1,289 research outputs found

### Determining physical properties of the cell cortex

Actin and myosin assemble into a thin layer of a highly dynamic network
underneath the membrane of eukaryotic cells. This network generates the forces
that drive cell and tissue-scale morphogenetic processes. The effective
material properties of this active network determine large-scale deformations
and other morphogenetic events. For example,the characteristic time of stress
relaxation (the Maxwell time)in the actomyosin sets the time scale of
large-scale deformation of the cortex. Similarly, the characteristic length of
stress propagation (the hydrodynamic length) sets the length scale of slow
deformations, and a large hydrodynamic length is a prerequisite for long-ranged
cortical flows. Here we introduce a method to determine physical parameters of
the actomyosin cortical layer (in vivo). For this we investigate the relaxation
dynamics of the cortex in response to laser ablation in the one-cell-stage {\it
C. elegans} embryo and in the gastrulating zebrafish embryo. These responses
can be interpreted using a coarse grained physical description of the cortex in
terms of a two dimensional thin film of an active viscoelastic gel. To
determine the Maxwell time, the hydrodynamic length and the ratio of active
stress and per-area friction, we evaluated the response to laser ablation in
two different ways: by quantifying flow and density fields as a function of
space and time, and by determining the time evolution of the shape of the
ablated region. Importantly, both methods provide best fit physical parameters
that are in close agreement with each other and that are similar to previous
estimates in the two systems. We provide an accurate and robust means for
measuring physical parameters of the actomyosin cortical layer.It can be useful
for investigations of actomyosin mechanics at the cellular-scale, but also for
providing insights in the active mechanics processes that govern tissue-scale
morphogenesis.Comment: 17 pages, 4 figure

### Extra-Dimensions effects on the fermion-induced quantum energy in the presence of a constant magnetic field

We consider a U(1) gauge field theory with fermion fields (or with scalar
fields) that live in a space with $\delta$ extra compact dimensions, and we
compute the fermion-induced quantum energy in the presence of a constant
magnetic field, which is directed towards the x_3 axis. Our motivation is to
study the effect of extra dimensions on the asymptotic behavior of the quantum
energy in the strong field limit (eB>>M^{2}), where M=1/R. We see that the weak
logarithmic growth of the quantum energy for four dimensions, is modified by a
rapid power growth in the case of the extra dimensions.Comment: 18 pages, 4 figures, 2 tables, several correction

### Information-disturbance tradeoff in estimating a maximally entangled state

We derive the amount of information retrieved by a quantum measurement in
estimating an unknown maximally entangled state, along with the pertaining
disturbance on the state itself. The optimal tradeoff between information and
disturbance is obtained, and a corresponding optimal measurement is provided.Comment: 4 pages. Accepted for publication on Physical Review Letter

### Interacting classical and quantum ensembles

A consistent description of interactions between classical and quantum
systems is relevant to quantum measurement theory, and to calculations in
quantum chemistry and quantum gravity. A solution is offered here to this
longstanding problem, based on a universally-applicable formalism for ensembles
on configuration space. This approach overcomes difficulties arising in
previous attempts, and in particular allows for backreaction on the classical
ensemble, conservation of probability and energy, and the correct classical
equations of motion in the limit of no interaction. Applications include
automatic decoherence for quantum ensembles interacting with classical
measurement apparatuses; a generalisation of coherent states to hybrid harmonic
oscillators; and an equation for describing the interaction of quantum matter
fields with classical gravity, that implies the radius of a Robertson-Walker
universe with a quantum massive scalar field can be sharply defined only for
particular `quantized' values.Comment: 31 pages, minor clarifications and one Ref. added, to appear in PR

### Symmetry-preserving Loop Regularization and Renormalization of QFTs

A new symmetry-preserving loop regularization method proposed in \cite{ylw}
is further investigated. It is found that its prescription can be understood by
introducing a regulating distribution function to the proper-time formalism of
irreducible loop integrals. The method simulates in many interesting features
to the momentum cutoff, Pauli-Villars and dimensional regularization. The loop
regularization method is also simple and general for the practical calculations
to higher loop graphs and can be applied to both underlying and effective
quantum field theories including gauge, chiral, supersymmetric and
gravitational ones as the new method does not modify either the lagrangian
formalism or the space-time dimension of original theory. The appearance of
characteristic energy scale $M_c$ and sliding energy scale $\mu_s$ offers a
systematic way for studying the renormalization-group evolution of gauge
theories in the spirit of Wilson-Kadanoff and for exploring important effects
of higher dimensional interaction terms in the infrared regime.Comment: 13 pages, Revtex, extended modified version, more references adde

### Fractal Characterizations of MAX Statistical Distribution in Genetic Association Studies

Two non-integer parameters are defined for MAX statistics, which are maxima
of $d$ simpler test statistics. The first parameter, $d_{MAX}$, is the
fractional number of tests, representing the equivalent numbers of independent
tests in MAX. If the $d$ tests are dependent, $d_{MAX} < d$. The second
parameter is the fractional degrees of freedom $k$ of the chi-square
distribution $\chi^2_k$ that fits the MAX null distribution. These two
parameters, $d_{MAX}$ and $k$, can be independently defined, and $k$ can be
non-integer even if $d_{MAX}$ is an integer. We illustrate these two parameters
using the example of MAX2 and MAX3 statistics in genetic case-control studies.
We speculate that $k$ is related to the amount of ambiguity of the model
inferred by the test. In the case-control genetic association, tests with low
$k$ (e.g. $k=1$) are able to provide definitive information about the disease
model, as versus tests with high $k$ (e.g. $k=2$) that are completely uncertain
about the disease model. Similar to Heisenberg's uncertain principle, the
ability to infer disease model and the ability to detect significant
association may not be simultaneously optimized, and $k$ seems to measure the
level of their balance

### Unsharp Quantum Reality

The positive operator (valued) measures (POMs) allow one to generalize the notion of observable beyond the traditional one based on projection valued measures (PVMs). Here, we argue that this generalized conception of observable enables a consistent notion of unsharp reality and with it an adequate concept of joint properties. A sharp or unsharp property manifests itself as an element of sharp or unsharp reality by its tendency to become actual or to actualize a specific measurement outcome. This actualization tendency-or potentiality-of a property is quantified by the associated quantum probability. The resulting single-case interpretation of probability as a degree of reality will be explained in detail and its role in addressing the tensions between quantum and classical accounts of the physical world will be elucidated. It will be shown that potentiality can be viewed as a causal agency that evolves in a well-defined way

### Uncertainty reconciles complementarity with joint measurability

The fundamental principles of complementarity and uncertainty are shown to be
related to the possibility of joint unsharp measurements of pairs of
noncommuting quantum observables. A new joint measurement scheme for
complementary observables is proposed. The measured observables are represented
as positive operator valued measures (POVMs), whose intrinsic fuzziness
parameters are found to satisfy an intriguing pay-off relation reflecting the
complementarity. At the same time, this relation represents an instance of a
Heisenberg uncertainty relation for measurement imprecisions. A
model-independent consideration show that this uncertainty relation is
logically connected with the joint measurability of the POVMs in question.Comment: 4 pages, RevTeX. Title of previous version: "Complementarity and
uncertainty - entangled in joint path-interference measurements". This new
version focuses on the "measurement uncertainty relation" and its role,
disentangling this issue from the special context of path interference
duality. See also http://www.vjquantuminfo.org (October 2003

### Effective Actions, Boundaries and Precision Calculations of Casimir Energies

We perform the matching required to compute the leading effective boundary
contribution to the QED lagrangian in the presence of a conducting surface,
once the electron is integrated out. Our result resolves a confusion in the
literature concerning the interpretation of the leading such correction to the
Casimir energy. It also provides a useful theoretical laboratory for
brane-world calculations in which kinetic terms are generated on the brane,
since a lot is known about QED near boundaries.Comment: 5 pages. revtex; Added paragraphs describing finite-conductivity
effects and effects due to curvatur

### Thermal correlators of anyons in two dimensions

The anyon fields have trivial $\alpha$-commutator for $\alpha$ not integer.
For integer $\alpha$ the commutators become temperature-dependent operator
valued distributions. The $n$-point functions do not factorize as for quasifree
states.Comment: 14 pages, LaTeX (misprints corrected, a reference added

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