APS Physics Tip Sheet – Dec 17, 2013

In this issue: Plant Power, New Light On Uranium Chemistry, Not a Lumpy Universe

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Plant Power

Light-harvesting molecules in plants inspire a proposal for more efficient solar cells.

In solar cells, the energy of photons is absorbed by electrons, which become unbound and can generate an electrical current. But the electrons can also quickly recombine, which poses limits on the efficiency of most photoelectric materials. Molecules in plants, on the other hand, can, under certain conditions, convert photons to electrons with near perfect efficiency, and recent results suggest this is the result of quantum-mechanical effects. Researchers in Cambridge (UK) have proposed a model photocell inspired by how plants convert sunlight into useful energy. According to their calculation, their system of three molecules, thanks to quantum effects, could have a 35% higher efficiency than a cell that works based on classical physics only. 

* Celestino Creatore (contact author), MA Parker, S Emmott, and AW Chin, “An efficient biologically-inspired photocell enhanced by delocalised quantum states”, Physical Review Letters (expected publication date: Dec 18)

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New Light On Uranium Chemistry

The X-ray analysis of uranium oxides calls for a revision of the current thinking on uranium chemistry.

The storage of spent fuel poses one of the most challenging problems to the nuclear industry. Safe disposal, for instance in deep geological repositories, requires a thorough understanding of the possible chemical forms of uranium in a fuel rod: different oxides may have different properties, such as their solubility in water. At the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, a research team has used X-ray techniques to probe several oxides of uranium. The authors were able to study how uranium dioxide – the main component of nuclear fuel rods – transforms into other types of oxides. The results will help predict uranium’s behavior in a number of chemical reactions relevant to long term nuclear-fuel storage.

* Kristina O Kvashnina (contact author), SM Butorin, P Martin, and P Glatzel, “Chemical state of complex uranium oxides”, Physical Review Letters (expected publication date: Dec 17)

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Not a Lumpy Universe

Astrophysical observations suggest the expansion of the universe is accelerating – a puzzling fact attributed to the presence of the yet-to-be-deciphered dark energy. But there are alternative theories that do not invoke a new form of energy. A popular one explains the observed expansion rates by assuming the universe is “lumpy”, i.e. its density is not uniform on large scales. But a team of theorists at the University of Texas (Dallas) has now shown a lumpy universe model would be at odds with observations of the speed by which clusters of galaxies grow. Instead, the authors show that some form of dark energy would still be needed to model galaxy-cluster growth.  

* Mustapha Ishak (contact author), A Peel, and MA Troxel, “Stringent restriction from the growth of large-scale structure on apparent acceleration in inhomogeneous cosmological models”, Physical Review Letters (expected publication date: Dec 19)

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Journal articles and preprints are available to journalists on request. 

Contact: Matteo Rini Tel: +1 631 591 4224 (office), +1 646 288 5441 (cell), email: mrini@aps.org

Physical Review Applied Call for Papers APS is now accepting submissions for Physical Review Applied, the newest member of thePhysical Review family. Dedicated to publishing the highest quality research at the intersection of physics and engineering, Physical Review Applied will debut in early 2014. Troy Shinbrot, Professor of Biomedical Engineering at Rutgers University, is the Editor forPhysical Review Applied and will work closely with a distinguished and diverse Editorial Board. Professor Shinbrot received his Ph.D. in Physics from the University of Maryland and held positions in industry before joining the Rutgers faculty in 1998. His research focuses on computational biology and multiphase flow. He was selected as an APS Outstanding Referee in 2008. Physical Review Applied expands the current APS family of journals to provide comprehensive coverage of applied physics research. This new journal will include Letters, Research Articles, and Review Articles and follow the same high-quality peer-review process as the other Physical Review journals. More information about Physical Review Applied, including manuscript submission, is available online at journals.aps.org/prapplied. Contact: prapplied@aps.org The American Physical Society (www.aps.org) is a non-profit membership organization working to advance and diffuse the knowledge of physics through its outstanding research journals, scientific meetings, and education, outreach, advocacy and international activities. APS represents 50,000 members, including physicists in academia, national laboratories and industry in the United States and throughout the world. Society offices are located in College Park, MD (Headquarters), Ridge, NY, and Washington, DC.

APS Physics Tip Sheet – Dec 10, 2013

In this issue: Dark Matter Signatures?, Detecting Molecules on a Chip, Liquid-Crystal Flowers, Counting Atoms Up To 1200

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Dark Matter Signatures?

Results from one the leading dark-matter-detection collaborations have revealed potential signatures of dark-matter particles, but more recent experiments suggest dark matter has yet to be detected.

The Cryogenic Dark Matter Search (CDMS) experiment at Fermilab in Illinois and the Large Underground Xenon (LUX) experiment in South Dakota have reported results from their most extensive search for dark-matter candidate particles called Weakly Interacting Massive Particles (WIMPs). In underground facilities providing shielding from cosmic radiation, CDMS detectors have recorded three blips that could signal a dark-matter particle hitting the detector. The finding would point to the existence of WIMPs nine times as massive as protons. But more recently, a similar experiment at the Large Underground Xenon (LUX) experiment in South Dakota, which nominally features a much higher sensitivity, announced it has seen no evidence for dark matter. Scientists are still debating whether the new LUX result rules out the CDMS finding. 

* CDMS Collaboration (contact author: Enectali Figueroa-Feliciano) et al, “Silicon detector dark matter results from the final exposure of CDMS II”, Physical Review Letters (expected publication date: Dec 16)

* LUX Collaboration (contact author: Blair Edwards), “First results from the LUX dark matter experiment at the Sanford Underground Research Facility”, http://arxiv.org/abs/1310.8214

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Detecting Molecules on a Chip

A new setup allows the detection of molecules trapped on the surface of a microchip.

Modern labs-on-a-chip can perform experiments on atoms or ions trapped on the surface of microchips, with applications ranging from quantum computing to gravitation sensing. Molecules would offer much richer properties than atoms and ions, but molecule-chips have lagged behind, partly because molecules are more difficult to cool, control and observe. A research group at the Fritz Haber Institute of the Max Planck Society, Germany, has developed the first on-chip molecular detector. The researchers use a laser to ionize a few carbon-monoxide molecules trapped on the chip, and then image the resulting ion cloud on a phosphor screen several centimeters above the chip.

* S Marx, Gabriele Santambrogio (contact author) et al, “Imaging cold molecules on a chip”, Physical Review Letters (expected publication date: Dec 12)

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Liquid-Crystal Flowers

Researchers at the University of Pennsylvania have demonstrated a method for engineering flower-shaped liquid-crystal structures, in which the molecules are aligned around multiple ellipses resembling the petals on a daisy. The structures could be used to make microlenses that focus light efficiently. 

* DA Beller, Randal D Kamien (contact author) et al, “Focal conic flower textures at curved interfaces”, Physical Review X (expected publication date: Dec 10)

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Counting Atoms Up To 1200

Many experiments with cold atoms rely on the exact determination of the number of atoms, in particular high-precision metrology applications like atomic clocks. Researchers at the University of Heidelberg have set a record for the number of atoms that can be counted with single-atom precision. By monitoring the light emitted by an ensemble of cold, trapped atoms, the researchers were able to count up to 1200 atoms, improving on the previous record by almost an order of magnitude. 

* DB Hume (contact author) et al, “Accurate atom counting in mesoscopic ensembles”, Physical Review Letters (expected publication date: Dec 16)

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Journal articles and preprints are available to journalists on request. 

Contact: Matteo Rini Tel: +1 631 591 4224 (office), +1 646 288 5441 (cell), email: mrini@aps.org

APS Physics Tip Sheet – Dec 3, 2013

In this issue: Distributing Entanglement on the Cheap, Asking Photons Where They Have Been, Why Swimming Particles Aggregate

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Distributing Entanglement on the Cheap

Three new experiments demonstrate how entanglement can be shared between two distant parties without the need of sending an entangled carrier.

Entangled states lie at the heart of quantum physics and can be used as a powerful resource in emerging quantum technologies such as quantum key cryptography. The disruption of entanglement, which can be caused by any interaction with the environment, poses the hardest challenge to practical applications. But three different international research teams have now demonstrated experiments that distribute entanglement between two distant parties by sending a non-entangled carrier. Their arrangements place this carrier in a "cheaper," so-called separable state, which is still tied, or “correlated,” to the two parties, but in a way that is less fragile to environmental disturbance than entanglement is. The scheme may thus help realize communication schemes that are more robust to noise.

* Christian Peuntinger (contact author) et al, “Distributing entanglement with separable states”, Physical Review Letters (expected publication date: Dec 4)

* CE Vollmer, Roman Schnabel (contact author) et al, “Experimental entanglement distribution by separable states”, Physical Review Letters (expected publication date: Dec 4)

* A Fedrizzi, Tomasz Paterek (contact author) et al, “Experimental distribution of entanglement with separable carriers”, Physical Review Letters (expected publication date: Dec 4)

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Asking Photons Where They Have Been

A new experiment exposes the path taken by photons in an optical setup, revealing an unconventional quantum phenomenon.

A research team at the Tel-Aviv University has demonstrated a technique that is able to find out what path – among a few possible ones – photons have taken in an optical apparatus. The experiment allowed them to reveal a bizarre quantum mechanical phenomenon: photons pass through a section of the setup that they neither enter nor exit. The authors explain the effect by invoking an alternative interpretation of quantum mechanics called a “two-state vector formalism”, in which any quantum state in the present is described through a relationship between quantum states in the future and quantum states in the past.

* A Danan, D Farfurnik, S Bar-Ad, Lev Vaidman (contact author), “Asking photons where they have been”, Physical Review Letters (expected publication date: Dec 9)

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Why Swimming Particles Aggregate

Simulations explain why ensembles of swimming particles in solution sometimes aggregate in clusters.

Particles that move of their own accord, such as bacteria or beads propelled by chemical reactions, can organize themselves into diverse arrangements, such as coherently moving swarms or ordered arrays. A commonly observed behavior is the formation of a number of clusters, which may achieve a stable size or aggregate into a single dense phase. The reasons for the diverse behaviors are clarified by the work of a team of researchers from Europe (UK, Spain, Germany) and the US. The authors’ simulations explain why clustering may occur, depending on a fine balance of different forces: the attractive or repulsive forces between particles and the swimming intensity of the self-propelled particles. The model could be used to describe the behavior of biologically relevant systems or to design new ways of assembling tiny particles into well-defined structures.

* BM Mognetti, Chantal Valeriani (contact author) et al, “Living clusters and crystals from low-density suspensions of active colloids”, Physical Review Letters (expected publication date: Dec 6)

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Journal articles and preprints are available to journalists on request. 

Contact: Matteo Rini Tel: +1 631 591 4224 (office), +1 646 288 5441 (cell), email: mrini@aps.org

APS Physics Tip Sheet – Nov 19, 2013

In this issue: New Light on Spacetime Wormholes, Modeling How Cells Grow Old, Catching Dark Matter with an Electrical Circuit, Picking the Brain, How Long Does a Neutron Live, What Accelerates Electrons in the Earth’s Radiation Belt, Computing with Entangled Ions 

* Please note that there will be no tip sheet next week (Nov 26^th). The tip sheet will be back on Dec. 3^rd.

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New Light on Space-time Wormholes

(Image credit: APS/Alan Stonebraker)

New theoretical results suggest a connection between quantum mechanical entanglement and the space-time wormholes predicted by general relativity.

The concept of entanglement – a perplexing consequence of quantum mechanics – has been used to explain a number of experimental observations with photons, electrons and molecules and the basis of all quantum communication and computing schemes. Now, two independent studies, by researchers in the US and Canada, explain how entanglement may be tied to another paradoxical idea: the existence of “wormholes”, hypothetical shortcuts through space-time connecting black holes, a prediction of general relativity. The result supports the idea that entanglement is related to the very fabric of space-time, which in turn may provide clues on possible links between quantum mechanics and general relativity.

* Kristan Jensen (contact author), Andreas Karch, “The holographic dual of an EPR pair has a wormhole”, Physical Review Letters (expected publication date: Nov 20)

** Julian Sonner (contact author), “Holographic Schwinger effect and the geometry of entanglement”, Physical Review Letters (expected publication date: Nov 20)

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Modeling How Cells Grow Old

Researchers have developed a statistical model that describes how parts of the chromosomes called telomeres shorten with time - a process that may play a key role in senescence.

Telomeres are DNA sequences that protect the ends of chromosomes from deterioration or unwanted fusion with other chromosomes. They are thought to play a role in the aging of cells (senescence): since telomeres shorten at each cell division, a cell can only replicate a finite number of times. A research team in Paris has developed a statistical method that describes telomere shortening during cell division. The model predicts the distribution of telomere lengths in a population of dividing cells and relates such distribution to the presence of an enzyme called telomerase. The approach may help extract from experimental observations the key biological factors that regulate telomere length and thus the onset of senescence. 

* K Dao Duc, David Holcman (contact author), “Computing the length of the shortest telomere in the nucleus”, Physical Review Letters (expected publication date: Nov 25)

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Catching Dark Matter with an Electrical Circuit

Certain dark matter particles may be detected by a small superconducting circuit, according to a theoretical proposal.

Several ongoing experiments, based on massive detectors, are attempting to identify the faint fingerprints of dark matter. But one of the leading candidate particles for dark matter may be revealed by a much smaller benchtop detector, according to a proposal by a researcher at the University of Cambridge (UK). The author’s calculations show that axions – a proposed component of dark matter – could leave a detectable signal when passing through a Josephson junction, a device made of two superconductors separated by a thin insulating barrier. He also suggests such a signal may have already been observed in a 2004 experiments, which revealed a signal of unknown origin that could now be interpreted as the indication of an axion with a certain mass. 

  

* Christian Beck (contact author), “Possible resonance effect of axionic dark matter in S/N/S Josephson junctions”, Physical Review Letters (expected publication date: Dec 2)

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Picking the Brain

A mathematical analysis explains why the brain can be modeled using only a few key active regions.

To describe the complex architecture of the brain, researchers often employ a coarse model based on a few “regions of interest” – for instance those brain areas that are most active during a specific task. But is this approach, which is based on an empirical and arbitrary choice of such regions, sound? The theoretical work of a researcher at the University of Sydney (Australia) suggests the method can be put on more solid ground. Using techniques borrowed from quantum mechanics, he shows how the complex equations describing brain dynamics can be rigorously reduced to a representation based on few dominant modes (the region of interests). The theory provides a systematic approach for selecting such regions, which may lead to better brain models. 

* Peter A Robinson (contact author), “Discrete-network versus modal representations of brain activity: Why a sparse regions-of-interest approach can work for analysis of continuous dynamics”, Physical Review E (expected publication date:Nov 26)

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How Long Does a Neutron Live?

An isolated neutron lives for about 15 minutes, before decaying into an electron and a proton, but some controversy exists on the exact lifetime: values obtained by two types of experiments differ by 8 seconds. By analyzing a 2005 experiment, scientists at NIST have reduced the measurement error of one of such methods to about 2 seconds, which confirms the discrepancy with the other method. Solving such discrepancy will be crucial since the free neutron lifetime is related to fundamental questions in particle physics and astrophysics, such as the rate of nucleosynthesis during the big bang.

* Andrew T Yue (contact author) et al, “Improved Determination of the Neutron Lifetime”, Physical Review Letters (expected publication date: Nov 27)

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What Accelerates Electrons in the Earth’s Radiation Belt

The Van Allen Probes – NASA spacecraft designed to characterize the Earth’s radiation belt – have observed the formation of transient, intense bursts of so-called “double layers”, structures consisting of two parallel layers of particles with opposite charge that create a strong electric field. The observation suggests such double layers may provide one of the key steps through which electrons in the Earth’s magnetosphere are accelerated to relativistic speeds. Similar double layers may also be important in radiation belts of planets such as Jupiter, Saturn, Uranus, and Neptune, in the solar corona during flares, and in other astrophysical objects.

* Forrest S Mozer (contact author) et al, “Megavolt parallel potentials arising from double layer streams in the Earth’s outer radiation belt”, Physical Review Letters (expected publication date: Dec 2)

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Computing with Entangled Ions

Measurement-based quantum computing (MBQC) – a technique that processes information via sequences of measurements on single qubits - is expected to have significant advantages over other quantum computing approaches. Experimental MBQC demonstrations have so far used entangled photons, but these are hard to prepare in the large numbers needed for computation purposes. Now, for the first time, researchers In Austria have performed MBQC with trapped ions. The team entangled several ionic qubits and demonstrated a set of basic measurements that can be combined to produce any arbitrary logic function. 

* Ben P Lanyon (contact author) et al, “Measurement-based quantum computation with trapped ions”, Physical Review Letters (expected publication date: Nov 19)

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More from the APS Physics News Ticker:

- Market Turbulence is Like Fluid Turbulence

- Riding an electron wave into the future of microchip fabrication

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Journal articles and preprints are available to journalists on request. 

Contact: Matteo Rini Tel: +1 631 591 4224 (office), +1 646 288 5441 (cell), email: mrini@aps.org