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Home » Page 1620

Did a Chinese calligrapher use 'fractal expression'?

su.jpg Huai Su’s fractal calligraphy

By Hamish Johnston

In the scientific world, fractals were first identified in the mid-1970s by the mathematican Benoît Mandelbrot.

However, it’s possible that artists and artisans have long been using the fragmented shapes in their work.

In 1999, two Australian physicists famously showed that the “paint-drip” canvasses of Jackson Pollock could be dated by computing their fractal dimension — which tended to increase as Pollock matured as an artist.

Now, Yuelin Li of Argonne National Lab in the US has posted a paper on the arXiv preprint server claiming that calligraphy done by the “maniac Buddist monk” Huai Su more than 1200 years ago contains fractals. Li analysed a request for “bitter bamboo shoots and tea” written by the monk and found that it can be characterized by two different fractal dimensions.

Li believes that the fractal nature of some artworks “can be attributed to the artist’s pursuit of the hidden order of [the] fractal”.

LHC report confirms electrical fault

The cause of the huge magnet warm-up or “quench” at the Large Hadron Collider (LHC) a month ago that released masses of helium coolant into its tunnel was a faulty electrical connection, an official report has confirmed.

The report, which was completed by the CERN lab near Geneva on Wednesday and released publicly yesterday evening, reveals that the fault resulted in the loss of some six tonnes of liquid helium, broken anchors in the concrete floor and damaged “jumper” connections in the cryogenic distribution line. It also states that up to 29 magnets will have to be repaired.

But the report adds that there are enough spare components to allow the LHC to restart in 2009 at the full energy of 7 TeV after the accelerator’s standard winter maintenance period.

Robert Aymar, the director-general of CERN, said in a statement, “The incident was unforeseen, but I am now confident that we can make the necessary repairs, ensure that a similar incident cannot happen in the future and move forward to achieving our research objectives.”

We are taking this process slowly and carefully to ensure that we do not miss anything that could help in the final analysis Paul Collier, CERN

Resistive zone

The report describes in detail the sequence of events that lead to the 19 September quench, which was first revealed by physicsworld.com. It began soon after 11 a.m. CET (10 a.m. BST) when the operations team was commissioning the final eighth of the LHC, sector 3–4, to an energy of 5.5 TeV. To do this, the team had to ramp up the current in the circuits of the “dipole” magnets, which steer the machine’s proton beams. Unfortunately — and for reasons yet unknown — the higher current was met by resistance in an electrical link between a dipole magnet and a neighbouring “quadrupole”, or focusing, magnet.

This resistance put an unwanted load on the power supply. In response, the power supply switched off and triggered a batch of resistors in the circuit to curb the high current. It also triggered quenches in many nearby magnets — an automatic safety system that is in place to distribute excess energy over a wide area.

Meanwhile, an electrical arc sprung from the fault and punctured a hole in the magnets’ cooling enclosure, allowing liquid helium to escape into the outer, thermally insulating vacuum of the cryostat. Relief valves in the cryostat opened to let out the helium into the tunnel — initially about two tonnes, though over time another four tonnes. However, the pressure of the helium was so great that several cryostats shifted and broke their anchors in the concrete floor. It was this movement that was responsible for the damaged the cryogenic jumper connections, which exist every 107 m along the sector.

‘Safe powering in the future’

According to the report, engineers will have to remove at most five quadrupole and 24 dipole magnets for repair. But it adds that the engineers may be forced to lift even more magnets from the 100 m-deep tunnel to the surface for cleaning because of a “soot-like” dust that was sprinkled down the beam pipes.

CERN now plans to improve the pressure release devices and cryostat anchors, and add more measurements to the early-warning systems before the technicians attempt to feed in any more high currents.

The technical parameters of the LHC are beyond precedent, and the energy stored in the superconducting magnets huge Official CERN report

“The technical parameters of the LHC are beyond precedent, and the energy stored in the superconducting magnets huge,” the report concludes. “Consequently, operation of this machine will always comprise a certain technical risk. We are however convinced that the repair actions underway and the improved protection systems to be implemented will ensure safe powering in the future.”

Paul Collier, the head of the accelerator operations team, told physicsworld.com that those at CERN are still in the process of warming up the entire sector, though they have begun inspecting the interconnects between magnets to gather more information. “We are taking this process slowly and carefully to ensure that we do not miss anything that could help in the final analysis,” he says.

“We have not yet fully disconnected any magnets,” he continues. “This will happen in the next week or so. Once disconnected, the first magnets — initially [those in the] short straight sections — will be removed from the machine for further study and the repair process started.”

Aside from the collateral damage it caused, the problem on 19 September was a blow to CERN scientists who had only recently celebrated a highly successful “switch on” and who were just days from being able to collide proton beams at 5 TeV. While further analysis of the incident continues over the winter, the operations team will have to find ways to rekindle its morale.

• You can read the full copy of the CERN report here.

Doppler shift reveals spin currents

If you like fast cars, you may be all too familiar with the Doppler shift. It forms the basis of the police radar gun, which can work out your speed by measuring the shift in frequency of microwaves fired from the gun and reflected off your car.

Now, however, physicists in France have used a different take on the Doppler shift to measure the rate at which currents of spin-polarized electrons flow through a conductor. The technique could help in the development of spintronic devices, which use both the spin and current of electrons to store and process information more efficiently than conventional electronics.

Spintronics relies on the manipulation of spin currents — in which most electrons have spin magnetic moments pointing in a particular direction. Such currents could be used, for example, to reverse the direction of magnetization of a magnetic data bit.

However, physicists have struggled to come up with a simple way of making direct measurements of spin current. Most techniques involve inferring the spin current from measurements of magnetization or electrical current, which would be expensive and difficult to implement in practical spintronic devices

Spin-wave speed gun

The new technique, developed by Vincent Vlaminck and Mattieu Bailleul from the CNRS Institut de Physique et Chimie des Matériaux de Strasbourg and the Louis Pasteur University, involves measuring changes in the frequency of “spin waves” — magnetic fluctuations that can propagate through a material (Science 322 410).

They did this by placing two antennas on either side of a tiny, 2 µm wide strip of permalloy, which is a magnetic alloy of nickel and iron. A high frequency signal was passed through one antenna, causing spin waves with a wavelength of about 800 nm to propagate across the permalloy, where they were detected by the second antenna.

The researchers then sent a spin current through the permalloy along the same direction as the spin waves. This current “pulls” the spin waves along, causing its wave fronts to pile up at the second antenna. As a result, the spin-wave frequency is shifted upwards by an amount proportional to the spin current.

Spintronics expert David Awshalom of the University of California at Santa Barbara described the scheme as an “important tool for the field of spintronics — an elegant technique aimed at probing current-driven spin transport in ferromagnetic systems”.

Vlaminck told physicsworld.com that the technique could be extended to shorter wavelengths where the short-range quantum “exchange” interactions come into play. This could help physicists understand how spin currents are affected by short-range changes in the magnetization of a material.

For example, the spin of the flowing electrons could be misaligned with the local magnetization and understanding this misalignment could be of great importance for the development of current-controlled magnetic devices.

Detector has an ear for dark matter

A slight difference in the sounds created by neutrons and alpha particles as they travel through a liquid could lead to the first direct detection of dark matter, say physicists working on the PICASSO experiment at SNOLAB in Canada.

Dark matter is thought to constitute up to 95% of all matter in the universe, but has never been observed directly. Our own Milky Way galaxy is thought to have a dark matter halo, through which Earth is moving and therefore it may be possible for us to detect the stuff.

Some physicists believe that dark matter could take the form of weakly interacting massive particles (WIMPs), which interact with normal matter via the weak force. However, such interactions are expected to be exceedingly rare and therefore any successful dark-matter detector must be able to differentiate between a rare WIMP interaction and background noise.

Buried deep underground in a former mine to shield it from cosmic rays, PICASSO is a bubble chamber containing 80 g of C4F10 fluid that is superheated above its boiling point. It detects subatomic particles via the bubbles they create when they collide with a nucleus in the fluid. Bubble creation is detected through the sound waves that are created when a bubble forms.

Noisy contaminants

Most of the background noise in PICASSO comes from alpha particles that are given off by radioactive contaminants in the detector. As a result, it is crucial that the PICASSO team work out a way to tell the difference between WIMPs and alpha particles.

A ready source of WIMPs is of course not available, so the team used a source of neutrons instead. Like WIMPs, neutrons have no electrical charge and therefore have a low probability of colliding with a nucleus — although nowhere near as low as WIMPs. By contrast, alpha particles have an electrical charge and therefore are more likely to interact with nuclei (and electrons) in the fluid.

The team studied the acoustic signals produced when the detector was exposed to neutrons and then to alpha particles. They found that the neutrons produced significantly quieter sounds than alpha particles — and that this difference is large enough to distinguish between the different particles (New J. Phys. 10 103017).

According to PICASSO spokesperson Viktor Zacek of the University of Montreal, the team stumbled upon the idea while calibrating the response of the detector using neutrons and compared the acoustic signals to alpha-particle data. The difference was first interpreted as a fault in the detector. “However, when we checked the data and refined the analysis the discrimination effect became even more pronounced,” he explained.

Multiple bubbles

The team doesn’t fully understand why the alpha particles are “louder” than the neutrons, but believes it could be related to the fact that they tend to create multiple bubbles in a collision, whereas the neutrons tend to only create a single bubble.

As well as helping in the quest for dark matter, the team also believes that its discovery could lead to better detection of alpha-particle emitting nuclei in biological materials and boost the sensitivity of neutron detectors.

Physicists set new entanglement record

Researchers in China and Europe have entangled a record-breaking 10 quantum bits — an important breakthrough in the quest to develop practical quantum computers.

The quantum bits (qubits) were in a “Schrödinger’s cat state”, so-called because the state has an equal probability of having one of two extreme values until a measurement is made on it. The state is much like the cat of Erwin Schrödinger’s famous thought experiment, which is both dead and alive until a quantum measurement seals its fate.

Qubits are the basis for storing and processing information in quantum computers, which could someday perform parallel calculations on a massive scale. A qubit could be the polarization state of a photon, for example, with “1” corresponding to horizontal polarization and “0” to vertical polarization. If two photons are entangled, a measurement of the polarization of one photon would automatically reveal the polarization of the other.

Particles such as a photons can also be entangled simultaneously in terms of several different properties in a process dubbed “hyper entanglement”. This can be achieved, for example, by simply passing a polarization-entangled photon through a polarizing beam splitter — a device that deflects vertically polarized light in one direction (say “left”) and horizontally polarized in another direction (say “right”).

Until the position of the photon is measured, it will be in an entangled state of a photon that has been deflected “left” by the beam splitter and one that has been deflected “right”. And a measurement on the position of this photon will reveal the polarization of its entangled partner.

However, single-photon qubits are very difficult to manipulate and physicists have yet to build a practical device that can perform calculations using large numbers of entangled qubits.

Now, Jian-Wei Pan and colleagues at Germany’s University of Heidelberg claim to have boosted the number of entangled qubits to ten — up from the previous record of eight. The team also included physicists from University of Science and Technology of China in Hefei and the University of Innsbruck in Austria (arXiv:0809.4277v1).

Entangled pairs

The experiment involves shining pulses of infrared laser light through a crystal of lithium triborate (LBO), which shifts some of the light to ultraviolet frequencies. The ultraviolet light is then passed through two successive beta–barium borate (BBO) crystals, which very occasionally produce two pairs of infrared photons (one pair from each crystal). The photons in each pair are entangled in terms of polarization.

One photon from each pair is then sent to a polarizing beam-splitter, which entangles the two photons. This means that all four photons produced in the BBO are entangled with each other. One of these photons is then entangled in a polarizing beam splitter with a fifth photon from the original infrared beam.

The result of this is five entangled photons. Furthermore, if one photon is measured to have a vertical polarization, the other four will all be vertically polarized, while if one has a horizontal polarization so will all the others. These two extreme polarization states are indistinguishable until a measurement is made and therefore the five photons are in a “Schrödinger’s cat state”.

The next step is to send each of the five photons through a differently polarized beam splitter, which splits each photon into two possible spatial states. The result is a hyper-entangled 10 qubit state — the largest yet to be entangled.

High-fidelity entanglement

The team confirmed this by measuring the position and polarization of the photons and determined that the qubits were entangled at a fidelity of about 0.56. According to the team, a value greater than 0.5 means that entanglement has been achieved,.

While this 10 qubit entanglement is impressive, Schrödinger cat states are not very useful for practical applications because they are easily destroyed — the problem is that an accidental measurement of one qubit reveals the value of all the other qubits.

A more robust route to multi-qubit entanglement is the “graph state”, in which the entanglement is engineered such that an accidental measurement of one qubit would only reveal the value of a subset of other qubits and leave the others entangled.

While Pan and colleagues have not entangled graph states, they have proposed several ways that their scheme could be extended to do so.

Physics graduates at careers fair

By Margaret Harris

I spent last Friday milling about with thousands of besuited young people and about a hundred potential employers at the National Graduate Recruitment Exhibition in London. My main goal was to sniff out companies or fields that are hiring physics graduates, so we can highlight these opportunities in the Careers section of Physics World. Along the way, I also talked to several recruiters and recruitees — and was even “recruited” myself by the Royal Air Force.

One big traditional physics employer, the nuclear industry, won my award for the most eye-catching stall at the exhibition: recruiters from nucleargraduates.com spoke to a steady stream of would-be workers inside a huge, grey, inflatable dome. Both dome and recruiters were there to advertise a new “umbrella” graduate programme with links to more than 20 companies and government agencies, and designed to bring new talent to an industry that has experienced a “greying” of its workforce over the past decade.

(more…)

STFC gives extra money for grants

By Michael Banks

Ever since the Science and Technology Facilities Council (STFC) announced a £80m shortfall in its budget late last year — to the wide condemnation of physicists in the UK — here at Physics World we have been trying to cover every twist and turn.

In September — after an STFC programmatic review spelling out which programmes would be funded — we ran opinion pieces in the magazine about the debacle from Brian Foster, European director of the global design effort for the International Linear Collider (ILC), as well as Keith Mason, chief executive of the STFC itself.

We have known for a few months now which facilities would be funded by the STFC in full — and which, like the Gemini telescopes and the ILC, would see the UK’s involvement cut back. What was not known, though, was how much research grants would be slashed.

The STFC earlier in the year issued a warning that up to 25% of grants could be cancelled over the next three years and those that have been issued could even be recalled as a result of the budget deficit. As Foster underlined in his opinion piece: “never before have grants that have been already issued been recalled and cut.”

However, the STFC has now pulled a rabbit out of the hat and come up with £9m over the next two years to plough into the research grants programme. The extra cash will reduce the original shortfall in grants funding by a half. A spokesperson for the STFC told me the new money has come from within the budget allocation and not from any external source. “[It originated from] how we manage our risks and our exposure to foreign currency fluctuations,” the spokesperson added.

This seems surprising as exposure to currency fluctuations was one of the main reasons given by the STFC management for its woes. However, for the time being physicists seem happy. “The flexibility that STFC have employed in addressing the reduction in grants is very welcome,” says particle physicist Mark Lancaster from University College London, who has been campaigning against the STFC cuts.

The new money will be available for the next grants round, but the STFC do not yet know how it will be distributed across sub-disciplines.

Metamaterials boost MRI sensitivity

Physicists in Spain have showed that metamaterials — artificial materials with exotic electromagnetic properties — can dramatically increase the sensitivity of magnetic resonance imaging (MRI). As well as enabling the technique to probe into deeper tissue, the researchers say that metamaterials could also enhance image quality and cut the amount of time taken to acquire images.

MRI has become a standard technique in medical diagnostics and chemical analysis over the past three decades. It involves placing a substance in a fixed magnetic field while exposing it to radio waves, which become absorbed by the substance’s atomic nuclei. By mapping the variations in phase and frequency of the absorbed radio waves with a receiving coil, a scientist can create an image of the substance’s internal structure.

Ricardo Marques and colleagues from the University of Seville in Spain have found that metamaterials can extend the depth to which a receiving coil is sensitive. They are not the first researchers to discover that certain materials can increase the capabilities of MRI — in 2001 a team including John Pendry of Imperial College, London, found that a material with a very high refractive index can be placed on top of the substance to act as a “flux guide” so that the receiving coil can be positioned farther away. By contrast, Marquez’s team has used a metamaterial with a negative refractive index to shift the magnetic field deep inside a patient to the outside (arXiv:0810.1689).

Both knees

In their experiment, Marquez and colleagues first performed a standard MRI scan of a male patient with the receiving coil held beside his knees. They found that they could image a cross section roughly as deep as one knee, but not the other.

Next they tried placing a slab of negative-index metamaterial between his knees. The metamaterial, which was 27 cm square and 3 cm thick, consisted of a 3D cubic array of copper rings that were each loaded with a capacitor. As the researchers performed another MRI scan, they found that the resultant cross-section image extended through both knees (see figure).

The researchers say that the ability to probe significantly deeper could reduce the time it takes to scan a large volume, and could also lead to improved image quality. In fact, they have already patented their technique and are working with a Spanish company called PET Cartuja to develop it further.

However, Marques told physicsworld.com that his team still needs to make it more adaptable. In normal MRI scans the receiver coil has to be tuned to the type of substance being probed — but in the Spanish team’s technique, the metamaterial must also be tuned.

Hawking gets under Newton's skin

hawking.jpg
(Credit: Stewart Francis)

By Hamish Johnston

Would you get the face of your favourite physicist tattooed on your leg?

Jack Newton spent six hours getting a portrait of cosmologist Stephen Hawking emblazoned on his leg after reading the physicist’s popular tome A Brief History of Time — according to the UK paper Metro.

Newton — 23 and apparently no relation to Hawking’s distant predecessor at Cambridge — admits that he didn’t understand Hawking’s book, but respected the 66-year old physicist.

Interface superconductor turns up the heat

High-temperature superconductivity has been seen for the first time in a layer of material just a few atoms thick. The finding, made by physicists in the US, could allow ultrathin superconductors to be used in transistor-like devices that would require much less power than conventional circuits. The researchers also think the work could shed light on the origin of high-temperature superconductivity.

Most known high-temperature superconductors are “cuprates”, consisting of parallel layers of copper oxide, each just a few nanometres thick. While interactions between electrons within individual layers are believed to make these materials superconduct, a theory of exactly how this happens has proven elusive. Conventional “BCS” theory, which successfully describes conventional, low-temperature superconductors such as lead, simply does not work.

One challenge in creating a theory of high-temperature superconductors is knowing how to deal with interactions between electrons in neighbouring layers, which tend to complicate the calculations. Some researchers have therefore sought to make single-layer superconductors to find out what is going on experimentally. Unfortunately, it has not been easy to make a flat ultrathin film and prevent it from being contaminated by exposure to air.

Metal and insulator sandwich

Now, however, Ivan Bozovic and colleagues at Brookhaven National Laboratory along with researchers at Cornell University and the FEI Company in Oregon have got around these problems by creating an ultrathin layer of lanthanum-strontium copper oxide (LSCO) superconductor, sandwiched between two much thicker non-superconducting layers. As well as protecting the LSCO from the air, the two thicker layers ensure that the superconductor is extremely flat (Nature 455 782).

When LSCO contains very little (or no) strontium it is an insulator. But as more strontium is added, it becomes first a superconductor and then a metal at higher concentrations. Using molecular beam epitaxy, the team made a series of samples in which a thick layer of metallic LSCO was deposited onto a thick layer of insulating LSCO or vice versa. The interface between the metal and insulator layers was found to contain a superconducting layer about 2–3 nm thick — which corresponds to 1–2 unit cells of the LSCO lattice.

This ultrathin layer becomes superconducting below a critical temperature (Tc) of 50 K, which is 10 K warmer than the Tc seen in much thicker samples of superconducting LSCO. According to Bozovic, the greatest challenge facing the team was proving that the superconductivity was occurring only in the “quasi-2D” ultrathin layer, rather than in a much thicker 3D region of sample. They did this using several different techniques, including atomic-resolution transmission-electron microscopy, to show that there was an abrupt change in strontium concentration right at the interface, which confirmed that the thick LSCO layers were insulators and metals, not superconductors.

High-Tc metamaterial

Bozovic told physicsworld.com that the team is now looking at how ultrathin layers could be stacked on one another to create a superconductor “metamaterial”. He believes that the Tc of such metamaterials could be boosted well above 50 K by adjusting the separation between layers — and hence the interactions between electrons.

Ultrathin superconductors are also interesting from a technological point of view because, unlike their 3D counterparts, they should not completely screen electric fields. This means that they could, in principle, be used to make the superconducting equivalent of a field-effect transistor — something that Bozovic and colleagues are now trying to make. Thanks to the lack of electrical resistance, such devices could run faster than their semiconductor counterparts while consuming much less power.

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