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Per Bak 1948–2002

Per Bak was born in Denmark in 1948 and received a PhD from the Technical University of Denmark in 1974. He then joined the Brookhaven lab, where he began studying phase transitions in condensed-matter systems. Bak, Tang and Wiesenfeld developed the concept of “self-organized criticality” in 1987 while studying the theoretical behaviour of a sand pile, in which grains of sand are sprinkled onto the pile, one at a time. As the pile grows, its sides become steeper, eventually reaching a critical state when just one more grain would trigger an avalanche.

Bak and his co-workers realised that it was impossible to predict if a particular grain would cause an avalanche. The size of these avalanches is, however, distributed according to a “power law”, and they coined the phrase “self-organized criticality” to describe the pile’s natural growth to a critical state. Their work showed that many phenomena in Nature are so complicated that their large-scale behaviour cannot be predicted from their microscopic origin. Self-organized criticality has since been applied to many other natural systems, including the size of earthquakes, the spreading of forest fires, the fluctuations of stock-market prices, and X-rays emitted by solar flares.

Bak, who was based at Imperial College London since 2000, wrote the ambitiously titled How Nature Works in 1996. He also wrote “Why Nature is complex” with his second wife Maya Paczuski in the December 1993 issue of Physics World. Bak died on 16 October in Copenhagen.

First light for attophysics

In the experiment, the X-ray pulse first ejected an electron from an orbit close to the nucleus of the atom. The team measured the time it took this vacancy to refill by carefully controlling the time delay between the X-ray pump and subsequent probe laser pulses.

This has not been feasible before because the decay processes in the electron cloud take just a few femtoseconds – and the fastest laser sources have similar pulse durations. However, the attosecond source recently developed by Ferenc Krausz and colleagues in Vienna has opened up new possibilities for probing ultrafast atomic phenomena.

“It is essential that the exciting X-ray pulse is much shorter than the decay process being investigated to achieve good temporal resolution,” says Drescher. The pump pulse must be in the X-ray region because the pulse duration is fundamentally limited by its wavelength. “The limit for visible pulses, given by the light oscillation period, is about 2.5 femtoseconds,” he explains. “For our [X-ray source] the fundamental limit is pushed to 40 attoseconds.” Drescher says that the team’s next aim is to measure sub-femtosecond phenomena, such as ionization processes involving two or more competing reaction pathways.

Commenting on the work in Nature, Louis DiMauro of the Brookhaven National Laboratory likened the attosecond source to the shutter speed of a camera – but one which is fast enough to capture electrons moving near the nucleus. “There are few papers that announce the beginning of a new era, but [this paper] falls into that category,” he says. “We are entering a new realm of hyperfast measurement – the age of attophysics has begun.”

Molecules power nanoscale computers

The density of components in silicon microchips has grown exponentially for more than four decades, but this progress is likely to slow down as devices approach the nanometre scale. Heinrich and colleagues have shown how to overcome this problem, in principle, by using a pair of low-temperature scanning tunnelling microscopes to arrange pairs of carbon monoxide molecules on a copper surface.

They moved a single carbon monoxide molecule alongside one of these pairs, so that the three molecules formed a chevron – the shape of an arrow head. However, this formation was unstable because it raised the energy of the system. The molecule at the tip of the chevron therefore hopped to the next pair of molecules, creating a new chevron, which in turn decayed. This process cascaded throughout the pairs of molecules, in a similar way to the motion of falling dominoes.

The IBM researchers used this principle to make an AND gate. They placed three rows of molecule pairs in a Y shape, with a single molecule at the point where the rows met. Two rows acted as inputs and the third acted as the output. If there is a cascade in both rows – i.e., if there is a “1” in both inputs – molecules will hop along the rows to form a chevron with the single molecule that is already at the point where the three rows meet. This chevron would then decay, generating a cascade (i.e. a signal) at the output. The researchers used a similar arrangement to make an OR gate.

Heinrich and co-workers were then able to join several AND and OR gates together to make more complicated logic devices. One such device, a three-input sorter, would have an area of about 50 microns squared if made using current technology, but it measured just 200 nanometers squared when constructed from molecular cascades.

Unfortunately the molecular cascade devices made by the IBM researchers were very slow and could only be used to perform a single operation. To re-use the devices the researchers had to place the molecules back into their original position using one of the scanning tunnelling microscopes. To be useful, molecular cascade computers would need an automatic mechanism that would reset some of the molecules and leave the others intact to act as data registers.

Quantum logic: to be, or NOT to be?

In conventional electronics, a NOT gate inverts the value of a bit of information from 1 to 0 or from 0 to 1. This works because ordinary bits can only have a value of 1 or 0. Physicists have long believed that such binary information could also be stored in certain two-state quantum systems, such as the horizontal and vertical polarization states of photons, or the spin-up and spin-down states of electrons.

But unlike conventional bits, quantum bits – or qubits – can exist in a superposition of the two states. This makes it harder to invert the value of a qubit and limits the efficiency – or ‘fidelity’ – of a quantum NOT gate to 2/3.

De Martini’s team used polarized photons as qubits in their set-up, which was based on a crystal of barium borate with nonlinear optical properties. The researchers fired an ultraviolet photon into this crystal, and the photon split into two longer-wavelength photons by a process known as down-conversion. Some of these pairs of photons are ‘entangled’, which means that a measurement of the polarization of one photon reveals the polarization of the other one.

One of these entangled photons travelled to a detector, which measured its polarization, while a mirror reflected its partner back into the crystal. When this reflected photon emerged from the crystal, a second detector measured its polarization.

After repeating this process several hundred times, De Martini and co-workers found that the polarizations of the output photons were opposite to those of the input photons 63.0% of the time, compared with the maximum theoretical value of 66.7% – or 2/3. The researchers checked the ‘universality’ of their device by repeating the process using input photons with a range of different polarizations.

Although this demonstration of a quantum NOT gate is an important step in the field of quantum computation, it is not certain that optical methods would be used in a real quantum computer. But it is likely that such techniques would be used in quantum cryptography, in which encoded optical signals would be transmitted over long distances.

Lasers target hard drugs

Infrared spectroscopy can be used to detect these drugs in the liquid phase but no techniques have been developed to detect them in the much more common powder form. Now S N Thakur of the Banaras Hindu University in Varanasi and co-workers have applied laser photoacoustic spectroscopy to the problem, using samples of almost 100% purity supplied by the Central Forensic Science Laboratory in Calcutta.

Heroin, morphine and narcotine are large molecules containing 50, 40 and 53 atoms respectively. Like all molecules they preferentially absorb and emit radiation at certain wavelengths to give a distinctive molecular “fingerprint”. Thakur and co-workers recorded photoacoustic spectra using a carbon dioxide laser that could be tuned to wavelengths in the 9.6 and 10.6 micron regions of the spectrum.

The quality of the spectra were much higher than has been obtained with infrared spectroscopy in the past. In particular there were more peaks in the photoacoustic spectra, and the peaks were also sharper. Thakur and co-workers then went on to use ab initio quantum chemistry calculations to identify the particular molecular vibrations responsible for most of the peaks. The team claims that the technique could be used to detect trace amounts of all three drugs.

INTEGRAL takes off

Gamma rays provide information about physical processes within regions of the universe that do not emit radiation at radio, visible or X-ray wavelengths. However, gamma-ray photons are absorbed by the Earth’s atmosphere, so astronomers must put their telescopes into space to detect them.

A number of previous space-based telescopes – including NASA’s Compton Gamma Ray Observatory – have carried out gamma-ray astronomy but INTEGRAL will be the most sensitive gamma-ray telescope ever launched. And weighing in at more than four tonnes, INTEGRAL’s scientific payload is the heaviest ever launched by the agency. It was launched on board a Russian PROTON rocket.

INTEGRAL will exploit its greater sensitivity and higher angular and energy resolutions to make detailed studies of the most interesting sources discovered by Compton. Its two main instruments – the IBIS imager and the SPI spectrometer – will image gamma-ray sources and measure their spectra, while an X-ray monitor and an optical camera will help to identify these sources. The nominal lifetime of the observatory will be two years with a possible extension to up to five years.

INTEGRAL will be followed by two major gamma-ray missions to be launched by NASA over the next three years. SWIFT will focus on gamma-ray bursts when it takes off next year, while the larger GLAST mission – which is scheduled for launch in 2005 – will concentrate on the most energetic gamma-rays observed by the Compton mission, which finished two years ago. All these missions, combined with a new generation of ground-based gamma-ray telescopes, will give astronomers a much improved view of the high-energy universe.

The Milky Way’s dark secret

Since black holes do not by definition emit light, their existence has to be inferred some other way. The two most common techniques are to follow the motions of stars around the suspected black hole, or to detect a characteristic pattern of X-rays emitted by material that is heated before being swallowed by the black hole. Astronomers believe that there is a supermassive black hole at the centre of every galaxy in the universe. However, until now they had not been able to rule out the possibility that a dense cluster of dark stellar objects or a ball of massive, degenerate fermions were mimicking the effects of a black hole.

Rainer Schödel of the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching and co-workers elsewhere in Germany, France, Israel and the US followed the orbit of a star around the compact radio source Sagittarius A* – which is thought to surround the supermassive black hole at the centre of the Milky Way – over a ten-year period using a variety of ground-based telescopes.

Their most recent obseravtions – with one of the 8-metre telescopes that make up the European Southern Obsveratory’s Very Large Telescope in Chile – exploited adaptive optics to correct for atmospheric blurring and obtain images that were 20 times sharper than before. The team also used the VBLA array of radio telescopes to improve the accuracy of the observations.

The new data allowed Schödel and co-workers to calculate that the star had a highly elliptical Keplerian orbit with a period of 15.2 years and that it approached within 17 light hours of Sgr A*. By comparison it takes the Sun 230 million years to circle the Milky Way.

The team now hopes to study the motion of even fainter stars near Sgr A* and to probe for the first time various effects predicted by the general theory of relativity.

Caesium joins the condensates

Bose-Einstein condensation occurs when a gas of atoms is cooled until the de Broglie wavelength of the atoms becomes comparable with the inter-atom separation. The atoms all collapse into the same quantum ground state and the resulting condensate exhibits many unusual quantum properties such as superfluidity.

The first condensate was created in a gas of rubidium-87 in 1995 and since then condensates have also be produced in lithium, sodium, hydrogen, helium and potassium. Eric Cornell, Wolfgang Ketterle and Carl Wieman shared the 2001 Nobel Prize in Physics for their work on Bose condensates. Several groups have also managed to create so-called quantum degenerate Fermi gases – the equivalent of Bose-Einsetin condensation for atoms that obey Fermi-Dirac statistics.

Since 1995 the majority of condensates have been produced in rubidium. Now Rudi Grimm and co-workers at the University of Innsbruck have managed to produce a caesium condensate for the first time. They observed the Bose-Einstein condensation of caesium at a temperature of 45 nanokelvin and managed to produce pure condensates containing some 20 000 caesium atoms. Caesium condensates should be particularly useful for studying degenerate quantum gases in two dimensions.

Lithium joins the superconductors

Some 29 elements are superconductors under normal pressure conditions, and lithium brings to 23 the number that superconduct at higher pressures. In high-pressure experiments the sample is compressed between two diamond surfaces in a diamond anvil cell. However, lithium is highly reactive, which makes high-pressure experiments difficult.

Shimizu and co-workers have now managed to compress a ribbon of highly pure lithium in such a cell. They observe superconductivity as a drop in electrical resistance, and also find that the superconducting transition temperature rises to 20 Kelvin at 48 gigapascals. Although this is the highest observed transition temperature of any element, it is a factor of four less than theoretical predictions. The team also points out that it has failed to observe the Meissner effect – the expulsion of a magnetic field from the sample. Observation of the Meissner effect is often considered a more reliable indication of superconductivity than a dramatic reduction in resistance.

Meanwhile Russell Hemley and co-workers at the Carnegie Institution of Washington in the US have also observed evidence for superconductivity in lithium at extreme pressures. Hemley and co-workers have measured both the electrical conductivity and magnetic susceptibility of lithium to pressures above 80 gigapascals, and have observed transition temperatures between 9 and 16 Kelvin – again much lower than theoretical predictions. However, both sets of results appear to confirm that tentative claims of superconductivity in lithium at high pressures by a group at the University of California at Los Angeles in the mid-1980s were correct.

Hollow core cuts Raman thresholds

In stimulated Raman scattering incoming photons interact with gas molecules to produce photons with both longer wavelengths (so-called Stokes radiation) and shorter wavelengths (so-called anti-Stokes radiation). Team member Fetah Benabid says that the Bath group has set new low-threshold records for both the Stokes and anti-Stokes processes: 800±200 nJ for Stokes conversion and 3.4±0.7 µJ for anti-Stokes conversion. Benabid says that the dramatic reduction is due to the increased interaction length between the pump laser and the hydrogen gas used as the Raman active medium.

“In the case of free-space, the interaction length was limited to the only a few millimetres before the laser beam diffracted,” he explains. “People have used capillary fibers before but the losses were so high that pieces no longer than one or two centimetres could be used. All these limitations vanish if you use hollow-core photonic-crystal fibre. Light is confined in a hollow core and with losses of less than 1 dB per metre, it is contained over much longer lengths.”

The researchers fabricate their photonic-crystal fibre using a capillary-stacking technique. Seven missing capillaries result in a 15 µm hollow core, which is then filled with hydrogen. Pulses with a duration of 6 ns from a Nd:YAG laser operating at 532 nm are then coupled into the fibre’s core, which contains hydrogen under pressure. The Stokes and anti-Stokes photons have wavelengths of 683 nm and 435.2 nm respectively.

According to Benabid, the low-loss fiber also has a very broad transmission window. “The fiber transmits over more than 700 nm through the visible and IR,” he says. “This allows effective guidance of the pump and the different Raman components.”

The team is now trying to develop fibre with much lower losses. Benabid says this would allow a low-power continuous-wave laser, such as a diode laser, to act as the pump source. Fibers filled with hydrogen, nitrogen or methane could yield sources at previously unattainable wavelengths in infrared, visible and ultraviolet regions, he adds.

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