of high calibre into physics education.

This is where books like Mad About Physics can help. Many of its problems will be useful for teachers, both at senior level in schools and at universities, for discussion with students in small groups. Such tutorials should be used to encourage students to start talking about physics and “thinking like a physicist”.

This, I believe, is what physics teaching is all about, and being able to think like a physicist has benefits far beyond the confines of the discipline. The versatility and intellectual skills that physicists develop are not only invaluable in areas like pure and applied research but also in industry, commerce, government, education, policy making and a host of other fields. In short, society needs good physicists – lots of them.

Nikolai Basov 1922 – 2001

After four years of military service during the Second World War, Basov studied physics at the Moscow Institute of Physical Engineers. In 1948 he moved to the P N Lebedev Physical Institute, also in Moscow, where he worked under the supervision of Prokhorov. While the pair were searching for a technique to amplify microwave signals in spectroscopic experiments, they hit upon the idea of using a gas-filled cavity with reflectors at either end, in which the microwave beam would be intensified. Their discovery that this method produced microwaves with an extremely narrow range of frequencies led to the construction of a ‘maser’ – microwave amplification by stimulated emission of radiation – and, after further refinements, the laser. Working in the US, Townes simultaneously made the same breakthrough.

Basov later became a professor in the department of solid-state physics at the Moscow Institute of Physical Engineers. He was also appointed as vice-director of at the P N Lebedev Physical Institute in 1958, where he became director in 1973. He achieved further recognition in Soviet political life, serving in the Presidium from 1982 until 1989. Basov was head of the laboratory of quantum radiophysics at the Lebedev institute at the time of his death. He died on 1 July.

Solar neutrino puzzle is solved

The first results from the Sudbury Neutrino Observatory in Canada have finally solved a problem that has puzzled astrophysicists for 30 years: why do experiments detect less than half the number of solar neutrinos predicted by models of the Sun? The results confirm that electron neutrinos produced by nuclear reactions inside the Sun “oscillate” or change flavour on their journey to Earth. Neutrino oscillations are only possible if the three flavours of neutrino – electron, muon and tau – have mass. The SNO result therefore has important implications for cosmology and particle physics.

Although the SuperKamiokande experiment in Japan has seen strong evidence for the disappearance of “atmospheric neutrinos” – neutrinos that are produced when cosmic rays interact with nuclei in the Earth’s atmosphere (Physics World July 1998 pp17-18) – the SNO results are significant because, when combined with solar-neutrino data from SuperKamiokande, they show for the first time that the disappearance of one neutrino flavour is accompanied by the appearance of another. This is the key signature of neutrino oscillations. The new results are also in excellent agreement with the predictions of standard solar models.

The SNO collaboration includes physicists from 15 centres in Canada, the US and the UK, and the results were presented on 18 June at the annual conference of the Canadian Association of Physicists in Victoria, and at seminars at Oxford University in the UK and the University of Pennsylvania in the US. They have also been submitted to the journal Physical Review Letters. “It is incredibly exciting to see such intriguing results coming out of our first data analysis,” says the collaboration’s UK spokesman, David Wark of the Rutherford Appleton Laboratory and Sussex University, “and there is so much more to come.”

Charged reactions

Neutrinos are elementary particles of matter with no electric charge and very little mass. They only interact weakly with matter, which makes them very difficult to detect. Indeed, the SNO experiment detects a mere 10 or so solar neutrinos per day. Electron neutrinos are produced in the Sun’s core when boron-8 nuclei undergo beta decay: the Sun is not thought to produce muon or tau neutrinos. Previous experiments have detected less than half of the predicted solar-neutrino flux, but these experiments were only sensitive to electron neutrinos. The combined SNO and SuperKamiokande results make it clear that this shortfall arises because electron neutrinos have changed into muon or tau neutrinos.

“This result agrees perfectly with theoretical predictions and indicates that we really do understand the nuclear processes that are the source of the Sun’s energy”, says Lincoln Wolfenstein, a particle theorist at Carnegie Mellon University in the US.

The SNO experiment is located 2 km underground in a nickel mine and consists of 1000 tonnes of heavy water – water in which the hydrogen has been replaced by deuterium – monitored by nearly 10 000 photomultipliers. Two different reactions can occur when an electron neutrino strikes a deuterium nucleus: in a “neutral-current reaction” the neutrino splits the deuterium nucleus into a proton and a neutron; in a “charge-current reaction” two protons and an electron are produced. All three neutrino flavours experience neutral-current reactions, but only electron neutrinos undergo charged-current reactions. All three flavours also experience “elastic scattering” off electrons in the detector.

At SNO the electrons produced in charged-current reactions emit Cerenkov radiation as they travel through water. The intensity of this radiation is proportional to the energy of the neutrino, and this allows the energy distribution of the incoming neutrinos to be calculated. The Sudbury team compared its value of electron-neutrino flux with a very precise measurement of the total neutrino flux based on elastic scattering measurements at SuperKamiokande.

According to the SNO detector, the flux of electron neutrinos from the Sun is 1.75 million neutrinos per square centimetre per second. The SuperKamiokande experiment puts the total flux at 2.32 million in the same units (S Fukuda et al. 2001 Phys. Rev. Lett. 86 5651, 5656). By comparing these figures, physicists from SNO and SuperKamiokande calculated that the true solar-neutrino flux is 5.44 million neutrinos per square centimetre per second, which is in excellent agreement with the “standard solar model” of energy production in the Sun.

Proponents of “dark matter” will be pleased to hear that neutrinos have mass. Astrophysicists have struggled for years to understand why galaxies rotate as if they contain more matter than we can see, and many believe this can only be explained by “dark matter” that cannot be seen. “Our calculations show that neutrinos account for between 0.1% and 18% of the mass in the universe,” says Wark. “Neutrinos may not account for all the dark matter, but they could certainly represent some of it now that we know they have mass.” The new results limit the possible range of masses for neutrinos to between 0.05 and 0.18 eV.

Removing uncertainties

The new-found mass of neutrinos must also be incorporated into the Standard Model of particle physics. According to Wark, the neutrino could be the first ever example of a Majorana particle, a type of particle that is its own antiparticle. “If you could place a bet at the bookmakers on the next change to the Standard Model, the Majorana theory would be the front-runner,” he says.

Further results from Sudbury – in particular an accurate “home-grown” value for the total solar-neutrino flux – are expected later this year based on measurements of the neutral-current reaction, which is sensitive to all three flavours of neutrino. These reactions are detected through flashes of gamma rays that are emitted when the neutrons released during the reaction collide with nuclei. To make the detector more sensitive to neutral-current reactions, the SNO team have added some of the world’s purest salt to the heavy water in the detector. The large chlorine-35 nuclei in the salt result in more collisions and therefore more gamma rays.

“SuperKamiokande has produced some beautiful measurements, but comparing absolute rates between two experiments is always tricky,” explains Wark. “Comparing two results from our experiment will allow us to get an even better picture of the neutrinos coming from the Sun with much lower systematic uncertainties.”

Colourful electrons solve puzzle of ferromagnetism

The spins on electrons can be either up or down and, depending on the spin-spin interactions, materials can be ferromagnetic (i.e. all the spins pointing in the same direction) or antiferromagnetic (i.e. neighbouring spins pointing in opposite directions).

Lithium nickel oxide, however, is a very unusual material. Although it appears to be ferromagnetic, the spins fail to align completely. This behaviour has been an interesting but controversial issue for years. Recently, Anatole Stepanov at the University of Aix-Marseille in France and co-workers have applied an oscillating magnetic field to check the electron spins – a technique known as electron spin resonance. They found magnetic anomalies that imply that the spin-spin interaction changes from antiferromagnetic at high temperatures to ferromagnetic at about 300 K and back to antiferromagnetic again below 40 K (F Reynaud et al. 2001 Phys. Rev. Lett. 86 3638). The results indicate that the electrons have another property, in addition to spin, known as “colour”. This extra degree of freedom describes the orbit of the electron around the atom.

In the July issue of Physics World, Fu Chun Zhang of the University of Cincinnati, USA, explains how spin-colour coupling introduces new magnetic behaviour into a spin system.

Electronics turns over a new leaf

 

Packaging materials that display animated adverts and containers that sound an alarm if their contents go mouldy are just two possibilities that could result from a new area of technology called “paper electronics”. The substitution of paper for semiconductors in electronic components might sound bizarre, but Swedish physicists are currently developing devices that can be printed onto paper using conventional industrial-scale printing methods.

Many electronic devices rapidly become obsolete as technology continues to improve. There is also a growing trend towards adding electronics to items that were previously difficult to make electronically active, such as plastic and paper. Both factors are motivating the electronics industry to find cheaper ways to manufacture products. Some 50 years after the first transistor was developed, circuit boards are still being manufactured from a large substrate that undergoes about 50 different processing steps before being cut into smaller circuit boards. A substrate that can be manufactured cheaply and quickly is needed to satisfy the industry’s demands.

Paper seems an obvious replacement: it is flexible and we have produced more of it, in terms of surface area, than any other material. These days paper is manufactured in an environmentally friendly way and can be printed on in a fast “reel-to-reel” process. Indeed, a diverse range of printing, coating and lamination processes is used to convert paper into products.

An industrial revolution unfolds

Our group at the Swedish electronics and optoelectronics company, Acreo, has teamed up with physicists at Linköping University to investigate how electronic functions can be added to paper using conducting molecular materials and polymers (both organic displays and plastic electronics were featured in the March 1999 issue of Physics World). We are also working with paper and packaging manufacturers, including Modo Paper and Tetrapak, to ensure that we use standard industrial processes in order to be able to produce low-cost products in high volume.

Many materials have undergone an industrial revolution as high-speed manufacturing processes have usurped slow and expensive production methods. Paper used to be made one sheet at a time and cut into smaller pieces. Now newspaper is produced in mills on 10-metre wide rolls at a rate of about 100 kilometres per hour. In addition, the printing and conversion of paper into other packaging products is done at very high speeds in continuous-flow processes.

But we have to fulfil some fundamental conditions before electronics can undergo the same revolution. First, the substrate has to be flexible so that it can be transferred easily from reel to reel. Second, the electronic materials have to be in the form of a solution so that they can be printed or coated onto the substrate. Third, we must be able to print features that have the right thickness and width so that they can function as electronic devices. Finally, we need a technology that ensures the devices can be distributed onto the surface to form different electronic systems.

Our approach – which uses paper as a substrate and relies on existing printing and lamination techniques – meets these demands and allows devices to be manufactured with low environmental impact. We have chosen organic molecules and polymers as the active electronic materials because they can be processed from solution. Moreover, the various techniques for converting paper into a printed product are well suited to producing the layer thickness and linewidths required for electronic devices. For example, the offset printers that are typically used to print magazines can reproduce features 10 µm wide and with a well defined thickness as small as 1 µm. Based on these techniques, we have created electronic materials with the dimensions necessary to manufacture proper devices.

We carry out the fundamental development work on paper electronics in our lab at Linköping, while the actual manufacturing process for such devices is being developed at Acreo within the framework of the PAELLA project.

Posters, pigments and parcels

The first application we have targeted is existing paper products, such as packages and advertising billboards. Indeed, we have developed a simple paper display some 70 x 100 cm2 based on electronically active pigments, which updates information and images. An electrical signal controls the colour of the pigment in much the same way that a chameleon alters the tone of its skin. Currently the inks can turn from one colour to another, but we are also developing pigments that can change into any colour state.

The choice of pigment material and the device architecture depend on the application. Pigments that can switch rapidly from one colour to another are known as dynamic and require power to remain in each of the states. Dynamic inks have been successfully demonstrated in large billboards to introduce animated areas to an otherwise static image (see figure).

Meanwhile, pigments that stay in the same state and only need power to change them from one colour to the next are known as bistable. The prime focus for bistable display technology is for tags and stickers, as well as large paper billboards. Indeed, with bistable technology it becomes possible to produce price tags that can be updated electronically and address labels for parcels that can be changed.

Printed sensors are another potentially huge application. A simple communication circuit together with a sensor printed onto a package could be used to remotely test the condition of the stored goods. Imagine, for example, being able to detect a carton of sour milk in a warehouse from the electronic signal it emits. Supermarkets, which increasingly rely on haulage companies to transport perishable produce from the warehouse to the store, would benefit enormously from such devices. Any items that had deteriorated could be removed long before they reached their final destination, thus saving on transport costs.

A crucial part of any electronic device is the power source. Energy can be transferred to paper devices by connecting the electronic pigment cells to coil antennas and transmitting signals from a remote antenna. Alternatively, physical contacts or battery tags can be added to the product. The power demands of the devices we have tested are very low, as we have focused on devices that only consume power when they are updated and read out.

So far we have looked at introducing paper electronics into traditional paper items to extend the existing use of paper – “electronic paper”. As the technology develops we plan to make paper an integral part of traditional electronic products.

Molecular storage ring makes its debut

The multitude of energy levels found in molecules has so far defeated the use of laser-based techniques, which rely on exciting transitions that can spontaneously radiate and return to the original levels. Many other ingenious methods have been invented to slow, cool or trap molecules. Paramagnetic molecules have been cooled to sub-kelvin temperatures in a cold helium buffer gas and then magnetically trapped. Ultracold alkali molecules have been formed by the photoassociation (the reverse of photodissociation) of trapped ultracold atoms. What is missing, however, is a single technique for slowing, cooling and trapping any species of molecule.

Now Gerard Meijer and co-workers at the University of Nijmegen and the FOM Institute for Plasma Physics in the Netherlands have taken an important step towards this goal by developing a device that slows and confines polar molecules (F Crompvoets et al. 2001 Nature 411 174). They recently slowed bunches of deuterated ammonia molecules (ND3) from 275 m s-1 to 80 m s-1, and then confined them in a storage ring about the size of a dinner plate.

In the July issue of Physics World, Harvey Gould of Lawrence Berkeley National Laboratory, USA, examines the state-of-the-art in molecular storage and speculates where it may lead.

Organic metal challenges superconductor paradigm

Moreover, the material in question – an organic crystalline solid called lambda-(BETS)2FeCl4 – contains a large concentration of magnetic ions (S Uji et al. 2001 Nature 410 908).

The discovery by Shinya Uji of the National Research Institute for Metals in Tsukuba, Japan, and co-workers at the Institute for Molecular Science, the Electrotechnical Laboratory and the University of Tokyo, is one of the latest in a long line of results on organic metals. Three years ago, the number of papers published on organic crystalline metals overtook that on high-temperature superconducting cuprates – and the gap is continuing to widen.

In the July issue of Physics World, John Singleton of the University of Oxford, UK, and Los Alamos National Laboratory, USA, explains why organic conductors are currently such a hot topic.

Physics and work

The report was commissioned from the Institute for Employment Studies by the physics programme of the Engineering and Physical Sciences Research Council (EPSRC) to examine the views of organizations that employ postgraduate physicists. University physics departments in the UK produce about 800 new physics PhDs every year, about 200 of whom have been funded by the EPSRC. In comparison, the UK produces around 2400 physics graduates every year.

While 30%-40% of newly minted PhDs remain in academic research, the rest go on to work for a wide variety of employers. According to the report, growth sectors that have been experiencing problems in recruiting sufficient postgraduate physicists include the electronics and communications sectors, while skills that are particularly in demand include software development and modelling. Coincidentally, the supply of all sorts of scientists and engineers in the UK is currently the subject of a separate review being carried out for the Treasury by the physicist Sir Gareth Roberts .

Most employers interviewed for the EPSRC report were happy with the technical skills of the physics postgraduates they were employing, and in many cases took these skills for granted. Job interviews, therefore, tended to focus on “soft skills” such as communication, problem solving and team working. Not surprisingly, physicists scored highly on problem solving, but communication and team-working skills were described as “problematic and often not well developed amongst postgraduates”.

The very nature of the PhD, with its emphasis on the individual’s contribution to scientific knowledge, was seen as part of the problem. However, there is a strong case for improving the communications skills of all postgrad students, irrespective of whether they intend to remain in academic research or move into industry and business. Preparing grant applications, writing reports and giving presentations are generic skills that will benefit physicists of all types. Team-work, while difficult to teach, is something that most supervisors should encourage in their research groups.

Other skills that physics PhDs lacked, according to the report, included time management, business awareness, self management and the ability to sell bones to dogs. There have, of course, been exceptions such as David Potter, the plasma physicist who founded Psion (page 17, print version only), and the various entrepreneurs highlighted in the April issue of Physics World. And with a little tweaking of the traditional postgraduate experience, there could be a lot more.

What do you think of us?

Every few years Physics World asks its readers what they think about the magazine. Are we covering the right topics? What subjects would you like more articles on? What sections of the magazine are the most (and the least!) popular? Are the articles too easy or too difficult? We take the answers to the reader survey very seriously and do our best to address any blind spots or shortcomings. Please take the time to fill in the survey, which can be found on the Web at www.physicsweb.org/html/survey2001. The survey will run from 2 July to 23 July.

Surface acoustic waves go under the microscope

Surface acoustic waves are commonly found in the electronic circuitry of everyday appliances, such as mobile phones and televisions, where they are used to filter frequencies. Surface acoustic waves are also employed in transponders to transmit signals that are modulated in amplitude and frequency to remote locations.

In one recent advance, Thorsten Hesjedal of Stanford University in the US and Gerd Behme of the Paul Drude Institute in Berlin have developed a technique to study elementary wave phenomena at the highest resolution ever. By exploiting the properties of sound waves at short distances (i.e. the so-called near field) and the high resolution of an atomic force microscope, the Stanford-Berlin team obtained acoustical images of gallium-arsenide substrates with a spatial resolution better than 1 µm (Europhys. Lett. 2001 54 154).

In the July issue of Physics World, Walter Arnold of the Fraunhofer Institute for Non-destructive Testing, Saarbrücken, Germany, explains how these new acoustical-imaging techniques will help scientists to understand the macroscopic elastic properties of composite materials and could shed light on the elasticity of biological materials.

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