The sound of an electron spin

LK13359 - The spin of an electron is a property separate from its motion which can be used for high-precision magnetic sensing and, because it is intrinsically quantum-mechanical, as a "quantum bit" for a quantum computer.  The separation of spin and motion is not perfect, however:  the theory of relativity predicts a "spin-orbit" coupling which can influence how electrons move in a solid.  In this paper, we show that this effect strongly couples the spin of an electron trapped on a carbon nanotube suspended over a small trench to the vibrations the nanotube itself.  The nanotube acts like  a tiny guitar string, with a sound mode that can ring for a very large number of oscillation periods.  When the nanotube is tuned to ring in unison with the spin, a quantum of sound emitted by the electron can be reabsorbed and reemitted many times before it is lost.  This strong coupling has many interesting consequences and potential applications: e.g., for studying the quantum nature of nanoscale mechanical motion or for enabling long distance communication between quantum bits.

Super Rogue Waves: Observation of a Higher-Order Breather in Water Waves

Click the image for a video of an experimentally-generated
rogue wave capsizing a Lego boat.

Phys. Rev. X 2, 011015
(2012) -
Oceanic rogue waves are relatively large surface waves that appear spontaneously far at sea. They can suddenly develop from very calm and apparently safe sea states, cause serious damage to ships or offshore structures, and then disappear without a trace. Where these apparent anomalies come from is still a puzzle for scientists. A simple theoretical model for describing the evolution of these waves is a nonlinear Schrödinger equation. This equation, due to its nonlinearity, has a set of hierarchically ordered solutions known as rational breathers, or evolving solitons growing out of, and amplifying, a small localized wave perturbation. Recently, we created the lowest order rational breather, also known as the Peregrine soliton, in a laboratory-scale water tank. One open question was then: Could higher-order strongly amplifying breathers, or super rogue waves, be generated also in such a water tank? In this paper, we combine an experiment with the theory to show that the answer is an affirmative “yes.”

To generate these solutions in an open water tank, we start with a carrier wave that is a wave-tank analogue of the stable small ocean waves. The mathematical initial conditions describing small localized perturbations for the generation of the higher-order breathers are simulated exactly with a computer controlled paddle and stage-wise experiments are carefully designed to remove artifacts or get around the constraints imposed by the limited size of the tank. Indeed, large localized waves with an amplification factor of 5—super rogue waves in this laboratory setting—grow out of the small perturbation in the manner predicted by the equation.

We believe that our work not only suggests an easily accessible platform for exploring extreme water-wave dynamics, but may also stimulate similar experimental studies on high-order breather solutions in other fields such as optics, plasma physics, and superfluidity where nonlinear dynamics rules.

Two Atom Transistor

LN12557 - The fabrication of single-atom transistors is a breakthrough which is now possible both by top-down and bottom up approaches. Using standard microelectronics techniques we made a transistor where the current is controlled by the fine tuning of the ionization of two phosphorus atoms connected in series. Following single-atom devices, this 2-atom transistor is the next step towards controlled atomic functionalities in electronics. Here we use the discrete ground level of one donor as a sharp energy filter to probe the levels of a second donor. Our results show that the energy spectrum for electrons bounded to a phosphorus atom in a silicon nano device is reminiscent but differs slightly from the spectrum of an isolated donor in a bulk silicon crystal. The large energy separation (10 meV) between the ground and first excited state arises from the sharp atomic confinement potential is a good opportunity for clean atomic orbital's manipulation.

Particle-based model for skiing traffic

EM10763 - We develop and investigate a particle-based model for ski slope traffic. Skiers are modeled as particles with a mass that are exposed to social and physical forces, which define the riding behavior of skiers during their descents on ski slopes. We also report on position and speed data of 21 skiers recorded with GPS-equipped cell phones on two ski slopes. A comparison of these data with the trajectories resulting from computer simulations of our model shows a good correspondence. A study of the relationship between the density, speed and flow of skiers reveals that congestion does not occur even with arrival rates of skiers exceeding the maximum ski lift capacity. In a sensitivity analysis, we identify the kinetic friction coefficient of skis on snow, the skier mass, the range of repelling social forces and the arrival rate of skiers as the crucial parameters influencing the simulation results. Our model allows for the prediction of speed zones and skier densities on ski slopes, which is important in the prevention of skiing accidents.

Water-driven patterns on stalactites retain paleoclimatic records

LN12591 - The stunning variety of patterns shaped by water in karst caves always awakes admiration and wonder. However, although geochemistry of calcite deposition is well-known, the reasons of the astonishing morphological regularity exhibited by the superficial drape-like undulations remained unexplored so far. The short-scale wavelets on the surface of stalactites demonstrate that their scale-invariance is due to a very subtle interplay between fluid dynamics and geochemistry. The perturbations of the water film flowing on the stalactite in fact interact with the calcite deposition rate, thus originating upstream migrating corrugations with a well-selected wavelength. Stalactite accretion occurs by successive calcite stratifications shaped by waves remounting the stalactite itself. This work also shows the wavelength and speed of such waves depend on the hydraulic and chemical characteristics of the cave environment. Therefore, the geometry of the undulations imprinted in the stalactite cores retains the record of the local paleoclimate evolution. These theoretical findings could pave the road to novel morphological analyses of stalactite specimens, aiming to acquire data about paleo-flows in karst environments.

High-temperature plasma target to enhance nuclear reactions

CM10336 - The first experimental evidence that demonstrates a high-temperature plasma
formation (higher than 1 million K) in liquid metal with ultrasonic waves is
given. The key aspect of this experiment deals with a cavitating target in
the liquid Li formed by ultrasonic waves, and the target is bombarded with
deuteron beams (Ed ~ 50 keV) to explore nuclear environments where fusion
reactions can be enhanced very strongly. The striking result is that the
yield of the 2H(d,p)3H reaction increases substantially when cavitation is
turned on and the proton peak becomes broader with a small tail on the
high-energy side. Kinematical analyses indicate a temperature around 600 eV
(~7 million K) of deuterons in the interior of the bubble. The experimental
results obviously show strong reaction enhancement, although no bubble
fusion (or sonofusion) events are observed.

Walking with coffee: Why does it spill?

LM13443 - In our busy lives, almost all of us have to walk with a cup of coffee. While often we spill that precious liquid, this familiar phenomenon has never been explored systematically. Here we report on the results of an experimental study of the conditions under which coffee spills for various walking speeds and initial liquid levels in the cup. These observations are analyzed from the dynamical systems and fluid mechanics viewpoints as well as with the help of a model developed here. Particularities of the common cup sizes, coffee properties, and biomechanics of walking proved to be responsible for the spilling phenomena. The studied problem represents an example of the interplay between the complex motion of a cup, due to biomechanics of a walking individual, and the low viscosity liquid dynamics in it.

Holographic Storage of Biphoton Entanglement

LP13000 - Coherent and reversible storage of multi-photon entanglement with a multimode quantum memory is essential for scalable all-optical quantum information processing. Although single photon has been successfully stored in different quantum systems, storage of multi-photon entanglement remains challenging because of the critical requirement for coherent control of photonic entanglement source, multimode quantum memory, and quantum interface between them. Here we demonstrate a coherent and reversible storage of biphoton Bell-type entanglement with a holographic multimode atomic- ensemble-based quantum memory. The retrieved biphoton entanglement violates Bell's inequality for 1 microsecond storage time and a memory-process fidelity of 98% is demonstrated by quantum state tomography. Our work makes the first step towards holographic storage of multi-photon entanglement.

Yellowing of Ancient Paper

LK13320 - The hue of the yellowish color in ancient paper may be placed in precise relationship with the environmental conditions to which the artefacts were exposed during their life. This important outcome is the result of our experimental and theoretical investigation on the optical degradation of 15th century papers. 

Paper degradation results in a yellowing of the sheets mainly as a consequence of the oxidation of cellulose fibres. The oxidized products act as chromophores capable of selectively absorbing light and give rise to the yellow coloration of ancient paper sheets. The complex chemical and physical properties of cellulose have prevented a detailed identification of chromophores up to now.

To solve this problem, we measured the optical properties of both ancient and, as comparison, modern paper samples artificially-aged in several environmental conditions and interpreted the collected data by means of ab-initio theoretical calculations based on time-dependent density functional theory. Through this approach the relative concentrations of chromophores responsible for the yellowing of the ancient paper were clearly identified and quantified. A given set of chromophores produced upon aging is strongly related to the environmental conditions to which the artefacts were exposed during their life, such as dry, humid or closed storage.

Our work contributes to a contemporary Preservation Science which attempts to provide conservators a rationale based methods of artefacts analysis and treatment.

Sonic Screwdrivers Serve up Fundamental Physics

LN13325 -  When the scriptwriters for Doctor Who imagined a futuristic device, they came up with the Sonic Screwdriver. Now we have taken equipment designed for MRI-guided focused ultrasound surgery and demonstrated a real Sonic Screwdriver, used to lift and spin a free-floating 10 cm rubber puck.  Energy from an ultrasound array forms a beam that carries momentum and can push objects away objects in its path. If the beam is designed as a vortex, then rotation is also possible.

The helical, phased spiral wavefronts of vortex beams mean there is a rotating, angular component of momentum that can exert torque on an object.  In this paper, we show how to generate vortex beams with many intertwined helices using a 1000-element ultrasound transducer array. These beams are strong enough to levitate and spin the 90 g puck made of ultrasonic absorber in water.

We use the screwdriving effect to test the theory that the ratio of angular momentum to energy in a vortex beam is equal to the ratio of the number of intertwined helices to the frequency of the beam.  This theory is used in topics from quantum physics to biophotonics but it has not previously been proved in a single experiment.  For the first time, our experimental results confirm directly the validity of this fundamental theory.

It may not be the Higgs Boson, but for those working on related topics, it's just as important!

Trouble with the Lorentz Law of Force

LN12869 - Using a simple thought experiment involving a magnetic dipole in the vicinity of an electrically-charged particle, we show that the Lorentz law of force, in conjunction with the Amperian current loop model of a magnetic dipole, violates the fundamental tenets of special relativity. This fact, taken together with the well-known phenomenon of "hidden momentum" in certain magnetic systems (first pointed out by William Shockley nearly 50 years ago), calls into question the applicability of the standard form of the Lorentz law to problems involving magnetic materials, in general, and to radiation pressure problems in magnetic media, in particular. There exists, however, a variant of the Lorentz law, proposed by Albert Einstein and Jakob Laub in 1908, which not only conforms with special relativity, but also is consistent with the conservation laws of energy, momentum, and angular momentum. We argue that a complete and consistent basis for classical electrodynamics is provided by (i) Maxwell's macroscopic equations, (ii) the Poynting postulate for energy, (iii) the Einstein-Laub force and torque densities, and (iv) the Abraham postulate for electromagnetic momentum density.

Demonstration of the Interaction between Two Stopped Light Pulses

LH12745 - This study reports the first experimental demonstration that two light pulses were made motionless and interacted with each other through a medium. The scheme with motionless light pulses maximizes the interaction time and can achieve a considerable efficiency even below single-photon level. To demonstrate the enhancement of optical nonlinear efficiency, the experiment in this study used the process of one optical pulse switched by another based on the effect of electromagnetically induced transparency. Moving light pulses activate switching at an energy per area of 2 photons per atomic absorption cross section as discussed in [Phys. Rev. Lett. 82, 4611 (1999)]. This study demonstrates that motionless light pulses can activate switching at 0.56 photons per atomic absorption cross section, and that the light level can be further reduced by increasing the optical density of the medium. The result of this work enters a new regime of low light physics.

Heaviest Oxygen Discovered: 26O

LK13350 - 26O, consisting of 8 protons and 18 neutrons has been predicted to beta-decay to 26F, however, extensive searches have found no evidence for the existence of 26O nor for this decay mode. In this paper we demonstrated for the first time that 26O after it is formed breaks up into two neutrons and 24O. The data suggest that the neutrons are emitted simultaneously, possibly as a di-neutron, an exotic decay mode which was discovered only a month ago in the decay of 16Be. The measured mass of 26O is a critical milestone towards the understanding of the question why oxygen can bind only 14 neutrons (24O) while fluorine with only one additional proton can bind at least six more neutrons (31F has 22 neutrons and 9 protons).

Lasers available in 2015 will tear apart the vacuum to create a new state of matter

LJ13498 - In this paper we have shown that next-generation lasers will tear apart the vacuum to generate a new state of matter: a quantum electrodynamic (QED)-plasma.  This will usher in a revolution in laser plasma physics leading to many exciting new applications from the generation of large quantities of antimatter, to the being the basis of the world's most intense gamma-ray source.  Quantum electrodynamics (QED) underpins our understanding of what happens when elementary particles such as electrons are accelerated to very high energies.  QED predicts that a strong electromagnetic field applied to the vacuum can be converted into mass, tearing the vacuum into electron-positron pairs.  We have shown that this will be possible by firing 10PW lasers (10PW = 10,000 times the electrical generating capacity of the US), due to be completed in 2015, at solid targets. When the laser strikes the solid the electrons are rapidly stripped away from their atoms, the solid is ionised and a dense plasma is created.  An entirely new state of matter is generated in the laser focus, defined by a complex interplay of the QED processes and classical plasma physics: a 'QED-plasma'.

Electric-field control of switchable and non-volatile magnetization at room temperature

LJ12852 - With the fast development of information storage, exploiting new concepts for dense, fast, and non-volatile random access memory with reduced energy consumption is a significant and challenging task. To realize this goal, electric-field control of magnetism is crucial. A promising way to control magnetism via electric fields is using the converse magnetoelectric effect, which is important for realizing high speed and low power writing memories. We report the first example of a large, switchable and non-volatile bipolar-electric-field-controlled magnetization at room temperature in a Co40Fe40B20/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 ferromagnetic/ferroelectric (FM/FE) two-phase structure. Through investigations of the ferroelectric domains and crystal structures with in situ electric fields, we demonstrate a novel mechanism for electric-field control of magnetization involving the combined action of 109¡ã ferroelastic domain switching and absence of magnetocrystalline anisotropy in Co40Fe40B20. This work provides a route to realize electric-field control of large, switchable and non-volatile magnetism at room temperature, and similar phenomena should occur for other comparable FM/FE structures.

Making the smallest heat engine

LG14179 - There is nothing that envisages the idea of classical physics more than a heat engine, but how can this stem from quantum mechanics. The time reversible nature of quantum mechanics suggests that the most simple ingredient of a heat engine, the system being in thermal equilibrium, is not possible.

We present a simple scheme of atoms confined to a double well, that demonstrates thermalization of the atoms and is simple enough to perform a heat engine cycle. By modifying the trapping confinement and well depths, one well acts as the heat engine and the other as the heat reservoir. Most notably, the heat engine, reservoir and hence the energy transfer is described fully quantum mechanically in this finite isolated system. The whole system could be as little as 10 microns in size, so if experimentally realization, it would be the smallest heat engine ever.

Smart surface structuring can help creating good electrical contacts

LH13420 - In light switches, in relays, on car batteries and in light bulk sockets
– many devices require a well conducting contact between two metals, at
the least possible cost and effort. Up until recently, it was common
practice to assume linearity between conductivity and normal force – in
other words, it was believed that one would have to press twice as hard
if one wished to channel the double amount of current. In our paper
though, we showed that the characteristic of the surface roughness has a
significant influence upon this behavior by altering the surface's
contact stiffness. With the help of computer simulations, we compared
surfaces which had a rather fuzzy look to more wavy ones and found out
that the later performed significantly better at low forces as can be
found in most technical applications.

A new, brief formula will help technicians and engineers estimating the
electrical resistance by taking into account the surface characteristic
from their manufacturing method. A better prediction may help them save
material or abandon a costly surface treatment.

Swirling airflow helps stabilize flapping-wing flyers

LG14064 - As the Wright brothers demonstrated one hundred years ago, the key challenge of flight is maintaining balance. Although insects took to the air 400 million years earlier, their flight stability remains a mystery because of the complex aerodynamics of flapping wings. We approach this problem by discovering the conditions needed to achieve stable hovering in mechanical flyers. Our system consists of pyramid-shaped bugs constructed from paper that hover when placed in an oscillating column of air, mimicking the effect of flapping wings. To our surprise, we find that top-heavy bugs hover stably: if the body tilts to the side, the swirls of fluid ejected from the wings automatically adjust to keep the bug upright. By providing this connection between wing shape and flow features, these findings offer a blueprint for achieving stability in highly maneuverable flapping-wing robots.

One-atom thick film absorbs light completely

LE13613

 - Black bodies are idealized objects capable of absorbing all
light impinging into them. In this paper, we engineer a black body made
of a single layer of carbon atoms (graphene). Our design achieves total
light absorption for selected colors, which can be easily tuned by means
of electrical connections placed nearby. This study has direct
application to improved photodetection, particularly in the difficult
range of infrared detection, for security sensing, nocturnal imaging,
etc. It has also direct application to infrared lighting, with great
potential for manufacturing sources with unprecedented levels of
brightness in this range of the optical spectrum, which has been
difficult to cover so far. Our design can also be used to collect
infrared light for improved photovoltaics, because this part of the
solar spectrum is generally lost due to the unavailability of good
infrared absorbers as ours. And there are several other directions of
application, including bionsensing, chemical detection, etc. And last
but not least, the new black bodies have fundamental interest as a
unique structure that is capable of absorbing light completely within
record-thin one-atom layer (the graphene) that is just several thousand
times thinner than the ligh wavelength.

TIMELIKE ENTANGLEMENT AND TELEPORTATION

AJ10688

 - It has been known since the 1970's that quantum entanglement exists
between regions of empty space at a given time – this underlies many
exotic phenomena such as the evaporation of black holes.  In this
paper, we show that the same type of empty-space entanglement also
exists between different times at a single location in space, and that
this “timelike entanglement” can, in principle, be used as a resource
for quantum technology.  We show how a particle detector operating at
11:45am could in principle be made to share quantum entanglement with
another particle detector at the same location in space at exactly
12:15pm, but at no other times (e.g. not 12:10 or 12:20).  The
now-standard technology of quantum teleportation uses ordinary
entanglement as a resource for moving qubits of information from one
point to another in space – we propose that the use of timelike
entanglement could be used to teleport qubits of information from one
time to another, without being present during the time in between.

APS Physics News Ticker on Hiatus . . .

. . . until January 3, 2012.

Sensors using sound amplification by light

LF13479A

- The quest to measure exceedingly small motions of test masses for gravitational wave detectors has, over the past 40 years, led to the development of successive generations of ever more sensitive instruments limited by the laws of nature. Physicists studying kilometer scale laser interferometer gravitational wave detectors have turned a problem - triple resonance of two light fields with a single sound field - into a new instrument. They have experimentally shown that the instrument can measure a tiny vibration as small as a thousandth of the diameter of the nucleus of a gold atom...about one part in 10 to the power of 17 of a meter! They show that simple improvements can make the instrument 1000-fold more sensitive.

The new instrument operates by shining laser light onto a flexible mirror. When the light reflects off the mirror, the mirror recoils and the tiny recoil is enough to convert the light into a new light beam of a slightly different colour, and in a pattern that allows it to build up by resonance as it reflects back and forth between the flexible mirror and a second mirror. This resonance enhances the conversion of the vibration to the new light. By measuring the intensity of the new light, the mirror vibration is measured to extreme accuracy. The trick in making this deceptively simple concept work is the tuning of the resonance for the new light, which the team achieves by using a secondary laser to precisely deform one of the mirrors.

In a slight variation the same instrument can amplify radio waves and turn these into light, or measure tiny magnetic fields down to a millionth of the strength of the earth’s magnetic field.

The researchers plan to make a sensor for airborne detection of magnetic minerals, as well as tiny devices usable in space for measuring the composition of asteroids that could be mined and used in future space exploration.

Melting Ice Avalanches

LG13252

- This letter describes a novel experiment exploring melting in ice-bearing granular flows such as rock fall from mountain faces previously stabilised by permafrost or collapse of glacier séracs. It is proposed that collisions between ice particles as they flow over each other cause the particles' surfaces to melt, even at temperatures well below freezing. The melted layer not only alters the friction between particles, but it leads to the formation of liquid bridges between neighbouring particles. Our experiment employs a narrow Perspex drum, half filled with ice particles, situated in a temperature controlled laboratory. As the drum rotates, a continual avalanche of ice particles is formed. By regulating the temperature of the experiment the energy required to melt the particle surfaces, the degree of melting and the moisture content of the granular ice avalanche are all controlled. This work shows that melting through collisions enhances ice avalanche velocities and defines a parameter space for detailed exploration of these processes. It pioneers a new approach to analyzing the evolving hazard posed by rock and ice avalanches.

Do quantum states truly jump instantly?

LG14011A

- Among the strange properties of quantum states is their behavior when a measurement is performed: according to the so-called orthodox interpretation they change instantly (collapse). In this work we assume that this collapse may last for a non-vanishing time and suggest a procedure to estimate its duration. This is important because the status of objective reality of quantum states is far from being consensual among physicists. Two competing views stand out: either the state is a mathematical tool that gives statistical information on sets of systems, being ultimately meaningless for a single particle; or it is an "existing" thing for each system, at the same footing as the velocity of a particle or the electric field generated by a charge. In the first scenario it is natural to conceive the collapse as a gain of information, which can, in principle, be sudden. However, if the quantum state is to be considered a physical entity, not only a mental construct, the idea of instantaneous collapse is, at least, awkward. Thus, accessing the time scale of collapse would deepen our knowledge on the very nature of quantum theory.

Artificial intelligence to search for new compounds

LF13329

- By combining quantum chemistry with artificial intelligence, researchers have achieved a scientific breakthrough expected to aid in exploring chemical compound space, i.e. the virtual space populated by all possible chemical compounds. The interdisciplinary team dramatically increased the speed of calculating energies of small molecules with quantum chemical accuracy. Quantum chemical methods permit scientists to calculate molecular properties on a computer from first principles (i.e., without having to conduct any experiments)—they are necessary for many chemical applications such as catalysis, or the discovery of novel materials. Previously, such calculations demanded intensive computational resources. Machine Learning, on the other hand, generates predictive models based on examples. While common in daily life, such as in Google's internet search engines or Amazon's book suggestions, it is also used in scientific domains, such as genetic research or brain computer interfaces. When applied to quantum chemistry, thousands of quantum chemical reference energies have been calculated in order to learn a molecular model. The resulting artificial intelligence machine permits the prediction of molecular properties with comparable accuracy within milliseconds, instead of hours. Such speed-up paves the way for highly accurate calculations of unprecedentedly many molecules.