Estimating dates for the emergence of new species over the course of
evolution is a pressing problem, and a common method used to address it
is the so-called "molecular clock" technique, which uses differences in
DNA or protein sequences across species to infer their lineage. However,
molecular clocks are often erratic: in many cases, they disagree with
each other and with evidence from fossil records. One explanation for an
erratic molecular clock is the variation over generations in the number
of harmful mutations that destroy gene or protein function. These
"deleterious" mutations leave no trace in evolutionary histories because
organisms that host them do not produce offspring. In this paper, I
develop a molecular clock model in the presence of such variation. An
implication of this work is that the best molecular clocks will be
proteins that can withstand many different mutations at any site, but
suddenly become non-functional if these mutations occur simultaneously
at many sites. LE11715
***
Criticality Signatures in Human Behavioral Organization
The domain of individual human behavior appears to be excluded from
treatment by physics, since actions are subject to the individual's
constant conscious deliberation and psyche, resulting in a continuously
changing type and level of activity, arising from interaction with
dynamically changing environmental demands. Yet, in our recent paper
we demonstrate that individual motor activity, covering the majority
of behavioral dynamics, follows strictly physical, universal behavior,
which can be generalized across individuals. It is particularly
striking, that the statistical law which we observe for the resting
periods between instances of activity (bursts) belongs to a
universality class analogous to that of the critical branching of
avalanche propagation experimentally determined in living neural
networks [1] and considered theoretically in a critical branching
process [2] as a model of neural avalanches. Therefore, a critical
branching paradigm analogous to that found in neural activity may
constitute the underlying mechanism of the universality we observe
in the motor activity of humans. Furthermore, deviation from the
critical scaling law has been observed in depression patients,
associated with more episodes of slowing down of movement. These
findings put in a different perspective and shed light on the
underlying mechanism of the recently observed phenomenon of
universality in human communication dynamics [3]. This universality
is of a similar class to that in periods in human behavior and in
neural activity dynamics. In addition, the findings are expected
to point towards a new direction in depression research, the illness
which is currently the leading cause of disability in North America
as well as other countries, and is expected to become the second
leading cause of disability worldwide (after heart disease) by the
year 2020, according to the World Health Organization [4]. LD11772
***
Atomistic simulations reveal new evolutionary pathway for vacancy
voids in FCC metals
Using LANL-developed accelerated molecular dynamics (AMD) methods,
atomistic-scale simulations may be extended in timescale beyond the
nanoseconds achievable with conventional molecular dynamics. Through
AMD simulations, we have discovered a new evolutionary pathway for
vacancy voids in FCC metals. We have found that they can thermally
transform to stacking fault tetrahedra (SFTs) at moderate temperatures
(about 400 K) without first passing through the Frank loop stage, as
is commonly assumed. Remarkably, this transformation occurs via a
process with an extremely large potential energy barrier: over 2 eV
for a 20-vacancy void and over 4 eV for a 45-vacancy void, barriers
that are typically associated with timescales of over one million
years at 400 K. Instead, extremely large entropy differences between
the void and the SFT lead to effective rate prefactors tens of orders
of magnitude greater than what is typically observed for atomic
processes in FCC metals. Moreover, the transformation mechanism
predicted in these simulations differs wholly from any
previously-conceived pathway. Because of this outstandingly high
prefactor, we observe events on experimentally relevant time scales
that would normally be dismissed because of the extremely high
barrier. LC11793
***
Nuclear antenna
A conventional antenna is known to play an important role in any radio
device (TV sets, cell phones etc). Surprisingly, the antenna is
found to play a crucial role in an absolutely different situation, when an
atomic nucleus is exposed to a radiation of a super-strong laser. Call
this type of antenna the "nuclear antenna", which only purpose is to help
a nucleus to accumulate and transform the energy of a laser into other
forms of energy. Start from the well known process. A nucleus exposed to a
laser field can create electron-positron pairs. Note that other particles
cannot be created this way because they are heavier, which presumes
absorption of extremely high energy from the laser that proves be
impossible. The idea of the present work is that the electron-positron
pair created plays a role of the nuclear antenna. Firstly, this pair is
accelerated to sufficiently high energies by the laser field. Then this
energy is released via electron-positron collision, which results in
creation of muons (for example). Net result is that a nucleus exposed to a
laser field can create heavy particles, a process that would be absolutely
impossible if not the nuclear antenna discussed. LF11543
***
RENORMALIZING RESOURCES FOR QUANTUM COMPUTING
One of the main obstacles towards realizing a fully fletched, large scale
quantum computer is to find a feasible strategy of how to deal with errors
and imperfections. Early on in the field, the seminal ideas of quantum error
correction and fault tolerance were introduced. Clearly, one cannot simply
look a the system, as this would necessarily destroy the quantum state.
By using a suitable encoding, however, one can make sure that reliable
computing is possible without having a classical signal available on what
error has happened.
But even if one does know what errors have occurred, a key challenge has to
be overcome: How can one compensate them, realizing effectively
deterministic computation? Even in a quantum computer operating based on
gates operating only with a certain probability of success, one may consider
the unwanted outcomes as faulty ones. Unfortunately, exactly such
situations are rather ubiquitous in many proposals of quantum computing,
ranging from linear optical approaches to ones based on atoms in cavities,
coupled via light, to ones based on atoms in optical lattices. Knowing what
error has occurred, it can of course in principle be corrected. But in just
the same type of architecture - needless to say - one definitely wants to
avoid adaptive measurements, rerouting, and what is called feedforward as
much as possible. This is specifically daunting in the linear optical
setting as light, after all, travels with the speed of light.
In Ref. [1] a new idea is introduced of how to deal with probabilistic known
errors in quantum computation, by establishing a novel link between quantum
computation and ideas of percolation, classical phase transitions, and
renormalization.
The basic idea is simple: If a resource state that could be used for
measurement-based computation [2] is faulty: Could one "renormalize it", and
find a new effective lattice which is almost certainly perfect? Indeed, this
is possible. Remarkably, as is rigorously shown, the state preparation for
quantum computation is achieved using essentially the same scaling in
resources as if fully deterministic gates were available. Hence, a novel way
to cope with randomness in quantum computing is introduced, giving the
recommendation: Do not try to compensate the errors. Just think in terms of
larger units, and hope for the best. It will (almost certainly) work. LZ10621
***
Quantum Interference Enables Low-Loss Negative Refraction
Negative refraction of electromagnetic radiation (light, radio waves,
etc.) is currently a very active area of research, motivated by goals
such as the development of "invisibility cloaks" and "perfect lenses"
in which imaging resolution is not limited by the wavelength of the
radiation. Despite remarkable recent progress in demonstrating
negative refraction in the lab using technologies such as
metamaterials and photonic crystals, a key challenge to practical
application in the optical regime remains the realization of negative
refraction in materials with low loss. In a paper appearing in the
August 17 issue of Physical Review Letters, scientists propose a
promising new approach to this problem: the use of quantum
interference effects, similar to those used to realize
electromagnetically induced transparency (EIT) and slow light, to
suppress loss while enabling negative refraction at optical
wavelengths. As with EIT, the proposed method may be applicable to a
wide range of gas and solid-state systems. LC10990
***
Our measurements show that by adding a modest amount of small plastic microspheres to water, acoustic group velocities in the resulting suspension can exceed the speed of light and enter the negative speed regime. Using a 3% aqueous suspension of 160 micrometer diameter spheres, the coherent scattering effects of the randomly positioned spheres give rise to a surprising amount of dispersion, which is the phenomenon responsible for the existence unusual group velocities. For over 25 years it has been known that electromagnetic pulses, from microwaves to visible light, propagating in dispersive materials can exhibit superluminal (i.e., greater than c, the speed of light in vacuum) group velocities (although this phenomenon is also known to comply with the constraints of Einstein’s relativity). However, for acoustic waves, there is a gap over 5 orders of magnitude wide between ordinary ultrasonic velocities in water (about 1 mile per second) and the speed of light (about 190,000 miles per second). For our work, the keys to the superluminal ultrasound effect lie in the unique scattering properties of the individual spheres, and the very narrow size distribution of the spheres in suspension, leading to a strong coherent response in spite of their random spatial arrangement.
Some background (including animations) on the theory and phenomenology of superluminal group velocities of ultrasonic waves in suspensions are detailed in a recent peer reviewed paper in an open access journal (“The time-domain signature of negative acoustic group velocity in microsphere suspensions”, Journal of the Acoustical Society of America Express Letters, Vol. 122, Issue 1, pp. EL8-EL14) and a non-technical description of earlier work (also including animations) on the web at http://www.acoustics.org/press/150th/Mobley.html. LC11172
***
Election results: do voters behave like particles?
When in the polling station, voters are completely free
to cast their vote for the candidate they prefer,
but their overall behaviour is predictable.
This is the conclusion of the manuscript by Fortunato and
Castellano, revealing that,
in proportional elections, the distribution of votes for
candidates in a party list is the same in all countries
and years. The universal voting behaviour has nothing to
do with social, economic or political issues, but only depends on
how the word of mouth spreads from the candidate to the voters. LB11253
***
Atomic Bonds Under the Microscope
A new method for extracting the signal of electrons interacting with a material has brought groundbreaking resolution to the examination of atomic bonds. The method was developed and tested using basic electron microscopes but the results surpassed previous measurements with far more sophisticated and expensive instrumentation.
Many properties of materials are sensitively governed by how atoms bond together. Therefore, accurate measurements of atomic bonds are of fundamental importance to materials science. Adequate resolution was only thought possible with the most complex and very expensive electron optics. The new method, in addition to surpassing the resolution of the established techniques, opens the field to mainstream electron microscopes and a much larger research base.
This method is surprisingly simple. It uses the opposed behaviours of the desired and undesired components of the electron signal to almost completely remove the unwanted part and simultaneously accentuate the crucial information. The method does this more comprehensively than is possible with the latest generation of instrumentation alone. This approach presents a new way of thinking in electron microscopy as it has other applications aside from the present demonstration. LE11550
***
Nonlinear waves trap themselves on local bound states
The scattering of waves by local medium discontinuities is a fundamental aspect
of physics. A well-known fact is that, in the linear-wave regime, it is
completely impossible for unbound waves “going through” local structures to
interact with bound states of these structures, as they are orthogonal. In a
first detailed experimental study of nonlinear wave scattering by local
photonic potentials, the researchers in Tel-Aviv and their collaborators in
Quebec have demonstrated the novel effect of scattering-wave trapping on the
bound states, and resonant scattering by the unbound states, of local
potentials. The observation of selective, power-dependent, wave trapping
between nearby scatterers, and between the different bound states of the same
scatterer, can pave the way to the realization of novel electro-optical
switching devices. LZ10607
***
Unraveling the Tangles of Turbulence
Lewis Fry Richardson (1883-1951), the grandfather of modern metrology,
described the cascade of energy through length-scales of ever-decreasing
size that occurs in turbulent flows with the whimsical rhyme:*
"Big whorls have little whorls
Which feed on their velocity,
And little whorls have lesser whorls
And so on to viscosity ..."
Research that is soon to be published in Physical Review Letters**
suggests for the first time that Richardson's tangle of whorls can be
forced to unwind; that is, to evolve backward in time so that large-scale
order is reestablished from complexity. The work involves nuclear magnetic
resonance (NMR) studies of spin-turbulence in highly magnetized samples of
3He dissolved in superfluid liquid 4He. Interactions between individual
nuclei and the magnetic field created by the remainder of the sample cause
initially uniform nuclear spin distributions to progressively break up
into "whorls" of ever-decreasing size; that is, to evolve in a manner
analogous to classical turbulent flows. By applying a complex pattern of
oscillating magnetic fields to these samples, Michael Hayden and
collaborators have managed to force turbulent "magnetization whorls" to
unwind, in effect causing small whorls to produce larger and larger
whorls, until uniformity is once more established. Although the
effectiveness of this time-reversal process is ultimately limited by
diffusion, it is expected to provide valuable new insight into the manner
in which turbulence is established, a problem that has challenged
scientists and engineers for centuries. LD11580
***
SCIENTISTS DETECT SINGLE SPINS IN ELECTRICAL MEASUREMENTS
The task of manipulating the spin of single electrons is a hugely daunting
technological challenge, but has the potential, nonetheless, to open up new
paradigms of nanoelectronics. In a paper recently accepted for publication
in Physical Review Letters (PRL) [1], we have demonstrated a novel approach
that allows us to easily trap, manipulate, and detect single-electron spins,
in a scheme that has the potential to be scaled up in the future into dense
integrated circuits. The approach that we use involves making use of a
special class of nanodevices known as quantum point contacts, which are
nanoscale constrictions in an electrical conductor. It was recently
predicted theoretically [2] that is should be possible to use such
constrictions to trap single spins, and in our paper to appear in PRL we
provide evidence that such trapping does indeed occur and that it may also
be manipulated electrically. Consequently, our results could have
significant implications for the future development of single-spin
electronics, with possible applications to spin-logic/memory devices, as
well as spin-based quantum computing. LE11481
***
Cooling a silicon cantilever with radio waves
Scientists in recent years have employed a variety of techniques to
lower the temperature of mechanical objects comprised of many atoms,
such as microscopic beams and cantilevers, in pursuit of observing
manifestly quantum effects in these objects. In this paper we
demonstrate cooling of the ¿diving board¿ motion of a miniature silicon
cantilever by capacitively coupling it to a higher frequency, resonant
radio-frequency circuit. By driving the circuit below its resonance
frequency, the capacitive force on the cantilever acts in a way that
opposes its motion, leading to cooling, here by 249 degrees C below room
temperature. Similar experiments using optical cavities and laser light
have reached lower cantilever temperatures, but the use of radio
frequency or microwave resonant circuits may be easier to implement in
certain applications. Moreover, refinements in this technique eventually
should allow cooling to near the quantum mechanical ground state of the
cantilever¿s motion. LE11101
***
Turbulence is unravelled in superdense bodies
Superdense bodies, for instance the next generation of intense laser-solid dense
plasmas, superdense astropphysical objects such as the interior of white dwarfs and
neutron stars, quantum diodes, quantum dots, micromechanical systems, and nanorwires
are described by a set of quantum hydrodynamical equations including
electrostatic and
quantum forces in dense plasmas. In this paper, we have, for the first time,
explored
new aspects of turbulence in a superdense Fermi plasma to understand
the energy transfer and electron transport properties. A non-universal behavior
of the
energy transfer between different scale
fluctuations ate nanoscales has been discovered
that depends upon the quantum tunnelling effect. The latter is
characterized by a ratio between
the energy density of the electron plasma oscillations and the electron Fermi
kinetic
energy.
The higher this ratio is, the scales condensate into large scale
structures. This property of quantum plasma turbulence can potentially unravel the
physical mechanism that leads to the formation of superdense astrophysical objects,
such as the white dwarf and neutron stars. On the other hand, the weaker tunneling
effect results in a co-existence of large and small scales in a dense quantum fluid. LB11324
***
Lift and bidirectional motion of colloidal particles in an
electrically driven nematic liquid crystal
Colloidal dispersions in a liquid crystal are a fascinating class of soft
matter. Colloidal particles distort the director of the liquid crystal
matrix, creating a dipole-like field. In a confined sample, these
distortions allow the colloidal particles to levitate in the liquid crystal
fluid: Elastic repulsion of the colloid from the bottom plate overcomes the
gravity force and stabilizes the particle in the bulk of the sample. By
using a three-dimensional visualization technique (Fluorescence Confocal
Polarizing Microscopy), we demonstrate the effect of levitation and then
expand the research to study the dynamics of colloids with dipole-like
distortions. By applying the electric field across the cell, the
particles are moved to one of the substrates, depending on the direction of
their elastic dipoles. By switching the field on and off, the particles
with opposite dipoles are moved into opposite directions. We demonstrate
that the phenomenon is caused by the backflow effect, i.e., a coupling
between the director reorientation and mass flow in the nematic bulk. This
unique experimental system with two subsets of identical particles moving
into opposite directions represents a wonderful model to
study non-equilibrium processes such as driven diffusion of species
against each other (traffic and jamming problems). It can be also used in
the development of new microfluidic devices. LD11821
***
Femtosecond filaments produce efficient THz radiation
A group of French researchers at ENSTA/ecole polytechnique in Palaiseau has
demonstrated that femtosecond laser pulses undergoing filamentation in air
produce efficient THz radiation (see PRL 98, 235002 (2007) and to be
published in PRL). This THz radiation is emitted along a narrow cone in the
forward direction. An attractive feature of this new THz source is its
extreme simplicity . To produce a nearly collimated THz pulse, just
launch a short pulse from commercially available femtosecond laser in air;
if the pulse peak power exceeds a critical value (about 5 GW for IR pulses)
the pulse will undergo self focusing until the beam collapse is arrested by
ionization of air molecules. It is the forward moving ionization front
which is responsible for the THz emission. Vladimir Tikhonchuk from the
University of Talence in France has interpreted it as a Cerenkov-type of
emission.
Another attractive feature is that it is easy to bring the beginning of
filamentation close to a remote sample. In this way, the problem
encountered in many THz applications, namely the strong attenuation of THZ
radiation in air due to water vapor is solved. In a proof of principle
demonstration D’Amico et al;(PRL 98, 235002 (2007) have detected a strong
signal 30 m from the laser itself. Furthermore it is easy to raster the
beam, hence the THz source over a large target. This may be useful for
applications in medicine or security.
In a recent development (see Yi Liu et al to appear in PRL), the same group
led by Andre Mysyrowicz has shown that the THz radiation can be enhanced by
one order of magnitude (at leasrt in the 100 GHz region where it is
detected) by sending a sequence of two identical laser pulses, each forming
a filament. The key to the enhancement is to produce two closely spaced
plasma columns. In this way a bimodal waveguide for THz is achieved which
can support a TM THz radiation mode. The enhanced TM emission is now
linearly polarized, instead of being radially polarized. Its polarization
direction is fixed by the alignment of the two produced plasma columns. LF11194
***
Revised theory of the magnetism of the electron
Theoretical understanding of the nature of electron's magnetism
has been advanced by a big step by the authors.
The electron, the simplest elementary particle, is a tiny magnet
whose strength is calculable
as a power series of the fine structure constant $\alpha$
(one of the fundamental constants of physics)
by quantum electrodynamics (QED).
The calculation involves a large number of Feynman diagrams
which describe in detail how electrons and photons interact.
The authors have developed an automated system to perform
this enormous and tedious calculation on a computer.
Applied to the evaluation of 518 diagrams
contributing to the $\alpha^4$ term,
it worked extremely rapidly.
This calculation confirmed the validity of previous results of
490 diagrams but uncovered errors in two integrals representing
the remaining 28 diagrams.
Correcting this error, the coefficient
of the $\alpha^4$ term is now firmly established.
Together with the exactly known coefficients of $\alpha$, $\alpha^2$,
and $\alpha^3$ terms, it provides the overall theoretical
precision that exceeds the precision of the recent measurement,
which is $\pm$ 0.000~000~000~000~66.
Comparison of this theory and experiment yields
$\alpha$ with the uncertainty of $\pm$ 0.000~000~000~71,
which is far more precise than all other measurements
of $\alpha$.
It has thus a great impact on all branches of physics
involved in high precision measurements. LF11635
***
Direct observations of small scale plasticity by a new in-situ diffraction technique
Uniaxial compression of single crystalline metallic volumes with dimensions of only a few microns has revealed that a metal can get stronger when the sample volume is reduced, casting doubt on classical deformation theories that lack such lengthscale dependencies. The suspicion that structural defects, i.e. deviations from perfect crystalline structures would play an important role in the smaller is stronger effect, especially since samples are usually made using ion sputter techniques, could not be verified because of the lack of an appropriate measuring technique. In “Time resolved Laue diffraction of deforming micropillars” the microstructure of micron sized Au pillars is followed in real time in a microfocused white X-ray beam of the Swiss Light Source (see figure 1 for experimental setup). The dynamics of the Laue patterns demonstrate the occurrence of crystal rotation and increased strengthening of the smaller pillar can be explained by plasticity on a slip system that is geometrically not predicted but selected because of the character of the pre-existing defect structure. Time resolved Laue diffraction presents a different picture on the “smaller is stronger” paradigm and urges the use of in-situ methods to study small scale plasticity, providing the correct input for the development of predictive mesoscopic models. LD11170
