800 research outputs found
Cluster derivation of Parisi's RSB solution for disordered systems
We propose a general scheme in which disordered systems are allowed to
sacrifice energy equi-partitioning and separate into a hierarchy of ergodic
sub-systems (clusters) with different characteristic time-scales and
temperatures. The details of the break-up follow from the requirement of
stationarity of the entropy of the slower cluster, at every level in the
hierarchy. We apply our ideas to the Sherrington-Kirkpatrick model, and show
how the Parisi solution can be {\it derived} quantitatively from plausible
physical principles. Our approach gives new insight into the physics behind
Parisi's solution and its relations with other theories, numerical experiments,
and short range models.Comment: 7 pages 5 figure
A Solvable Model of Secondary Structure Formation in Random Hetero-Polymers
We propose and solve a simple model describing secondary structure formation
in random hetero-polymers. It describes monomers with a combination of
one-dimensional short-range interactions (representing steric forces and
hydrogen bonds) and infinite range interactions (representing polarity forces).
We solve our model using a combination of mean field and random field
techniques, leading to phase diagrams exhibiting second-order transitions
between folded, partially folded and unfolded states, including regions where
folding depends on initial conditions. Our theoretical results, which are in
excellent agreement with numerical simulations, lead to an appealing physical
picture of the folding process: the polarity forces drive the transition to a
collapsed state, the steric forces introduce monomer specificity, and the
hydrogen bonds stabilise the conformation by damping the frustration-induced
multiplicity of states.Comment: 24 pages, 14 figure
Photo-induced dynamics of the heme centers in cytochrome bc 1
The ultrafast response of cytochrome bc1 is investigated for the first time, via transient absorption
spectroscopy. The distinct redox potentials of both c1- and b-hemes allow for a clear differentiation of
their respective signals. We find that while the c1-heme photo-product exhibits the characteristics of a
5-coordinated species, the b-hemes presumably undergo photo-oxidation at a remarkably high
quantum yield. The c1-heme iron–ligand recombination time is 5.4 ps, in agreement with previous
reports on homologous cytochromes. The suggested photo-oxidized state of the b-hemes has a lifetime
of 6.8 ps. From this short life-time we infer that the electron acceptor must be within van der Walls
contact with the heme, which points to the fact that the axial histidine residue is the electron acceptor.
The different heme-responses illustrate the flexibility of the c1-heme ligation in contrast to the more
rigid b-heme binding, as well as the higher electronic reactivity of the b-hemes within the bc1 complex.
This study also demonstrates the remarkable connection between the heme local environment and its
dynamics and, therefore, biological functio
Hierarchical Self-Programming in Recurrent Neural Networks
We study self-programming in recurrent neural networks where both neurons
(the `processors') and synaptic interactions (`the programme') evolve in time
simultaneously, according to specific coupled stochastic equations. The
interactions are divided into a hierarchy of groups with adiabatically
separated and monotonically increasing time-scales, representing sub-routines
of the system programme of decreasing volatility. We solve this model in
equilibrium, assuming ergodicity at every level, and find as our
replica-symmetric solution a formalism with a structure similar but not
identical to Parisi's -step replica symmetry breaking scheme. Apart from
differences in details of the equations (due to the fact that here
interactions, rather than spins, are grouped into clusters with different
time-scales), in the present model the block sizes of the emerging
ultrametric solution are not restricted to the interval , but are
independent control parameters, defined in terms of the noise strengths of the
various levels in the hierarchy, which can take any value in [0,\infty\ket.
This is shown to lead to extremely rich phase diagrams, with an abundance of
first-order transitions especially when the level of stochasticity in the
interaction dynamics is chosen to be low.Comment: 53 pages, 19 figures. Submitted to J. Phys.
Video-Assisted Thoracoscopic Surgery in Patients With Clinically Resectable Lung Tumors
To investigate the feasibility of thoracoscopic resection, a pilot study was performed in patients with clinically resectable lung tumors. In 40 patients, Video-assisted thoracic surgery (VATS) was performed because of suspicion of malignancy. There were 29 men and 11 women with a median age of 54.8 years (range 18 to 78). Preoperative indications were suspected lung cancer and tumor in 27 patients, assessment of tumor resectability in 7 patients, and probability of metastatic tumors in 6 patients. The final diagnoses in the 27 patients with suspected lung cancer were 12 primary lung cancers, 6 lung metastases, and 9 benign lesions. The success rates for VATS (no conversion to thoracotomy) were 1 of 12 (8.3%) for resectable stage I lung cancer, 8 of 12 (66.7%) for metastatic tumors, and 9 of 9 (100%) for benign tumors. With VATS, 6 of 7 patients (85.7%), possible stage III non-small cell lung cancer, an explorative thoracotomy with was avoided, significantly reducing morbidity. The reasons for conversion to thoracotomy were 1) oncological (N2 lymph node dissection and prevention of tumor spillage) and 2) technical (inability to locate the nodule, central localization, no anatomical fissure, or poor lung function requiring full lung ventilation). The ultimate diagnoses were 19 lung cancers, 12 metastatic lung tumors, and 9 benign lung tumors. Our data show the limitations of VATS for malignant tumors in general use. These findings, together with the fact that experience in performing thoracoscopic procedures demonstrates a learning curve, may limit the use of thoracoscopic resection as a routine surgical procedure, especially when strict oncological rules are respected
Cryogenic setup for trapped ion quantum computing
We report on the design of a cryogenic setup for trapped ion quantum
computing containing a segmented surface electrode trap. The heat shield of our
cryostat is designed to attenuate alternating magnetic field noise, resulting
in 120~dB reduction of 50~Hz noise along the magnetic field axis. We combine
this efficient magnetic shielding with high optical access required for single
ion addressing as well as for efficient state detection by placing two lenses
each with numerical aperture 0.23 inside the inner heat shield. The cryostat
design incorporates vibration isolation to avoid decoherence of optical qubits
due to the motion of the cryostat. We measure vibrations of the cryostat of
less than 20~nm over 2~s. In addition to the cryogenic apparatus, we
describe the setup required for an operation with
Ca and Sr ions.
The instability of the laser manipulating the optical qubits in
Ca is characterized yielding a minimum of its
Allan deviation of 2.410 at 0.33~s. To evaluate the
performance of the apparatus, we trapped Ca
ions, obtaining a heating rate of 2.14(16)~phonons/s and a Gaussian decay of
the Ramsey contrast with a 1/e-time of 18.2(8)~ms
Survey propagation at finite temperature: application to a Sourlas code as a toy model
In this paper we investigate a finite temperature generalization of survey
propagation, by applying it to the problem of finite temperature decoding of a
biased finite connectivity Sourlas code for temperatures lower than the
Nishimori temperature. We observe that the result is a shift of the location of
the dynamical critical channel noise to larger values than the corresponding
dynamical transition for belief propagation, as suggested recently by
Migliorini and Saad for LDPC codes. We show how the finite temperature 1-RSB SP
gives accurate results in the regime where competing approaches fail to
converge or fail to recover the retrieval state
Bicoloring Random Hypergraphs
We study the problem of bicoloring random hypergraphs, both numerically and
analytically. We apply the zero-temperature cavity method to find analytical
results for the phase transitions (dynamic and static) in the 1RSB
approximation. These points appear to be in agreement with the results of the
numerical algorithm. In the second part, we implement and test the Survey
Propagation algorithm for specific bicoloring instances in the so called
HARD-SAT phase.Comment: 14 pages, 10 figure
Slowly evolving geometry in recurrent neural networks I: extreme dilution regime
We study extremely diluted spin models of neural networks in which the
connectivity evolves in time, although adiabatically slowly compared to the
neurons, according to stochastic equations which on average aim to reduce
frustration. The (fast) neurons and (slow) connectivity variables equilibrate
separately, but at different temperatures. Our model is exactly solvable in
equilibrium. We obtain phase diagrams upon making the condensed ansatz (i.e.
recall of one pattern). These show that, as the connectivity temperature is
lowered, the volume of the retrieval phase diverges and the fraction of
mis-aligned spins is reduced. Still one always retains a region in the
retrieval phase where recall states other than the one corresponding to the
`condensed' pattern are locally stable, so the associative memory character of
our model is preserved.Comment: 18 pages, 6 figure
REVISITING ANNA MOSCOWITZ\u27S KROSS\u27S CRITIQUE OF NEW YORK CITY\u27S WOMEN\u27S COURT: THE CONTINUED PROBLEM OF SOLVING THE PROBLEM OF PROSTITUTION WITH SPECIALIZED CRIMINAL COURTS
This article explores New York City\u27s non-traditional, judicially based response to prostitution. This article first recounts the history of New York City’s Women’s Court. It then examines the work of the Midtown Community Court, the “problem-solving court” established in 1993 to address criminal issues, like prostitution, in Midtown Manhattan. It also discusses the renewed concerns about sex work in New York and describe the movement, propelled by modern reformers, to address prostitution through specialty courts. It then contrasts the shared features and attributes of the Women’s Court and Midtown Court models. Finally, the article urges modern reformers to step back from the problem-solving court movement and their call for the creation of more such specialized criminal courts
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