Lasers That Mimic Nerve Cells, The Dark Side of the Higgs, An Entanglement Laser Pointer

APS Physics Tip Sheet – May 6, 2014

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Lasers That Mimic Nerve Cells

Researchers have engineered semiconductor lasers that behave like nerve cells and that could be used to build computing structures based on neural circuits.

The brain’s information processing ability has inspired researchers to build computing architectures from building blocks that behave like neurons. To accurately mimic neural behavior, these individual units should exhibit similar characteristics: neurons respond to small stimuli with small, linear responses, and to large stimuli—above a critical threshold—with large, nonlinear responses. Essential to their operation is a so-called “refractory period” following the nonlinear events, during which any response is temporarily inhibited. Researchers from France have now engineered micropillar semiconductor lasers that can reproduce such refractory behavior and thus could form the basis of neuron-like optical computing architectures. 

* F. Selmi, Sylvain Barbay (contact author), et al., “Relative refractory period in an excitable semiconductor laser,” Physical Review Letters(expected publication date: May 07)

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The Dark Side of the Higgs

The decay of the Higgs boson into “invisible particles” delivers no evidence of physics beyond the standard model, putting new limits on dark matter theories.

The recently discovered Higgs boson may act as a link between particles we are familiar with and particles that have so far avoided detection, such as dark matter. To investigate this possibility, the ATLAS collaboration (one of the two groups that discovered the Higgs) has searched through data from the Large Hadron Collider (LHC) for events where a Higgs boson decays into “invisible particles,” like neutrinos or dark matter particles. Such particles would leave no trace in the LHC’s detectors, but their presence could be inferred by analyzing the collision products. The researchers found that the frequency of these invisible decays does not exceed values predicted by the standard model of particle physics. They use this result to place the strongest limits to date on the strength with which Higgs bosons may interact with dark matter particles.

* ATLAS Collaboration (contact: atlas.publications@cern.ch), “Search for invisible decays of a Higgs boson produced in association with a Z boson in ATLAS,” Physical Review Letters (expected publication date: May 8)

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An Entanglement Laser Pointer

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One of the main challenges for future quantum information technologies is the miniaturization and integration of components such as sources of entangled photon pairs. A research team in France has now built the first electrically powered semiconductor source of photon pairs. The scheme is much more compact than existing schemes and may be easily integrated into a fiber-optic communications network.

* F. Boitier, Sara Ducci (contact author) et al., “An electrically injected photon-pair source at room temperature,” Physical Review Letters(expected publication date: May 7)

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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

Matteo Rini, PhD 

Deputy Editor, Physics 

mrini@aps.org  

http://physics.aps.org

In this issue: Sliding Sand, A Tractor Beam Made of Sound, Neutron Star Goes Wobbly, Element Z=117 Confirmed, Remote Controlled Entanglement, The Arrow of Time in Physics and Psychology

APS Physics Tip Sheet – Apr 29, 2014

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Sliding Sand

​Adding a small amount of water to sand can significantly reduce the sliding friction.

Pouring some water—but not too much—on sand makes it easier to slide objects thanks to the formation of capillary bridges. This is the conclusion of a study by a team of researchers from the Netherlands, Iran, France, Germany and India. The authors tested the sliding friction of dry and wet sand when a weighted sled was pulled across the surface. As water was added, both the force needed to pull the sled and the friction coefficient were found to decrease below that of the dry sand, reaching a minimum before increasing again as more water was added. The authors explain that this is due to capillary liquid bridges that start to form between the sand grains when water is added, facilitating the sliding. But if too much water is added the capillary bridges merge and disappear. 

* A. Fall, Daniel Bonn (contact author) et al., “Sliding Friction on Wet and Dry Sand,” Physical Review Letters (published Apr 29)

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A Tractor Beam Made of Sound

Sound waves can be engineered to realize a tractor beam – a device able to pull centimeter-size objects.

A “tractor beam”—a term coined in the 1931 science-fiction novel Spacehounds of IPC by E.E. Smith—is a device that can attract one object from a distance. Researchers have had some success at a microscopic level, demonstrating optical tweezers that can pull nanoparticles opposite to the light propagation direction. Now a team of researchers from the UK and the US has reported a step towards a macroscopic device. The scheme is based on a sound beam that generates low-pressure zones towards which an object can be attracted. The authors were able to scale up the tractor beam to handle centimeter-size objects, a million times larger than those manipulated by previous schemes.

* Christine E.M. Démoré (contact author) et al., “Acoustic tractor beam,” Physical Review Letters (expected publication date: Apr 30)

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Neutron Star Goes Wobbly

The x-ray emission from a magnetar has revealed that the star’s huge magnetic field has distorted its shape, causing it to wobble.

Magnetars are neutron stars with a colossal magnetic field. Having  magnitudes up to 10¹¹ tesla, their fields are a billion times stronger than the most powerful magnets on Earth. Possible evidence of an even stronger field comes from recent x-ray observations of one of the brightest magnetars (4U 0142+61, located in the constellation Cassiopeia at 13000 light-years from Earth). A team of researchers from Japan and the US has reported the detection of a time-varying x-ray signal, indicating a wobble, or precession, in the magnetar’s rotation caused by an internal field of about 10¹² tesla. The authors suggest that a strong internal toroidal field deforms the magnetar into a prolate shape, like a football, which wobbles as it spins. 

* Kazuo Makishima (contact author) et al., “Possible evidence for free precession of a strongly magnetized neutron star in the magnetar 4U 0142+61,” Physical Review Letters (expected publication date: Apr 30)

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Element Z=117 Confirmed

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An international collaboration has used an intense isotope beam provided by the GSI research facility in Darmstadt, Germany, and a target material of radioactive berkelium supplied by Oak Ridge National Lab in Tennessee to produce two atoms of the superheavy element with atomic number Z=117 (ununseptium). The result confirms the initial observation published in 2010 –an essential step towards the recognition of the discovery of the element. In the process, the researchers also discovered a new isotope (lawrencium-266). 

* J. Khuyagbaatar (contact author) et al., “Study of the ⁴⁸Ca+²⁴⁹Bk fusion reaction leading to element Z =117: long-lived-decaying ²⁷⁰Db and discovery of ²⁶⁶Lr,” Physical Review Letters (expected publication date: May 01)

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Remote Controlled Entanglement

Researchers from California and India have created entangled states of two superconducting qubits separated by more than a meter of coaxial cable. While it is now routine to entangle photons at a distance, the authors have managed to entangle two superconducting circuits using microwave radiation transported by 1.3 meters of coaxial cable. The result shows that quantum entanglement can be established between distant systems that interact only through a signal propagating along low-loss electrical wires. This functionality may be exploited in future quantum networks.

* Nicolas Roch (contact author) et al., “Observation of measurement-induced entanglement and quantum trajectories of remote superconducting qubits,” Physical Review Letters (published Apr 28)

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The Arrow of Time in Physics and Psychology

Researchers from Caltech and University of South California have presented a conjecture of why we perceive time as flowing from past to future—the same direction determined by the laws of thermodynamics. According to their theory, the psychological arrow of time is determined by a principle they call “generality”: this states that what defines a memory is the correlation with the things it remembers. A memory can thus be correlated with our past, but not with our future, since the correlation with the future would be contingent on different possible outcomes. This, suggest the authors, is what makes a prediction psychologically different from a true memory, forming the basis of our perception of time.  

* Leonard Mlodinow, Todd A. Brun (contact author), “On the Relation between the Psychological and Thermodynamic Arrows of Time,”Physical Review E (expected publication date: May 02)

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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

Matteo Rini, PhD 

Deputy Editor, Physics 

mrini@aps.org  

http://physics.aps.org

Sterile Neutrino as Dark-Matter Candidate, Amber Does Not Act Its Age, Stopping Light in a Waveguide, An Undulator Made of Microwaves

APS Physics Tip Sheet – Apr 22, 2014

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Sterile Neutrino as Dark-Matter Candidate 

A dark matter particle in the form of a so-called sterile neutrino could explain the recent detection of an x-ray emission line from galaxy clusters.

Recent astronomical observations of the Andromeda Galaxy and the Perseus Cluster have revealed an x-ray emission line whose origin cannot be explained. A researcher from UC Irvine has now suggested that such x rays could be generated through the decay of dark matter particles in the form of sterile neutrinos—hypothetical neutrinos that do not interact via any of the fundamental interactions except gravity. The researcher calculates that the decay of sterile neutrinos would produce x rays exactly at the observed wavelength. Further, he shows that a cosmological model based on this sterile-neutrino dark-matter particle would solve two key problems faced by the currently most popular dark-matter models: correctly predicting both the number of Milky Way satellite galaxies and their central densities.

* Kevork N Abazajian (contact author), “Resonantly-Produced 7 keV Sterile Neutrino Dark Matter Models and the Properties of Milky Way Satellites,” Physical Review Letters (expected publication date: Apr 24)

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Amber Does Not Act Its Age

110-million-year-old amber samples surprisingly retain the same thermodynamic properties as much younger glasses.

Amber is a unique example of a glass because it has “hyperaged,” that is, it has undergone thermodynamic stabilization for millions of years—a process that is impossible to replicate in a lab. This allows researchers to study how the properties of a glass change with age. A team of researchers from Spain has measured the specific heat and other thermodynamic properties of a 110-million-year-old amber sample, finding that such features are identical to those of younger samples. The result implies that the thermodynamic properties remain fossilized in the glass—much like a trapped insect—and are surprisingly unchanged by 110 million years of aging.

* T Pérez-Castañeda, RJ Jiménez-Riobóo, Miguel A Ramos (contact author), “Two-level systems and boson peak in 110-million-year-aged amber glass,” Physical Review Letters (published Apr 22)

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Stopping Light in a Waveguide

A new proposal suggests light could be stopped in a compact, solid-state device working under ambient conditions.

Researchers have recently demonstrated a variety of methods to stop light—a phenomenon that could be used, for instance, to process information in an optical network.  But most available techniques are based on complex setups using ultracold gases and can only stop light in a very narrow band of frequencies. A new method, theorized by a group at Imperial College, in London, could decelerate light pulses down to 20 meters per second inside a waveguide made of a silicon core surrounded by a metal layer. The scheme, which could lead to practical devices working under ambient conditions, could stop light for a time long enough for applications like fast optical switches, nanolasers, and solar cells. 

* Kosmas L Tsakmakidis (contact author) et al, “Completely Stopped and Dispersionless Light in Plasmonic Waveguides,” Physical Review Letters (expected publication date: Apr 25)

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An Undulator Made of Microwaves

Researchers at SLAC National Accelerator Laboratory have used a microwave cavity to build an undulator—a device that causes electrons to jiggle and emit a beam of intense and coherent x rays. Instead of using the field of fixed magnets as in conventional undulators, the scheme exploits the magnetic and electric fields of intense microwaves in a cavity. The new type of undulator may outperform conventional ones in the generation of x-ray radiation at very short wavelengths for high-resolution imaging and crystallography applications.

* S Tantawi, Muhammad Shumail (contact author) et al, “Experimental demonstration of a tunable microwave undulator,” Physical Review Letters (expected publication date: Apr 23)

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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

Matteo Rini, PhD 

Deputy Editor, Physics 

mrini@aps.org  

http://physics.aps.org

The Physics of Voting, Dark Matter as a Trigger for Comet Impacts, Neutrons Put Limits on Dark Matter and Energy, The Key to Thin-Film Solar-Cell Efficiency

APS Physics Tip Sheet – Apr 15, 2014

In this issue: The Physics of Voting, Dark Matter as a Trigger for Comet Impacts, Neutrons Put Limits on Dark Matter and Energy, The Key to Thin-Film Solar-Cell Efficiency

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The Physics of Voting

A computer simulation of voter behavior is able to reproduce and explain long-term electoral patterns observed in US presidential elections.

In recent years, researchers have developed a number of mathematical theories to analyze the behavior of voters in elections, but no model has been shown to reproduce real-world data. Now, a team of researchers from Poland and Spain tested a voter model with data from U.S. presidential election results. The model is based on the idea that the influence of one voter on another is similar to diffusion of particles in liquids. The simulations yielded a county-level map of electoral outcomes strikingly similar to actual U.S. presidential election results between 1980 and 2012, and could reproduce a number of observed statistical properties of the data. The authors suggest that, while the model cannot predict election outcomes, it explains what is behind who's winning. 

Juan Fernandez-Gracia (contact author) et al, “Is the voter model a model for voters?”, Physical Review Letters (expected publication date: Apr 18)

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Dark Matter as a Trigger for Comet Impacts

An increased likelihood of life-threatening comet impacts could occur when the Sun passes through a hypothetical dark matter disk in the plane of our galaxy.

Physicists have recently suggested that there's a thin, dense disk of dark matter running along the mid-plane of our Milky Way galaxy. Our solar system would then trace an up-and-down, wavy motion through that plane as it travels around the galaxy. According to a study by researchers at Harvard, the passage through such dark matter disk could disturb the path of certain comets or asteroids - and send them hurtling towards our planet, causing impacts of the kind that likely killed off the dinosaurs. The hypothesis is corroborated by the observation that comet impacts occur with a 35-million-year periodicity that closely matches the rate at which the Sun passes through the plane of the galactic disk.

* Lisa Randall and Matthew Reece (contact author), “Dark Matter as a Trigger for Periodic Comet Impacts”, Physical Review Letters (expected publication date: Apr 21)

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Neutrons Put Limits on Dark Matter and Energy

The behavior of a neutron bouncing in the gravitational field of the Earth can improve what we know about dark energy and dark matter. 

A team of researchers from Austria, France and Germany has studied the behavior of neutrons in the gravitational field of the Earth to put constraints on dark energy and dark matter scenarios. The authors made an “artificial atom” by confining neutrons in a thin cavity between two mirrors.  Similarly to light-induced electronic transitions in an atom, varying the distance between the mirrors can cause the neutrons to climb up and down the quantized energy levels in this gravitational box. The interaction of dark matter or dark energy particles could cause shifts in the energy levels. The lack of observation of such shifts allowed the authors to derive some of the most stringent limits on parameters of potential dark matter and dark energy particles.

* T Jenke, Hartmut Abele (contact author) et al, “Gravity resonance spectroscopy constrains dark energy and dark matter scenarios”, Physical Review Letters (expected publication date: Apr 16)

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The Key to Thin-Film Solar-Cell Efficiency

Thin-film solar cells are now a serious competitor to silicon cells for power generation, with comparable efficiencies and rapidly decreasing costs. Cadmium telluride (CdTe) is one of the most promising thin-film materials, with a record efficiency of 20%, obtained by including a stage during synthesis in which the material is treated with cadmium-chloride. But why such treatment leads to better efficiency has remained a mystery and progress has been driven by incremental trial and error. Now, a team of researchers from the UK and the US, through atomic imaging experiments and new calculations, showed the large photovoltaic efficiency is due to the polycrystalline nature of the films, which helps separating photogenerated electrons and holes and preventing unwanted recombination.

* Chen Li (contact author) et al, “Grain-boundary-enhanced carrier collection in CdTe solar cells”, Physical Review Letters (expected publication date: Apr 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

Matteo Rini, PhD 

Deputy Editor, Physics 

mrini@aps.org  

http://physics.aps.org