Page 1501 – Physics World

DAMA results go through the looking glass

By Jon Cartwright

The debate as to whether the DAMA/LIBRA team has detected dark matter, as it claimed in April, will no doubt persist until fresh data can say either way. But in the meantime, Robert Foot, a physicist from the University of Melbourne, suggests an alternative interpretation: “mirror matter”.

I’ll take a step back for a moment in case you aren’t familiar with the story. (Alternatively, you can see Physics World’s feature DAMA/LIBRA is an underground experiment based at the Gran Sasso laboratory in Italy. It looks for dark-matter particles known as WIMPs (weakly interacting massive particles) — a class favoured by theorists for the mysterious substance — by monitoring for flashes that occur when the particles collide with nuclei in 250 kg of sodium-iodide detectors. The idea is that the frequency of flashes should modulate over the year as the Earth changes its speed through our galaxy’s “halo” of dark matter: in June, when the Earth’s orbit takes us faster through the halo, one would expect to see more flashes; in December, when we are moving slower, one would expect to see fewer.

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Bubble-fusion researcher loses professorship

Purdue University in the US has announced how it will reprimand Rusi Taleyarkhan after an internal committee ruled in July that he is guilty of scientific misconduct. The committee has also denied an appeal from Taleyarkhan about the misconduct verdict.

Taleyarkhan, a nuclear engineer who claimed the discovery of “bubble fusion” in 2002, will lose his title of Al Bement Jr Professor of Nuclear Engineering and will not be able to be thesis adviser to graduate students for at least three years.

Taleyarkhan retains his position as a member of the Purdue University graduate faculty, but with the reduced rank of “special graduate faculty”.

In July the university concluded that he had cited a paper by researchers in his own lab as if it were an independent confirmation of his alleged discovery of bubble fusion.

Proportional punishment

“In considering the sanctions to impose, I have been guided by the principle that the sanctions should address and be proportional to the specific findings of the research misconduct,” Purdue provost Randy Woodson wrote in a letter to Taleyarkhan that outlined the disciplinary actions.

Woodson added that the university will review Taleyarkhan’s conduct after three years, to determine whether he can apply for reinstatement as a full faculty member.

Taleyarkhan believes that the decision is unreasonable. “The sanctions are unfair and egregious in their severity,” he told physicsworld.com. He pointed out that a previous Purdue committee had exonerated him of misconduct charges in 2006, and that the latest committee absolved him of most charges of research misconduct.

Political motivation

The university started its latest investigation, involving “new allegations” of falsifying the research record, he said, “following political pressure from Congress motivated by articles in Nature.” Overall, he continued, “the university system has failed miserably and taken the expedient way out.”

What will happen next? Taleyarkhan, who has initiated a civil lawsuit against the university, does not entirely discount further legal action. “As a faculty member and a US citizen,” he said, “I have a right to appeal the findings along with seeking redress from the courts of the United States for the extensive damage caused to me and several others.”

Legal options

His lawyer, Indianapolis attorney John Lewis, said that “given the way Purdue administrators have handled this matter, Dr. Taleyarkhan has many options in the judicial system.” However, Lewis continued, “after years of fighting and being overwhelmingly successful against his detractors, he may not want a further part in this aspect.” Rather he may choose to focus on his teaching and research.”

Taleyarkhan argues that the affair has not damaged the credibility of his research. “The matter . . . will not affect bubble fusion research,” he declared, “as the final two allegations have nothing to do with the science which, as a consequence of this overall ordeal has been further vetted and strengthened in terms of its credibility.”

Six-year saga

The controversy began in 2002 when Taleyarkhan, who was then working at the Oak Ridge National Laboratory in Tennessee, co-authored a paper in Science in which he reported firing a barrage of ultrasound waves into a liquid mixture of benzene and acetone (Science 295 1868). He claimed that bubbles of gas, which emit flashes of light when the sound waves force them to expand and collapse, could reach such high temperatures and pressures that during this process fusion reactions are initiated.

Several groups, however, failed to replicate the research, while two other Purdue engineers — Lefteri Tsoukalas and Tatjana Jevremovic — complained that Taleyarkhan had tried to prevent them publishing their negative results. An internal investigation last year cleared Taleyarkhan of that charge, but when critics argued that the panel had not taken their views into account, Purdue began a second investigation. Although the committee completed its work in April 2008, Purdue did not issue the report until July when the Office of Naval Research, which funded Taleyarkhan’s research, had accepted it.

Quantum repeater demonstrated

An international team of physicists has taken an important step on the road to global quantum communication by demonstrating the basic principle of a quantum repeater. The breakthrough, which marks the first time that two atom clouds have been entangled remotely, could someday be used to counteract decay in quantum signals.

Quantum communication provides a means to transmit information that is fundamentally secure. It requires two parties to be entangled over a quantum channel, over which a “key” for decoding encrypting information can be established. Because this key becomes corrupt as soon as it is used once, the intended receiver can always tell if the key has been intercepted by an eavesdropper.

Although quantum communication has been used already over distances of up to 100 km or so, it is difficult to create entanglement over larger distances because of signal degradation. In classical communication the simple remedy would be to amplify the signal periodically, but this is impossible for quantum keys because of the “no cloning theorem” which precludes a quantum signal from being copied.

Building up entanglement

The answer is the quantum repeater — and now Jian-Wei Pan, Yu-Ao Chen and colleagues from the University of Heidelberg, the University of Science and Technology of China the Vienna University of Technology have demonstrated a crude version of such a device. The idea is that the quantum channel is split up into segments, each of which is easier to entangle. Once the segments are entangled individually they can then be entangled together via the same process.

Pan and colleagues have shown how to entangle one of the segments. They begin with two ultra-cold clouds of rubidium atoms, each of which they entangle with a single photon. The two photons are then sent towards each other via fibre-optic channel. At the middle the photons cross at a beam splitter, and finally the researchers perform a measurement on them to entangle the atom clouds (Nature 454 1098).

The team states in its report in Nature that it has only managed the entanglement of a fibre-optic channel 300 m long, which was limited by a 10 µs lifetime of the atom cloud. However, Chen told physicsworld.com that they have since increased the atom-cloud lifetime to 1 ms, thereby opening the door to a quantum channel 100 km long.

The researchers are presently trying to increase the efficiency from 10% towards unity so that they can couple several of the entanglement segments together.

Gamma-ray telescope renamed after Fermi

NASA’s Gamma-ray Large Area Space Telescope has been successfully calibrated and has begun to map gamma-ray sources throughout the universe. The international mission — which was launched into Earth-orbit in June — has also been renamed the Fermi Gamma-ray Space Telescope in honour of the Italian-American physicist Enrico Fermi.

NASA marked the occasion yesterday by releasing the first data from the satellite: a gamma-ray image of the entire sky taken over four days by Fermi’s Large Area Telecsope (LAT). According to the agency, an image with comparable resolution taken by Fermi’s predecessor (the Compton Gamma-ray Observatory) took several years to obtain.

Working fine

The four-tonne observatory was built by researchers in the US, France, Germany, Italy, Japan and Sweden. It also contains the GLAST Burst Monitor (GBM) to detect transient sources such gamma-ray bursts and solar flares.

Ronaldo Bellazzini of Italy’s University of Pisa — which was involved in building the LAT — told physicsworld.com that the telescope is “working as expected” and has already made several new discoveries regarding pulsars, active galactic nuclei and gamma-ray bursts. “These results will be published shortly”, he said.

However, Bellazzini added that astrophysicists will have to wait a little longer before Fermi begins to deliver meaningful data on one of its prime objectives — shedding light on the nature of dark matter. Certain dark matter particles, called Weakly Interacting Massive Particles, or WIMPs, could annihilate to produce gamma rays, which would show up as tiny gamma-ray signals from the dark-matter haloes that surround galaxies. “It will take about one year to understand the subtleties of the instrument and gather the data,” said Bellazzini.

Proud in Pisa

Bellazzini is particularly pleased with the telescope’s new name because Fermi was a student at the University of Pisa. “Fermi worked at the frontier of particle physics and astrophysics,” he said, citing Fermi’s pioneering work on the acceleration of cosmic rays.

Born in Rome on 29 September, 1901, Fermi won the Nobel Prize for Physics in 1938 for his work on nuclear reactions. That same year he left Italy for the US, where he played an important role in the Manhattan Project. Fermi died in Chicago on 28 November, 1954.

Cold atoms explode like cloverleafs

Physicists in Germany have created spectacular, cloverleaf-shaped explosions in a gas of ultracold atoms trapped by magnetic fields. The cloverleaf shapes were formed by finely tuning the magnetic interactions between the atoms, which had formed a Bose-Einstein condensate (BEC) and so were all in the same quantum state. Although such “bosenovas” have been seen before, they have previously always been the same shape in all directions.

Bosenovas were first created about 10 years ago by adjusting the magnetic-field strength between the atoms in a BEC so that the short-range “van der Waals” forces between the atoms are attractive, rather than repulsive. This causes the BEC to collapse in on itself much like a dying star. It then explodes like a tiny supernova and throws off many of its constituent atoms.

Physicists believe that the explosion occurs when the atoms are close enough for short-range interactions to affect groups of three atoms (“three-body” interactions) rather than just pairs. Until now the attractive forces between atoms were isotropic, which meant that the explosions ejected atoms equally in all directions.

Preferred directions

Now, however, Tilman Pfau and colleagues at the University of Stuttgart have created the first bosenovas in which the attractive forces between atoms are dipolar and therefore depend on the relative orientation of the atoms. This caused clover-leaf shaped explosions, which the team says reflect the underlying symmetry of the attractive forces (Phys Rev Lett 101 080401).

The team used a BEC of chromium-52 atoms, which have large magnetic dipole moments. The magnetic forces between the atoms are attractive and are normally much weaker than the van der Waals forces. However, the team adjusted the magnetic field so that the van der Waals forces were near zero, allowing the magnetic forces between atoms to take over.

According to Pfau, this caused the BEC to contract until the atoms were close enough for three-body interactions to cause an explosion. In this process, some of the atoms are thrown out of the BEC, while the remaining BEC expands outwards with a distinctive cloverleaf pattern. Such a pattern is expected for an expanding BEC with dipolar interactions in the presence of a magnetic field, said Pfau.

A gentler kind of implosion

Because the implosion was driven by the much weaker magnetic interactions, it was much more “gentle” than implosions caused by van der Waals interactions, according to Pfau. This he said, made it easier for Masahito Ueda and colleagues at the University of Tokyo to use current theories of BECs to describe the bosenova process — something that had proved difficult in the past.

Computer simulations by Ueda’s team suggest that the BEC collapse involves the formation of two “vortex rings” that spin in opposite directions. The physicists are now keen to see if they can create stable vortices by switching the repulsive van der Waals interactions back on before the BEC explodes.

Pfau believes that the insights gained into how to create and control chromium-52 BECs could someday be technologically relevant. Chromium is already used in a number of nanotechnologies and chromium-52 BECs could form the basis of “atom lasers” that could deposit tiny amounts of chromium to an extremely high degree of spatial precision.

Simulating magnetism

On a more fundamental level, the dipole interactions in the BEC are the same as those found in magnetic materials and Pfau believes that the system could be used as a “quantum simulator” to study magnetism.

Dave DeMille of Yale University agrees. He told physicsworld.com that such BECs could allow physicists to make “a new, deep connection between ultracold atom experiments and … many interesting magnetic systems in real-world materials”.

LHC kicks in both directions

(Credit: Olaf Behrendt)

By Jon Cartwright

Could it be — touch wood — that the Large Hadron Collider (LHC) will make it to the official 10 September start-up date without any further hiccups?

On Friday scientists at the European laboratory CERN were able to tick off two more items on the accelerator’s commissioning list. First, they managed to feed a bunch of protons from the transfer line of the Super Proton Synchrotron (SPS) into the LHC and then steer it some three kilometres round the beam pipe in a counter-clockwise direction. Second, a detector at LHCb — one of the four main experiments at the LHC — got the first taste of collision debris.

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Bush honours US atomic physicist

The pioneering atomic physicist David Wineland is one of eight US researchers to be awarded the 2007 National Medal of Science. Wineland, a physicist at the National Institute of Standards and Technology (NIST), was cited for his “outstanding leadership in developing the science of laser cooling and manipulation of ions”. He will receive the award from President George W Bush in a ceremony in Washington, D.C., on 29 September.

The National Medals, which are administered by the National Science Foundation (NSF) and awarded each year, were established by the US Congress in 1959 to honour scientific research that “enhances understanding of the world and leads to innovations and technologies that give the US a global economic edge”.

Based in Boulder, Colorado, Wineland is credited for his work on “applications in extremely precise measurements and standards, quantum computing, and fundamental tests of quantum mechanics, and for his major impact on the international scientific community through training scientists and outstanding publications.”

Helped isolate a single electron

Wineland’s long record of breakthroughs began in 1973, when as a postdoctoral researcher with Hans Dehmelt at the University of Washington he helped isolate a single electron using a Penning trap. Five years later, Wineland — by then in charge of his own group at NIST in Boulder — succeeded in laser cooling magnesium ions to below 40 K.

This early work on trapped ions was aimed at developing alternatives to the caesium beam clock, which then formed the basis of international time and frequency standards. However, the ion cooling and trapping techniques Wineland developed had a much wider impact, sparking work on neutral atom laser cooling, which formed the foundations of both the 1997 and 2001 Nobel Prizes.

Later, Wineland helped launch the field of experimental quantum computing by demonstrating the first two-bit “controlled-NOT” quantum logic gate. Recent work has focused on overcoming barriers to turning such few-qubit systems into a full-scale quantum computer.

“Wineland is an outstanding scientist who has made revolutionary contributions to the development and application of laser cooling and trapping,” said Katherine Gebbie, head of NIST’s Physics Laboratory. “We are thrilled by this well-deserved recognition for him”.

‘Invented ion trap quantum computing’

There was also praise from Winfried Hensinger, senior lecturer and head of the Ion Quantum Technology group at the University of Sussex in the UK. “Dave Wineland really invented ion trap quantum computing, which has been the most successful experimental method used to date,” he said. Hensinger described the award as “well overdue” and noted that Wineland “stands out in that he’s immersed in all the results that come out of his group – he always knows exactly what’s going on in the lab”.

Seeing animals in a new light

(Credit: Chris Lavers)

(Credit: Chris Lavers)

By Jon Cartwright

At first glance these images look like snapshots from that classic eighties sci-fi flick Predator.

It turns out, though, that the pre-eminent being responsible for them is not a brawny, gun-toting alien, but Chris Lavers, a technology lecturer at Britannia Royal Navy College in Dartmouth, UK. The images are infrared portraits of various animals dwelling at Paignton Zoo in Devon.

“I have been involved in thermal imagery for about 10 years now, and thermal imagery of wildlife with Paignton Zoo since 2002,” writes Lavers in an email. “My interest is concerned with highlighting the plight of endangered species under pressure from both man and climate change, deteriorating environments, etc.”

Lavers explains that thermal images can be used to observe animals without stressing them. “It enables a healthy baseline assessment of animals to be established and thereby aids veterinarian diagnosis,” he writes.

Aside from giving giraffes, tortoises and other creatures a once-over, Lavers is also interested in using thermal imaging to copy some of nature’s designs, such as iridescent butterfly wings. These could be employed in future stealth devices, he says.

If you want to see more of Lavers’s images — and are out and about in the south-west of the UK — you can visit his exhibition. It starts on 15 September at Paignton Zoo, and moves onto the Living Coasts zoo in Torquay until the third week of October.

Spin-flip speed is pushed to the limit

Researchers in Germany have devised an extremely fast way of changing the value of a magnetic data bit using a current of spin-polarized electrons.

They claim that their technique could soon be used to create magnetic random access memories (MRAMs) that are as fast as conventional memory chips and have storage densities that are just as high. The big advantage of MRAMs is that they retain their data when switched off, which also means they could be used to create energy-efficient electronic devices.

Conventional fast memory chips, such as dynamic and static random access memories (DRAMs and SRAMs), store data bits in the form of electrical charge in tiny capacitors. When their power is shut off, the data rapidly leak away.

As well as being responsible for the annoying “boot time” when a computer is first switched on, in which data get transferred from the hard-drive to the memory, conventional memory also needs lots of energy just to store information.

Magnetic nanopillars

Most chipmakers believe that MRAM — in which data bits are stored in tiny “nanopillars” of magnetic material — offer the best way of creating a fast memory that does not need to be powered all the time.

Most MRAMs use a tiny magnetic coil near to the nanopillar to switch the direction of its magnetization and so flip the bit from, say, “0” to “1”. However, it is a tricky business making coils small enough to achieve MRAM chips with bit densities as high as those found in DRAM or SRAM.

One solution is to forget about having a bulky coil and instead flip the nanopillar by passing a pulse of spin-polarized electrons through it. Most spins in such a pulse point in a specific direction (up or down) and their magnetic moment exerts a “spin torque” on the magnetization of the nanopillar.

In prototype spin-torque MRAMs made to date, however, this process is much too slow. The snag is that the magnetic moment oscillates for about 10 ns before settling into its new orientation, which is about 10 times too long to be practical.

This has frustrated physicists, because in theory it should be possible to flip the magnetization in about 1 ns.

Speedier flip

Now Hans Werner Schumacher and colleagues at the PTB standards lab in Braunschweig and the University of Bielefeld have shown that by carefully controlling the temporal shape and length of the pulse — and by applying a small constant magnetic field — a nanopillar can be flipped in just 1 ns (Phys Rev Lett 101 087201).

Shumacher told physicsworld.com that the magnetic field “puts the magnetization into a well-defined position where switching is easiest”.

The team carried out their experiments using commercial prototype spin-torque MRAMs made by Singulus Nano Deposition Technologies in Frankfurt. While external coils were still used to generate the magnetic field, Schumacher believes that a similar magnetic field could be created in the vicinity of each nanopillar by carefully designing the shape of the memory bit.

Commercial technology

“Our technique is the first way of getting reliable switching to one nanosecond”, said Schumacher, who believes that the technology could be included in commercial MRAMs by 2010.

Massive young stars defy tidal forces

Astrophysicists have long wondered why the centre of our galaxy is populated by massive young stars that, one might think, would be unable to form close to the supermassive black hole that lies at its heart. Now, however, that mystery may have been solved by two researchers in Scotland.

They have carried out computer simulations that suggest that some of the gas swirling into the black hole can form dense clumps that orbit — but do not disappear into — the hole before turning into massive stars. The research could improve our understanding of both star formation and supermassive black holes.

Startling discovery

Astronomers have known for about a decade that the cores of many galaxies — including our own Milky Way — contain supermassive black holes. The discovery was made by charting the motions of the very bright and very massive stars that populate the region near to the centre of the Milky Way.

But it wasn’t long before astrophysicists started to wonder how those stars got there in the first place. Conventional theories of star formation suggest that violent tidal forces would prevent stars from forming close to a supermassive black hole — and the stars appear to be too young to have been created elsewhere and then pulled into the galactic core.

The new simulations, carried out by Ian Bonnell at St Andrews University and Ken Rice from the University of Edinburgh, suggest that the stars were in fact formed near to the black hole. They also found that the process favours the creation of massive stars over their smaller counterparts, which could explain why the core of our galaxy — but not the rest of it — is dominated by very large stars (Science 321 1060).

Two different clouds

In their simulations, Bonnell and Rice used over a year of computing time on a SGI Altix supercomputer to predict what would happen when two differently-sized gas clouds (one about 10,000 times the mass of the Sun and the other about 100,000 times the mass of the Sun) are sucked into a black hole.

The simulation suggested that the intense gravitational field of the black hole (which itself is about one million times the mass of the Sun) heats the incoming gas, creating shock waves that transfer energy outwards from the black hole. These waves appear to deflect as much as 90% of the incoming gas into an oval-shaped disk that orbits the black hole – with the remaining gas disappearing into the abyss.

The simulations also tracked the progress of the gas in the disk. They found that the black hole’s violent tidal forces heat the clumps, which increases their internal pressure. Only larger clumps have enough gravitational energy to overcome the pressure effects of tidal heating, which is why — say Bonnell and Rice — the formation of larger stars is favoured.

Close eccentric orbits

The researchers have so far tested two possible types of incoming clouds. The first, which involved 100,000 solar masses of gas, suggested that about 200 stars with masses between about 10 to 50 times that of the sun would be formed in close eccentric orbits around the black hole. However, a second simulation, involving 10,000 solar masses of gas, failed to generate a preponderance of massive stars and instead suggested that mostly Sun-sized stars would be created further away from the black hole.

“These results match the two primary properties of the young stars in the centre of our Galaxy: their high mass and their eccentric orbits around the supermassive black hole,” says Bonnell.

One drawback of the study is that it only considered two of the many possible shapes and sizes of clouds that could fall into a black hole. The pair now plans to simulate a few more scenarios to understand the relationship between star formation and the initial conditions of the cloud.

Reinhard Genzel , one of the astronomers who originally discovered the Milky Way’s supermassive black hole, told physicsworld.com that the simulations will further our the understanding of supermassive black holes and their relationship to the formation of galaxies and exotic structures such as quasars.

Genzel believes that theoretical work on star formation near black holes is “now becoming sophisticated and realistic enough such that …we can expect to obtain a fairly robust understanding of these wonderfully puzzling phenomena in the very near future”.

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DAMA results go through the looking glass