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Taking a closer look at light

One of the problems that arises when measuring an optical field is that the measuring device can disturb the field being studied. The Munich team overcome this problem by using a single calcium ion in a radio-frequency trap to measure the intensity of a standing light wave inside a cavity (G Guthöhrlein et al 2001 Nature 414 49). The standing wave causes the ion to fluoresce at a certain wavelength, and the intensity of this fluorescence is proportional to the strength of the optical field in the cavity.

By detecting the fluorescence from the ion when it is at different positions inside the cavity, it is possible to map out the intensity of the optical field in three dimensions. Lange’s team achieved a resolution of about 60 nanometres in measurements of the standing wave produced by radiation with a wavelength of 397 nanometres. ‘This approach takes all the probabilistic elements out of the atom-field interaction,’ Lange told PhysicsWeb. The team plans to use the technique in fundamental tests of quantum theory where it is important to have maximum control over the position of single ions.

Meanwhile, van Hulst and co-workers have used measurements of the light fields on surfaces to track the progress of laser pulses in a silicon-based waveguide (M Balistreri et al 2001 Science 294 1080). Laser pulses that last just femtoseconds – or 10^-15 seconds – are used in a wide range of optoelectronic and optical fibre systems. But these pulses get distorted as they travel through devices.

Existing methods compare the final shape of the pulse as it leaves a device with its original shape. But such ‘black box’ methods cannot pinpoint when or where the shape changes. Now van Hulst and colleagues have found that surface light waves produced by the laser pulse as it travels through the waveguide have the same group and phase velocities as the pulse itself.

Using a fibre-optic probe to measure intensity variations in these surface fields, the Twente team was able to monitor changes in the shape of a laser pulse as it travelled through the waveguide. Van Hulst and colleagues believe that their method will allow physicists to see how nonlinear effects – which can be both helpful and destructive – emerge in different systems.

US group calls for new linear collider

The group, a subpanel of the High-Energy Physics Advisory Panel (HEPAP) of the Department of Energy and the National Science Foundation, recommends that the US should ‘take a leadership position in forming the international collaboration needed to develop a final design, build, and operate this machine.’ Its report, presented this week at a two-day meeting of the full HEPAP panel chaired by Frederick Gilman of Carnegie Mellon University’s department of physics, also calls on the US to prepare a bid to host the linear collider.

In addition the subpanel recommends that:
* The US should take steps to remain a world leader in particle physics, through a broad programme of research focused on the frontiers of matter, energy, space, and time.
* A 20-year roadmap should be organized to chart advances in the field, replete with a mechanism to update the roadmap and set priorities across the entire field.
* Vigorous, long-term R&D aimed at future high-energy accelerators should be organized as a high priority.

The subpanel’s draft does not represent the final word in the long-range plan for high-energy physics. The full HEPAP panel will release the final report in January 2002. Theoretically, the panel could make wholesale changes to the subpanel’s draft. Most likely, however, the panel will merely tweak parts of it.

Quantum Hall effect gets new dimensions

‘The higher-dimensional generalization of the quantum Hall effect has been sought for a long time, but no one has succeeded before,’ Zhang told PhysicsWeb. ‘The mathematical structure could be very relevant to string theory, but the model is far, far from a realistic model of the universe.’

Most condensed matter systems can be explained by ignoring the interactions or correlations between electrons and calculating the properties of charged excitations in the ‘sea’ of electrons in the system. However, there is a growing number of strongly correlated systems – such as high-temperature superconductors – in which electron interactions are important and the conventional approach breaks down. Most of these systems develop long range order in their ground state. However, the quantum Hall effect and the ‘Luttinger liquid’ are the only known examples of quantum disordered ground states.

The quantum Hall effect is observed when the resistance of a two-dimensional gas of electrons is measured in a magnetic field. In 1980 Klaus von Klitzing discovered that the resistance of the gas is quantized when the magnetic field is high and the temperature is very low. This integer quantum Hall effect could be explained without electron correlations, but this conventional approach failed when the fractional quantum Hall effect was discovered in 1982. This effect was subsequently explained by Robert Laughlin in terms of electron correlations leading to fractionally charged excitations. The Luttinger liquid can also be understood in terms of fractionally charged excitations.

When Zhang and Hu extended the theory of the quantum Hall effect to four dimensions, they found that the equations that described excitations at the boundary of the system were similar to Maxwell’s equations of classical electromagnetism, and also to the linear version of Einstein’s General Theory of Relativity. They also found that the excitations could be used to model relativistic particles without mass, such as the photon and the graviton, and also other particles without analogues in high-energy physics. The results suggest that it might be possible to think of special and general relativity as theories that emerge from quantum mechanics, rather than as completely different theories.

‘Although this work is still very limited,’ write Zhang and Hu, ‘we hope that this framework will stimulate investigations on the deep connection between condensed matter and elementary particle physics.’

Leicester imports student hotshots

“I’ve been very impressed by the four students we’ve got,” says Pounds. “They’re extremely bright and can’t help but benefit their fellow undergraduates.” The students, from Moldova, Bulgaria, Azerbaijan and Indonesia, have all entered the degree at the start of the second year and will therefore complete their studies in three years. A scholarship has also been awarded to a student from Finland to start next year.

The scholarships are jointly funded by the British Physics Olympiad Committee and the university. They pay for the students’ air fares as well as their tuition and board for the three years. Pounds say that without such schemes students rarely get the opportunity to come to the UK to study physics because it is so expensive to do so without the national support that tends to favour students studying vocational degrees.

Leicester’s new stars from overseas told PhysicsWeb that the teaching facilities and methods were better in their new university than at home. “Seminars, workshops and experiments are good for people to get real-life practice of working in groups,” says Ismail Hasanov from Azerbaijan. “In my country there are just lectures and examinations.”

Evghenii Gaburov from Moldova says that many of his country’s best scientists have moved to the West following the collapse of the former Soviet Union. As a result, he says, physics education has suffered because there are very few scientists left to teach in the universities. In addition, he says that lecturers are more open in the UK. “If one has a question here it is possible to ask without any problems. But in Moldova it is not so easy to ask a professor or lecturer.”

Halim Kusumaatmaja from Indonesia says he looks forward to living in the UK. “I want to experience studying in a foreign country that has produced famous scientists, from Sir Isaac Newton to Stephen Hawking. And the scholarship is such a good opportunity that you can hardly reject it.” Commenting on his first few weeks at Leicester, he adds that “the weather is strange and life is expensive. But it is not a bad city at all”.

Pounds says he hopes the scheme can be extended. He is currently trying to secure funding from “other sources” to continue the scheme in years to come and perhaps expand it nationwide.

Super shock absorber could protect buildings

Granular materials, including sand and soil, have long been used to absorb impacts, but if the grains are all the same size, the shock waves are not always dispersed effectively. Instead, Sen’s team simulated a shock wave travelling along a chain of several hundred spherical elastic beads of ever-decreasing size. The beads at one end of the chain were around ten centimetres in diameter, and became progressively smaller.

After the shock wave has passed through the large sphere at the beginning of the chain, it proceeds to the next – slightly smaller – sphere. But the wave cannot be transmitted symmetrically into this sphere. To ensure that its energy is conserved, the wave is forced to stretch out. Its leading edge accelerates away from its trailing edge and this effect occurs every time the wave moves from one bead to the next. As the beads get smaller, the energy of the impulse is distributed and successive beads carry less and less kinetic energy.

Sen’s group found that the smallest bead at the other end of the chain feels the initial large impact as a long series of very small shocks. The amplitudes of these mini-shocks are less than 10% of the original impulse. “This very simple system demonstrates that theoretically, any size shock can be absorbed with assemblies of appropriately tapered chains”, explains Sen.

This kind of nonlinear wave propagation is still poorly understood, according to Sen. But he is optimistic that his team’s demonstration could one day be exploited to reuse the energy from unwanted man-made mechanical vibrations and even natural shocks from geological activity.

Dynamo effect forces energy from black holes

Some black holes contain the mass of a billion Suns compressed into a space the size of our solar system. Their gravity is so intense that even light cannot escape. Matter swirls around the black hole before it is pulled in, and this build-up of gas and dust is known as an accretion disk. The friction encountered by the material in the accretion disk heats it up and makes it emit X-rays.

An earlier study of the X-ray spectrum of the matter around the black hole in galaxy MCG-6-30-15 showed that it contained iron. But an international team led by Wilms used the XMM-Newton X-ray observatory to analyse the iron signal in much greater detail. ‘This broad line was first detected in 1995, but we have never seen it so clearly – and it is full of surprising features’, says Wilms.

The researchers found that the iron signal was very intense, which showed that the matter had absorbed much more energy than current theories allow. The signal also contained features that suggested the X-rays originated from matter very close to the black hole. In contrast, all previous studies have suggested that the X-rays arise from the outer edge of the accretion disk.

To explain these unexpected results, the researchers turned to a theory proposed in 1977. Roger Blandford and Roman Znajek predicted that a strong magnetic field could exert a braking force on a spinning black hole, and the excess rotational energy would be transferred to the accretion disk as heat. ‘We are probably seeing this electric dynamo effect for the first time’, explains Wilms. ‘Energy is extracted from the black hole’s spin and conveyed into the innermost parts of the accretion disk, making it hotter and brighter in X-rays’.

The idea that a strong magnetic field could lead to these unusual results is controversial, and Wilms and co-workers admit that further studies are crucial. ‘But there is no disputing the strong iron line in the spectrum – it is extremely puzzling and an explanation must be found’, says Wilms.

X-ray device sets pulses racing

X-rays have long been used to study the structure of matter, but some reactions take place in just a few picoseconds – that is, 10^-12 seconds. Pulses of x-rays just femtoseconds long – 10^-15 seconds – are needed to probe such reactions as they proceed.

Synchrotrons produce trains of such pulses, but physicists need to be able to tailor the pulse trains to their own experiments. The simple device developed by DeCamp’s team – which is based on a crystal of germanium – allows scientists to do this by switching the pulses on and off as required.

The X-ray beam from the synchrotron passes through the crystal, and at a certain angle of incidence, it splits into two as it leaves the crystal. The intensities of these beams depend on the exact structure of the crystal lattice, and DeCamp and co-workers realised that they could distort the crystal to change the relative intensities.

To do this, they fired ultrashort laser pulses at the crystal to briefly displace the atoms in the crystal lattice. When the laser was at a certain angle, the team found that the distorted lattice channels nearly all of the energy from the original X-ray beam into just one of the output beams. This momentarily creates a very intense coherent beam and a very dim beam – so the crystal behaves as a switch.

Although the X-ray pulses created by DeCamp and colleagues are currently too long to probe fast chemical reactions, the researchers claim they can produce femtosecond pulses by using shorter laser pulses to distort the crystal. But they admit that the technique will ultimately be limited by how fast the atoms in the crystal move in response to the laser pulses.

OSA elects British president

Knight, who is 54 and currently a ‘director at large’ of the OSA, says that he has three main aims for his time in office: to make sure the society is fully international in all its activities; to ensure that the society’s journals remain affordable and maintain their scientific quality; and to ensure that the society manages the huge success of the Optical Fiber Communication (OFC) conference and exhibit, which was attended by over 37 000 people last year.

Even though the OSA’s headquarters is in Washington, DC, Knight points out that about one third of the society’s 14 000 members work outside the United States and that its premier journal, Optics Letters, currently publishes more papers by European authors than by American authors. Knight himself has published more than 350 papers on theoretical quantum optics and quantum information and is head of the quantum optics and laser science group at Imperial.

Organic magnetism hots up

The magnetic properties of electron-doped carbon-60 compounds have intrigued physicists since they were first reported in 1991. Ferromagnetism has previously been observed in a handful of other organic materials, but only at very low temperatures. Indeed, the highest temperature previously reported for an organic magnet is 65 kelvin for a sulphur-based compound under a pressure of 16 kilobar.

Pure carbon-60 exists in a crystalline state in which the isolated molecules are bound together by weak van der Waals forces. But under high pressures, the material can transform into polymers where the molecules are held together tightly by covalent bonds. The exact crystal structure of these polymers depends on the pressure and temperature applied.

Makarova and co-workers from Russia, Sweden, Germany and Brazil prepared a series of samples under different conditions and found that only the two-dimensional “rhombohedral” phase is ferromagnetic. In this phase, the covalently bonded carbon-60 molecules are arranged in highly oriented layers like graphite. The magnetic characteristics persisted up to 500 kelvin ? well above room temperature. The team even demonstrated that the magnetization of the organic magnets is strong enough for a small magnet to lift them off a surface.

Makarova and colleagues were careful to protect the carbon-60 crystals from magnetic impurities, such as iron, nickel and cobalt, and believe that the results can only be explained by the intrinsic properties of the carbon-60 polymer. Although the origin of the ferromagnetism remains a mystery, the team speculates that structural defects or the creation of unpaired electrons during the polymerization process might be the answer. The group now plans to investigate samples prepared under different conditions in greater detail to determine the precise cause of the magnetic behaviour.

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