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New director of Brookhaven unveiled

Praveen Chaudhari joined IBM in 1966, working initially at the T J Watson Research Center in New York. He was appointed director of physical science in 1981 and was made vice-president for science a year later, responsible for the Watson centre and the company’s other main labs in California and Switzerland. He returned to full-time research in 1991. With a doctorate in physical metallurgy from the Massachusetts Institute of Technology, Chaudhari has published over 160 research articles in condensed-matter and materials physics and holds 22 patents

Brookhaven is one of ten national laboratories funded by the US Department of Energy. It employs more than 2800 staff and has an annual budget of $463m. It carries out a wide range of research in the physical, biomedical and environmental sciences. It is also the site of the Relativistic Heavy Ion Collider (RHIC), which aims to create and study quark-gluon plasmas. Brookhaven has been run by interim director Peter Paul since October 2001.

New planet challenges theorists

More than a hundred extra solar planets have been found in recent years but until now all were discovered by the “radial velocity method”, which detects the gravitational pull a planet exerts on its parent star. Sasselov and co-workers have used an independent method, which measures the periodic dimming of starlight as a planet moves across — or transits — the line of sight between the observer and the star. Despite many searches, no transiting planets have been discovered this way before.

Sasselov and colleagues examined data from the Optical Gravitational Lensing Experiment and identified five stars that they thought could be transited by a planet. They used the high-resolution echelle spectrograph on the Keck telescope in Hawaii to obtain detailed spectra and precise velocities of these objects. The team found that one of these stars, known as OGLE-TR-56, showed a variation in brightness that was consistent with velocity changes measured using traditional radial velocity methods.

The results from the Keck study suggest that the planet is about 5000 light years from Earth — making it the most distant extra solar planet to be found to date. Radial velocity measurements show that it is about 0.9 times as heavy as Jupiter and orbits its parent star every 29 hours at distance of only 3.5 million kilometres — closer than any other known planet to its star.

The researchers say that the planet is the first to be found with an orbital radius much smaller than current models allow — some theorists believe that the planet should have been consumed by its star. The results, which have yet to be confirmed by further observation, suggest that the planet may belong to a small population of objects known as “Class II planets”. These planets have lost a large amount of their mass to the parent star but have still managed to survive.

Nobel laureates oppose war against Iraq

The declaration reads:

“The undersigned oppose a preventive war against Iraq without broad international support. Military operations against Iraq may indeed lead to a relatively swift victory in the short term. But war is characterized by surprise, human loss and unintended consequences. Even with a victory, we believe that the medical, economic, environmental, moral, spiritual, political and legal consequences of an American preventive attack on Iraq would undermine, not protect, US security and standing in the world.”

The signatories include Norman Ramsey, who worked on the Manhattan Project, and Charles Townes, a former research director of the Institute for Defense Analyses at the Pentagon. Townes was also chairman of a federal panel that studied nuclear warheads.

Kohn expects more laureates to sign this week but the current list of signatures is:

Physics

Philip W Anderson, Hans A Bethe, Nicolaas Bloembergen, Owen Chamberlain, Leon N Cooper, James W Cronin, Val L Fitch, Sheldon L Glashow, Leon M Lederman, Arno A Penzias, Martin L Perl, William D Phillips, Norman F Ramsey, Robert Schrieffer, Jack Steinberger, Joseph H Taylor Jr., Charles H Townes , Daniel C Tsui, Robert W Wilson

Chemistry

Paul Berg, Paul D Boyer, Robert F Curl Jr., Herbert A Hauptman, Alan J Heeger, Walter Kohn, Yuan T Lee, William N Lipscomb, Ahmed H Zewail

Economics

George A Akerlof, Lawrence R Klein, Daniel L McFadden, Franco Modigliani, William F Sharpe

Medicine

Robert F Furchgott, Roger Guillemin, Louis J Ignarro, Eric R Kandel, Har Gobind Khorana, Ferid Murad, George E Palade, Harold E Varmus

Molecular dots rise for information storage

Information-storage technology relies on systems that can be easily “switched” from one configuration to another by applying an external stimulus. Molecular systems, for example, can be switched by changing their shape or spin state.

The rotaxane molecules used by Cavallini and co-workers are shaped like dumb-bells. Each contains a ring structure that is mechanically locked onto a “thread” by two bulky “stoppers”. A film of these molecules lying side by side resembles an abacus, with the stoppers playing the role of the abacus beads. The molecules are “bistable” in that they can be made to switch by rotating the ring between two states of slightly different energies.

Cavallini and colleagues used the probe of an atomic force microscope to scan the surface of the molecular films using a load force of less than 2 nanonewtons. The researchers then increased the force on the probe and observed that the normally smooth surface of the film becomes perturbed — the rotaxane beads begin to spread out and form a string of regularly spaced stable pattern of dots that have a uniform width and height.

The team found that they could control the number of dots with the length of the scan and so fabricate any predetermined number of dots –- which will allow them to write information on the dots as strings of bits. They also found that thinner films led to denser and smaller dots.

The scientists say that their approach is better than existing methods of writing with a scanning probe as it allows multiple dots to be encoded at the same time. The team now hopes to speed up this process using parallel writing and to develop an efficient read-out method.

First quasars shed light on the early universe

Quasars are the oldest known astronomical objects and can thus provide important information on the state of the early Universe. They are thought to exist at the centres of giant host galaxies and may be powered by supermassive black holes, which would explain why they are the brightest objects in the sky.

There is now compelling evidence that galaxies as large as the Milky Way had already formed less than a billion years after the Big Bang. However, existing models of galaxy formation cannot explain how so much matter could have assembled at such a rapid rate.

Loeb and Barkana have now studied the absorption spectra of two very distant quasars, found at high redshifts of 4.79 and 6.28 by the Sloan Digital Sky Survey in 2001. The redshift is a measure of how fast an object is receding from the Earth due to the expansion of the Universe — a higher redshift means that the object is further away. The spectra contained emission lines resulting from excited hydrogen atoms with characteristic “double-horn” peaks that could not be explained.

Loeb and Barkana believe that these peaks are unique “signatures” which may provide direct evidence that quasars are embedded in massive host galaxies. Such a host galaxy gravitationally pulls in large quantities of gas from its surroundings, which would absorb some of the light from the quasar. The researchers say that, based on the absorption signature, they can estimate the amount of gas falling into the host galaxy. This enables them to calculate the gravitational force exerted by the host galaxy and consequently its total mass.

The researchers show that the two quasars lie in galaxies that weigh about 10¹² solar masses — which is about the size of the Milky Way. They calculate the total in-fall rate of gas into these galaxies to be about 1300 solar masses per year for the z=4.79 quasar and 2900 solar masses per year for the z=6.28 quasar. From these rates they estimate that the host galaxies of these quasars could have been formed in about 300 million years for the z=4.79 quasar and 900 million years for the z=6.28 quasar. This ties in well with the age of the Universe, which is estimated to be about 14 billion years.

The team admit that more observational data are needed to test their model and now hope to look at other quasars. “We are working on similar signatures for less massive galaxies – those that may host other sources of light such as gamma-ray bursts which are visible at greater distances,” Loeb told PhysicsWeb.

Adaptive optics may help opticians

Astronomers developed adaptive optics to overcome the blurring in their images caused by turbulence in the Earth’s atmosphere. In adaptive optics, atmospheric distortions in the light from a “reference beacon” are analysed by a sensor, which then sends electronic signals to a “deformable” mirror that changes its shape to correct for the distortions. The mirror’s shape can change several hundred times a second, which ultimately results in a sharper image of the object observed.

Miller and Thibos applied this principle to observations of the human eye: an optical instrument takes the place of the telescope, cells on the retina play the role of stars, while the inside of the eye distorts the image just like the turbulence in the Earth’s atmosphere. The reference beacon is a laser spot focused on the retina.

Thibos created an “ocular aberrometer” that measures the deviation in optical wavefronts reflected by the retina using a “Shack-Hartmann” sensor, and Miller developed technology that corrects these deviations. Combined with a retina camera, the researchers will be able to make high-resolution, non-invasive observations of cells at the back of the eye.

The scientists hope that their instrumentation will help doctors to diagnose retinal disease before actual symptoms appear.

Fires destroy world-renowned observatory

Mt Stromlo, which was established in 1924, is one of Australia’s leading astronomical research facilities. It is operated by the Australian National University and, together with the Sliding Springs Observatory at Coonabarabran, has contributed to several recent discoveries — such as the discovery of the oldest known star.

The fires burnt down Mount Stromlo’s historic 1.3 m Great Melbourne Telescope, which was built in 1868, along with a 1.9 m telescope and a A$5 m spectrograph built for the Gemini project. The fires also destroyed a workshop where a A$6.3 m camera for the Gemini South telescope was to be built. These losses are expected to delay the Gemini project by at least three years. The fires also mean that a five-year assignment to digitally map the southern skies that was recently started at Stromlo will have to be postponed.

Fortunately, however, two office buildings and the visitor centre were spared. This is good news for astronomers as it was here that most of the recently generated computer data were stored.

ANU vice-chancellor Ian Chubb hopes that Mt Stromlo’s work can continue at other facilities until the observatory is rebuilt. “Plans are already being put in place to rebuild at Mount Stromlo and restore the research school to its full capacity,” he said.

New look for Cerenkov radiation

A photonic crystal is a material that contains a periodic arrangement of air-filled voids that have a lower refractive index than the host material. It is this periodic variation of refractive index that gives the material its “photonic band gap” — the optical equivalent of the energy gap in a semiconductor. Just as the periodic potential experienced by the electrons in a semiconductor restricts their energy to certain ranges known as bands, the periodic variation of the refractive index in a photonic crystal means that only certain wavelengths of light are able to pass through the crystal.

Joannopoulos and co-workers solved Maxwell’s equations for the case of a charged particle moving through a two-dimensional photonic crystal. They calculated that Cerenkov radiation emitted by the crystal could be emitted backwards with respect to the particle’s direction of travel. Moreover, they found that there was no threshold velocity of the emission of Cerenkov radiation — unlike in conventional materials.

The simulations predict that between about 10 and 200 photons should be emitted per square centimetre and the theorists say that it should be possible to detect this signal from a silicon-based two dimensional photonic crystal structure in the laboratory. The modelling could also be easily extended to three-dimensional crystals, they believe.

If these results are backed up by experiment, they could lead to new applications, such as particle detectors that are sensitive to velocity, and as sources of radiation generation at selectable frequencies.

Bad news for code breakers

In cryptography, the data in a message is encoded by multiplying it with a certain number. To decode the message the receiver must apply a second number, related to the first, called a key. But in order for cryptography to work, the key must be kept secret.

In most forms of quantum cryptography, the key consists of a sequence of single photons. To set the value of the key, the sender, known conventionally as Alice, chooses the polarization of each photon from among several non-orthogonal directions, and then transmits the photon. Quantum mechanics dictates that the polarization of a given photon is retained if the receiver, known as Bob, measures it in the same direction as Alice. But if Bob measures it in a different direction, he will only measure the correct value about half of the time.

To work out the key, Bob measures the polarization of each photon in whichever direction he chooses and telephones Alice to tell her which measurements he carried out. Alice then tells him which of the polarization values they agree upon — it is the photons with these values that are used as the secure key. By publicly comparing a portion of their key they will be able to confound an eavesdropper, known as Eve, since any measurement she makes on the key is likely to alter the values of some of the polarizations. Quantum mechanics also means that Eve is unable to make a perfect copy of the key and then pass it on to Bob.

In their system, Grosshans and co-workers use the average values of the amplitude and phase of the electric field of a group of photons, rather than the polarization of individual photons. Like the polarization of an individual photon, these variables are constrained by the uncertainty principle. But unlike the polarization of an individual photon, which can only assume one of two values along each orthogonal direction, these variables can assume a continuous range of values.

Several groups are investigating such continuous variables for use in quantum cryptography, but Grosshans and co-workers are the first to demonstrate their potential experimentally, as well as carrying out the hardware and software steps needed for manipulation of the secret key. Because their system measures continuous variables, it does not depend on being able to measure every single photon that is transmitted. In fact the researchers were able to securely send 75, 000 key bits per second, even though over half of the transmitted signal was lost.

“Our scheme is potentially much faster than single photon counting,” says Philippe Grangier, leader of the group. “This makes it suitable for high secret bit rates over short distances – typically less than 15 km – but its behaviour over longer distances requires further analysis.”

The researchers’ next step is to build a demonstration device that can transmit photons along optical fibres.

Nanowire lasers go electric

Electrically driven semiconductor lasers are already used for applications but they are costly to fabricate. Moreover, the devices are usually made from compound semiconductors which makes them difficult to integrate into silicon-based systems. To overcome these problems, many researchers have started to investigate alternatives such as nanowire lasers.

A laser requires a population inversion between two energy levels in the system, with the upper level having a higher population than the lower level. This can be achieved by using an external laser to stimulate the material in a method known as “optical pumping”. For real applications, however, devices must be electrically, and not optically, driven.

To address this problem, Lieber and co-workers used a single crystal of cadmium sulphide, 80 – 200 nanometres in diameter, for their experiment. Such single crystal wires are attractive building blocks for creating electrically driven lasers because they are defect-free and exhibit superior electrical transport properties. The devices were built by assembling cadmium sulphide on heavily doped silicon.

The Harvard team slowly increased the current through the nanowire and observed a rapid increase in the intensity of the light emitted above about 200 microamps — which corresponds to the onset of lasing. “This is the first time an electrically-driven nanowire laser has been demonstrated,” Lieber told PhysicsWeb, “Moreover, it is also a first for electrically driven devices assembled from pre-made building blocks.”

The researchers believe that their approach could be extended to other materials, such as gallium nitride and indium phosphide nanowires. In this way nanoscale lasers could cover the ultraviolet through to the near-infrared parts of the electromagnetic spectrum. Lieber says that it will be essential to learn more about the basic physics of nanowire lasers before they are used in real-world devices, but some simple applications could be ready in less than five years.

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