Critical behavior and transmission of information of a swarm of units

LE13424

- This paper shows how a few lookout birds transmit to the whole swarm their reactions to either environmental scares or attractions. Cooperation generates organization as a phase transition from the random motion of single independent individuals to an order condition where all the units fly in the same direction. However, at criticality the flock organization is not permanent and undergoes collapses allowing the single individuals to recover their independence, thereby making the flock select randomly new directions. The danger/opportunity perceiver birds act as nucleation seeds generating a bias turning the free-will condition into wise decisions, namely the choice of the most convenient flying directions, according to the environment state: As a result of criticality the swarm of cooperating units behaves as a single "intelligent" individual. This article shows that criticality turns local interaction into a long-distance communication, with a time delay determined by the distance between two consecutive organizational collapses. The time delay is proportional to the flock size, thereby generating two challenging issues, (a) the choice of the optimal size for the swarm function and (b) the assessment of whether the occurrence of organizational collapses is related or not to the process of information transmission diffusion via diffusion.

Is religion doomed?

LA12632

- Much like animals competing for a limited supply of food or water,
social groups can be thought of as self-perpetuating entities that
compete for members. The group of religiously-unaffiliated individuals
has grown rapidly in recent times. We use research on social
conformity, together with methods from nonlinear physics, as the basis
for a mathematical model of this growth. We come to a startling
conclusion: coexistence of religious and irreligious segments of
society may be unstable! That is, our model suggests that religion
may be driven toward extinction in the long run. We test the model
with historical census data drawn from 85 regions of nine modern
secular democracies, and find that it fits data well. According to
the model, a slight advantage in the perceived utility of religious
non-affiliation should lead to the long-term decline of religion.

New metamaterial cloaking concept promises vessel motion without resistance or wake

LF13222

- When a vehicle or a vessel moves through air or water at a steady speed, most of its fuel is spent on fighting the hydrodynamic resistance. This is because the vessel needs to push the fluid out of its way in order to move forward. The disturbance of the fluid caused by the vessel motion normally spans the distances greatly exceeding the size of the vessel; enormous amounts of energy are needed to displace the huge mass of fluid in that range. Using computer fluid dynamics (CFD) simulations, we show that the volume where the fluid is pushed by a vessel can be reduced to a thin layer filled with a properly designed fluid-permeable porous metamaterial with anisotropic permeability. The structure, dubbed “fluid flow cloak,” moves through the fluid without generating any wake behind it, and it experiences zero resistance force, known as the drag force in hydrodynamics. The drag force normalized to the size of the vehicle, or the drag coefficient, is one of the most important measures of fuel efficiency. For relatively slow motions, we find that the drag coefficient can be made to vanish.

A new kind of nanoscale laser emits coherent photons with ultra-low energy consumption

LD13537

A thresholdless laser, which would produce coherent light the instant it is switched on, has been a concept pursued by many research groups for the inherent advantages in terms of energy consumption. The threshold represents the amount of energy expended before coherent light is emitted by a laser. Researchers have now reported a single nanowire polariton laser with an ultra low threshold energy of 92 nJ/cm2 at room temperature. This translates to a very small threshold optical power density of ~7W/cm2 or an electrical current density of ~1.75A/cm2, the lowest achieved so far in any laser. Polaritons are admixed particles resulting from the strong coupling between cavity photons and excitons. Polariton lasers, operating in the strong coupling regime may prove to be a new source of coherent light representing a regime in efficiency and performance beyond that of conventional semiconductor lasers. Coherent emission in a polariton laser results from stimulated scattering of polaritons into quantum degenerate polariton states and subsequent spontaneous radiative recombination. The polariton laser consists of a single defect-free and GaN nanowire of length and diameter equal to 750nm and 60nm, respectively, enclosed in a dielectric microcavity. The nanowires are epitaxially grown on silicon substrates before being selectively dispersed in the microcavity. The polariton dispersion characteristics and the nonlinearity, spectral linewidth narrowing, polarization, and coherence of the output were measured and analyzed. The threshold carrier density for polariton lasing is three orders of magnitude lower than that of photon lasing in the same devices.

How do our organs acquire their shape?

LD13645

Morphogenesis is the evolutionary process leading from the few cells in an
embryo to the very complex shapes of organs or animals; how it is guided
remains an open and fascinating question. In this paper, we show that
complex patterns observed in the intestine can be understood from
simple mechanical arguments.

When cells divide in an epithelial monolayer, they exert a pressure on
their surroundings, which is at the origin of the intestinal tube
wrinkling: the monolayer buckles. By forming large wrinkles, the area of
the tube is increased. This buckling theory predicts patterns strikingly
similar to those observed in the small intestine.

The shape of a tissue also has important effects on division : shape
can feedback growth. Adding this ingredient to the model allows us to
explain the patterns observed in the large intestine as well,
providing a comprehensive physical theory of the morphogenesis of the
intestine.

Intestinal shape and renewal seem to be at least partially controlled by a
mechanical balance. This fine balance is disrupted in the case of
intestinal diseases and should be studied more carefully in the future,
as it may shed some new light on these diseases.

FOREVER ENTANGLED

LD13369

Quantum entanglement between two separate macroscopic objects which is
maintained for as long as electricity runs in the lab is demonstrated.
This achievement disproves a popular belief that quantum entanglement,
the main component in quantum information processing, is a fragile
property which can only exist for a limited time. The novel method
used to produce this unusual result is based on employing dissipation
for generation of entanglement. Dissipation, or, in other words,
uncontrolled interaction with the environment, so far has been the
major reason for ruining quantum entanglement. We engineer dissipation
for atomic ensembles containing thousands of billions of atoms so that
it generates entanglement rather than impairing it. Our work
demonstrates that dissipation can be used for quantum information
processing. Applications range from quantum communication to quantum
sensing.

Graphene Spintronics: Realization of Nano-second spin relaxation times at room temperature.

LY12671

- Conventional electronic transistors involve the control of electronic charge at the nanoscale to realize memory, logic and communication functions. All these electronic charges, however, also carry a spin that remains unutilized in present commercial devices. This has motivated the search for new materials that propagate spin-polarized currents over large distances. Among them the most promising materials for spintronics has been graphene, a truly two-dimensional crystal of carbon atoms with relativistic carriers. Micron-scale spin relaxation lengths have been previously demonstrated in single-layer graphene. In this paper, we show that the two-layered cousin of graphene (called bilayer graphene) is a far more interesting candidate for spintronics. By fabricating spin valves on bilayer graphene we have achieved record room temperature spin relaxation times up to 2 nanoseconds, which are significantly higher than for single layer graphene. Furthermore, the presence of interlayer interaction between the two layers of bilayer graphene makes it a particularly lucrative (profitable) nanoscale material for spintronics – particularly since this interaction can be tuned by electric-field. Spin devices made from exactly two stacked layers of graphene turn out to be remarkably different not only from single-layer graphene but also from all other multilayer graphenes.Our work provides fundamental insight into the unique properties of bilayer graphene for spintronic applications.

Layers of rapidly moving media may repel or attract each other with Casimir force

AB10715

- When trains pass each other at a high speed, the air pressure
vibrations in the gap between the train bodies may not only be audible
by the passengers, but, in a worst case scenario, may push the trains
off track or pull them too close resulting in a crash. Now scale
everything down to microns and replace the trains with rapidly
moving layers of a dielectric, and the air pressure vibrations with
the quantum fluctuations of the electromagnetic field, and you will
get an idea of what is studied in this paper. With a rigorous
treatment we theoretically demonstrate a possibility for the Casimir
force to be repulsive in dielectric layers that rapidly slide one with
respect to another. The repulsion occurs in a triple-layer structure
where the exterior layers move in the same direction with respect to
the stationary middle layer of the same material. When the exterior
layers move in the opposite directions (like the trains in the
example) the Casimir force is attractive and is stronger than the same
force between stationary layers.

First Electrical Cable Built of a Single Molecule

BER1188B

- Can single molecules be deployed as reliable and effective conductors in the future electronic devices? Seeking the answer to this question we succeeded for the first time to create an electric cable from a single molecule in a systematic and reproducible way. The obtained wires are only 10 Ångstrom (1 nanometer) long, but they represent a prototypical example of a true molecular conductor clamped to the electrodes via the two well-defined “crocodile” clamps of atomic size. We create the wires by lifting one end of the single molecule lying on the atomically clean metal surface with the sharp metal tip vibrating with sub-Ångstom amplitudes. The vibrating tip on one hand plays a role of the second electric contact used for the conductance measurements at the same time it allows us to measure the stiffness of the wire thus controlling its conformation during the liftoff process.

Soda cans squeeze sound on tighter spots

LD13104


- On top of being convenient soda containers, cans are also good acoustic
resonators. Anyone can verify this by blowing air inside an empty can: an
almost pure tone can be heard. In this letter, we prove that a packed
bunch of soda cans behaves as a new sonic material with fascinating
properties. Subject to various tones, the ensemble of cans responds
collectively and creates complex sound patterns. Very interestingly, those
patterns oscillate on spatial dimensions of the order of a soda can, that
is, much smaller than the wavelength of the acoustic waves that excite
them. Generating broad spectrum sounds with low cost computer
loudspeakers, we demonstrate that 1 meter wavelength audible acoustic
waves can be squeezed thanks to the cans onto spots as tight as a few
centimetres. This experiment opens up the possibility to control acoustic
power or send audio messages on dimensions much smaller than the
wavelength. Our approach, evidenced with soda cans for the sake of
simplicity, is very general. It can be realized using many resonator and
is valid for acoustic and elastic waves at any frequency. We believe it
opens up exciting possibilities for the control of audible sound in new
exotic ways, but also more pragmatically for the design of smart sensors,
actuators and MEMS.

Stressed Workers Lead to Relaxed Colleagues

LC13228

- We study the nature of workplace stress from the aspects of human-human interactions. We investigated the distribution of Center for Epidemiological Studies Depression Scale scores, a measure of the degree of stress, in workplaces. We found that the degree of stress people experience when around other highly stressed people tends to be low, and vice versa. A simulation based on a model describing micro-level human-human interaction reproduced this observed phenomena and revealed that the energy state of a face-to-face communication network correlates with workplace stress macroscopically.

Bacteria Swim Faster in Groups

LD13593

- It is a well known fact that in nature many animals exhibit collective behaviours to move more efficiently and save energy. For example, birds fly in flocks when they migrate, fish swim together when they feed, and cyclists ride in a close group during a race to dramatically reduce drag. However, little is known about what happens when we consider collective behaviour in the microscopic world of bacteria. In particular, whether collective swimming is also the most efficient way of moving at the micro-scales, where viscous effects are dominant, is a question that until recently had no answer.

The mystery has been solved by researchers who investigated the collective swimming of bacteria. By measuring the three-dimensional velocity field, they discovered that by moving together bacteria can swim three times as fast, and that the mass transport in the suspension can be considerably improved.

Surprisingly, to achieve this enormous increase in speed, the bacteria used only a tiny amount of additional energy. The study was the first to discover that also for microorganisms collective swimming is the most energy-efficient way of moving and of absorbing oxygen and nutrients in nature.

A record-size quantum computing register

LC13841

- Quantum computing holds the revolutionary promise of exponentially speeding up such calculations as integer factoring and the simulation of quantum mechanical systems. Building a practical quantum computer will require a scalable number of individual quantum memory units ("Qbits" or "Qmodes"), all individually addressable and controllable with no error. In this paper, we demonstrated an experimental breakthrough: the generation of a record-size quantum register of 60 Qmodes, in 15 independently entangled (i.e. specifically correlated) sets, called cluster states. This massively scalable implementation of a quantum register was all-optical and based on an exotic laser, an optical parametric oscillator, which emitted quantum electromagnetic fields (the Qmodes) at equally spaced optical frequencies over an "optical frequency comb." Technical constraints limited the measured number of generated Qmodes to the reported 60 but we estimated the actual size of our quantum register to be of 180 to 600 Qmodes. This work is a major step toward entangling all these Qmodes together in a single cluster state, in order to achieve a scalable platform for a quantum computer.

Time-reversed optical parametric oscillation

LC13088

- The invention of the antilaser has attracted recently a considerable
attention from the scientific community and general media as well. The
antilaser is a device that acts like a laser in reverse and is thus
capable of completely absorbing coherent light beams instead of
scattering them as most other things do. Such a device might provide a
way to build miniature silicon optical switches or lead to new types
of photonic sensors.

In this Letter the author has shown rather generally that the main
idea of time-reversing a laser to perfectly annihilate coherent light
quanta is not peculiar to a laser system, and can be applied to other
kinds of optical instabilities. In particular, the time-reversed
process of optical parametric oscillation in a nonlinear crystal can
realize a kind of coherent perfect absorber for colored incident
signal and idler fields. In this device, two coherent light waves of
different colours can be fully annihilated in the crystal. As compared
to the antilaser, here photons are not converted into heat or some
internal energy of the medium, rather they are converted into photons
of a different colour. The idea of time reversing a generic kind of
optical instability is expected to be fruitful to built up a novel
class of optical devices similar to the antilaser but operating with
e.g. multicoloured light.

Solid-State Refrigerators and Materials to Convert Waste Heat to Energy

BB11707

- Innovations could improve the conversion of waste heat to energy.

By substituting some of the indium in CuInSe2 with manganese the thermoelectric properties of CuInSe2 have been improved by over two orders of magnitude. Thermoelectric materials work in two ways, by creating either electricity (Seebeck effect) or a temperature gradient (Peltier effect). These improved thermoelectric substances may be key in converting waste heat to energy, for example in automobile engines. Additionally, these materials may prove useful in developing new solid-state refrigerators, due to their advantages of being more compact, portable and environmentally friendly as compared to traditional liquid, ozone-depleting refrigerants. In this work, increased electrical conductivity has been observed in CuInSe2 by adding manganese, thus the thermoelectric figure of merit (ZT) of the material is increased. At the same time, the addition of manganese disrupts the normal atom locations in the material, resulting in a decrease in the thermal conductivity. This is exciting because most often the improvement in one of these properties is at the expense of the other. This work demonstrates that CuInSe2-based systems could be viable thermoelectric materials if the electrical conductivity could be even further enhanced. The improved system would be of interest to the automobile industry, for example, for the purpose of converting waste heat in the internal combustion engine to electricity.

Turbidity breaks the diffraction barrier

LB13391

“Turbidity” caused by multiple scattering is normally considered detrimental to optical imaging. For example, translucent media such as biological tissues and a ground glass make an object underneath invisible. In our study, we developed a method of extracting the original image information from the multiple scattering. This enabled us to see through the turbid media. More importantly, we made a counter-intuitive finding that optical turbidity, rather than being a hindrance to imaging, can in fact dramatically improve both the spatial resolution and the field of view of the target images. In essence, our method turns a turbid medium into a unique and unconventional lens. We believe that this will lead to great important applications in deep-tissue biological imaging and super-resolution imaging.

Figure caption:

Live cell imaging under a rat skin tissue. (a) the image of a microglia cell hidden under a skin tissue. (b) reconstructed image from (a).

A trick for visual stabilization in a hovering bird

EBJ1059

- Using fundamental mechanics principles, we interpret the complicated phenomenon of visual stabilization in a flying bird. For birds, mechanisms of visual stabilization are of great significance because blurred vision resulting from vigorous body vibrations caused by wing flapping could seriously jeopardize their survival. Most relevant studies have attributed the bird’s vision stabilization to their nervous and musculoskeletal systems that sense and reduce the vibrations. In our study, however, we found that a flapping passerine exploits a trick to fix the eyes: the production of a lift force exerted posterior to the center of mass of the bird’s body. This trick concurrently results in rotational and translational displacements of the bird's body. Such a complicated body motion does not deteriorate the stability of the eye; instead, the eye remains stabilized because the displacement caused by body translation becomes an offset due to the displacement caused by body rotation. This trick for visual stabilization can offer bio-inspired guidance for engineers to enhance the visual stability of surveillance cameras incorporated in micro aerial vehicles.

Cancer Cells as Fractals: possible new way of cancer diagnostics

LB12649


- Here it was shown that the surface of human cervical epithelial cells demonstrates substantially different fractal behavior when the cell becomes cancerous. Fractals are "self-similar" irregular shapes that repeat their pattern when zoomed in or out. These complex disorderly patterns are typically formed under far-from-equilibrium conditions, or emerge from chaos. Examples of fractal shape range from the large-scale structure of the Universe to the shape of trees and snowflakes. This paper demonstrated that the surface of human cancer cells could be treated as fractal. Analyzing the adhesion images of individual cells obtained with atomic force microscopy, it was found that cancer cells demonstrate a simple fractal behavior, whereas normal cells could only be approximated at best as multifractal. This leads to an unusually high accuracy in identification of cancer at the single cell level. Although some difference in the surface of cancer cell was expected, the observed unambiguous divergence of the fractal behavior was a surprise. This may shed light on the nature of cancer from a new physics prospective. Furthermore, it can be used for early detection of cervical cancer with accuracy surpassing the existing methods.

St. Elmo’s fire helps emerging organic electronics

LD13757

- For centuries, sailors observed St. Elmo’s fire on ship masts before a storm: this intriguing phenomenon visualizes strong increase of the electric field at sharp conducting edges. Here we find that a remarkably similar effect appears in organic electronic devices at micro-scales, and it may be controlled to improve the device performance.

Everyday electronics is expected to be revolutionized by the introduction of organic polymer semiconductors, which are new “green” materials with unique properties and low cost. These soft polymers can be transformed in-situ from an insulating to highly conducting state via electrochemical doping. When voltage is applied, doping zones spread in the polymers in a manner similar to flame fronts. However, electrochemical doping is rather slow, which represents a serious obstacle for many technical applications. In this paper we obtain a new fundamental effect, doping front instability, which speeds up the process considerably. We find that small humps at a doping front produce local increase of the electric field, similar to St. Elmo’s fire, which, in turn, leads to further growth of the humps. As a result, the front becomes strongly corrugated and moves much faster. This new effect exhibits deep similarities to the speed-up of corrugated flames in car engines and in powerful star explosions, Supernovae.

At the Edge: Why Drops Spread

LA13026


- The spreading of a liquid drop on a solid surface is a simple everyday phenomenon, yet much of the process is complex and remains under investigation. Hydrodynamic analysis of the spreading leads to a non-physical singularity at the contact line, or the triple point where air, solid, and liquid meet. In 1919, Sir William Bates Hardy discovered that the edge of a spreading drop emits a microscopically thin layer of fluid, invisible to the naked eye. The existence of this "precursor film" relieves the singularity issue. In the mid-1980s, Pierre-Gilles de Gennes and coworkers developed a theoretical model for the precursor film, considering intermolecular forces close to the contact line. Since then, physicists have striven to capture experimental evidence of its behavior and characteristics. However, due to the film's nano-scale features, it has been a challenge to overcome the limitations of many detection techniques. Researchers have recently measured the dynamic evolution of the precursor film using fluorescence microscopy. This work is the first to provide experimental support for the theory governing the precursor film's behavior with respect to time and space.

Trapping single electrons without barriers

LD12982

- In the last 20 years, many circuits have been realized where single electrons are trapped in small regions delimited by thin insulating barriers. Based on electron trapping, controlled transport of single electrons has been achieved, and spectroscopic measurements of quantum dots and molecules have been developed.

In our experiment, we show that single electrons can also be trapped without barriers. Our trap is an atomic-size contact between two superconductors with different macroscopic phases. Whereas most electrons are paired at the temperature of the experiment, we find that unpaired electrons tend to be trapped at the atomic contact and stay there for more than 100µs. The trapping of an electron is revealed by the suppression of the supercurrent that flows between the superconductors. The dynamics of trapping and untrapping displays surprising features yet to be understood.

While trapping single electrons in superconductors can be detrimental to the functioning of qubits and detectors, it could open the way to individual spin manipulation and to superconducting spin qubits.

Nanomagnetism takes energy efficiency to the limit

LC13625

- How efficient can a computer built out of magnets be? It turns out, the answer is maximally efficient. In our paper, we show that magnetic computers are able to process information at the theoretical upper limit of energy efficiency - great news for mobile devices and other energy-constrained computer applications. Some background: in 1961, Rolf Landauer of IBM discovered that any information processor – no matter how it is built – requires a minimum amount of energy to carry out a computation. This mysterious quantity is now called the Landauer limit. While the Landauer limit represents a clear technological goal for energy efficient computers, most existing computers consume far more energy than this. Enter nanomagnets. Our calculations reveal that computers built from these tiny magnetic elements use up the exact amount of energy required by the Landauer limit – nothing more. This result establishes that nanomagnetic computers are the first known technology capable of attaining the long-theorized maximum of computational energy efficiency.

Reducing Blood Viscosity with Magnetic Fields

Blood viscosity is a major player in heart disease. When blood viscosity increases, it damages blood vessel and increases the risk of heart attacks. Currently, the only method is to take drugs like Aspirin that has, however, several unwanted side effects. Here we report our new finding that blood viscosity can be reduced with magnetic fields of 1 Tesla or above in the blood flow direction. One magnetic field pulse of 1.3 Tesla lasting about one minute can reduce the blood viscosity by 20-30%. After the exposure, in absence of magnetic field, the blood viscosity slowly moves up, but takes a couple of hours to return to the original value. The process is repeatable. Reapplying the magnetic field reduces the blood viscosity again. By selecting the magnetic field strength and duration, we can keep the blood viscosity within the normal range at around 1 cp. In addition, such viscosity reduction does not affect red cells’ normal function. This technology has high potential for physical therapy.

Probing Atoms to Understand the Coexistence of Superconductivity and Magnetism

LB13449

- Superconductivity and magnetism seem antagonistic and only coexist under very restricted conditions. Explaining their coexistence requires a detailed understanding of the interaction between magnetism and superconductivity down to the atomic scale. Researchers have for the first time utilized atom probe tomography (APT) to directly image the three-dimensional atomic arrangement in newly discovered Fe-based superconductors that exhibit both antiferromagnetism and superconductivity. APT, a cutting-edge analytical microscopy technique that allows atom-by atom mapping of a material, revealed that dopants form nanoscale clusters. Complementary advanced quantum-mechanics based simulations demonstrated that these clusters underpin the unique properties of this material. These exciting results, soon to appear in Physical Review Letters, demonstrate the potential of combining nanoscale materials characterization and advanced simulations to unveil the fundamentals and advance superconductor science, exactly 100 years after its discovery, and facilitate the design of future generations of superconductor devices.

Dengue epidemics and human mobility

EB10856

- In this work we explore the effects of human mobility on the
dispersion of a vector borne disease. We combine an already presented
stochastic model for dengue with a simple representation of the daily
motion of humans on a schematic city of 20x20 blocks with 100
inhabitants in each block. The pattern of motion of the individuals is
described in terms of complex networks in which links connect
different blocks and the link length distribution is in accordance
with recent findings on human mobility. It is shown that human
mobility can turn out to be the main driving force of the disease
dispersal.