6,402 research outputs found
Transition from antibunching to bunching in cavity QED
The photon statistics of the light emitted from an atomic ensemble into a
single field mode of an optical cavity is investigated as a function of the
number of atoms. The light is produced in a Raman transition driven by a pump
laser and the cavity vacuum [M.Hennrich et al., Phys. Rev. Lett. 85, 4672
(2000)], and a recycling laser is employed to repeat this process continuously.
For weak driving, a smooth transition from antibunching to bunching is found
for about one intra-cavity atom. Remarkably, the bunching peak develops within
the antibunching dip. For saturated driving and a growing number of atoms, the
bunching amplitude decreases and the bunching duration increases, indicating
the onset of Raman lasing.Comment: 4 pages, 4 figure
An Algorithm for Cellular Reprogramming
The day we understand the time evolution of subcellular elements at a level
of detail comparable to physical systems governed by Newton's laws of motion
seems far away. Even so, quantitative approaches to cellular dynamics add to
our understanding of cell biology, providing data-guided frameworks that allow
us to develop better predictions about and methods for control over specific
biological processes and system-wide cell behavior. In this paper we describe
an approach to optimizing the use of transcription factors in the context of
cellular reprogramming. We construct an approximate model for the natural
evolution of a synchronized population of fibroblasts, based on data obtained
by sampling the expression of some 22,083 genes at several times along the cell
cycle. (These data are based on a colony of cells that have been cell cycle
synchronized) In order to arrive at a model of moderate complexity, we cluster
gene expression based on the division of the genome into topologically
associating domains (TADs) and then model the dynamics of the expression levels
of the TADs. Based on this dynamical model and known bioinformatics, we develop
a methodology for identifying the transcription factors that are the most
likely to be effective toward a specific cellular reprogramming task. The
approach used is based on a device commonly used in optimal control. From this
data-guided methodology, we identify a number of validated transcription
factors used in reprogramming and/or natural differentiation. Our findings
highlight the immense potential of dynamical models models, mathematics, and
data guided methodologies for improving methods for control over biological
processes
DIVERGENT ROLES OF THE TCF4 AND LEF1 WNT SIGNALING TRANSCRIPTION FACTORS IN COLON CANCER
The canonical WNT signaling pathway is necessary for guiding cell growth during embryonic development. In adults, WNT signaling maintains tissue stem cells and therefore plays an essential role in tissue homeostasis. In the colon, the WNT transcription factor, TCF4, is necessary for maintaining the intestinal stem cells. The initiating event in colon cancer is the aberrant activation of the WNT signaling pathway, which results in constitutive activity of TCF4. To determine how TCF4 influences colon cancer cell behavior, we silenced TCF7L2, the gene encoding TCF4, and used RNA sequencing and Hi-C to measure changes in transcription and nuclear structure in the SW480 colon cancer cell line. Loss of TCF4 resulted in A/B compartment switching, local chromatin reorganization, and a dramatic up-regulation in transcription. However, A/B compartment switching was not associated with changes in gene expression. We also found that loss of TCF4 resulted in the up-regulation of LEF1, another WNT transcription factor. Expressed LEF1 isoforms were found to be transcriptionally competent and over-compensated for WNT signaling activity upon loss of TCF4, suggesting a WNT-intrinsic feedback mechanism. Over-expression of LEF1 altered WNT signaling output to favor the expression of lymphoid genes, as opposed to a TCF4-based transcriptional program. ChIP-seq demonstrated that TCF4 and LEF1 bind distinct target genes, though they synergize to express MYC. TCF4 was found to bind the LEF1 promoter, indicative of direct repression, though LEF1 did not bind the TCF7L2 promoter. The CtBP1 protein, a known binding partner of TCF4, was found to be the most potent repressor of LEF1 expression. This demonstrates that despite the overall activation of WNT signaling in colon cancer, repressive functions of the WNT transcription factors are still intact, and the repression of LEF1 by TCF4 maintains a TCF4-centric transcriptional program in colon cancer cells
Radiative Corrections to and Decays
We reexamine radiative corrections to and decays. We
perform a matching calculation, including vector and axial vector resonances as
explicit degrees of freedom in the long distance part. By considering the
dependence on the matching scale and on the hadronic parameters, and by
comparing with model independent estimates, we scrutinize the model dependence
of the results. For the pseudoscalar meson decay constants, we extract the
values f_pi = (92.1 \pm 0.3) MeV and f_K = (112.4 \pm 0.9) MeV. For the ratios
R_pi and R_K of the electronic and muonic decay modes, we predict R_pi =
(1.2354 \pm 0.0002) 10^{-4} and R_K = (2.472 \pm 0.001) 10^{-5}.Comment: 8 pages, Latex, uses epsfig. Revised version: Major changes in the
presentaion, but no changes in the results. To appear in Phys. Lett.
Predicting the energy output of wind farms based on weather data: important variables and their correlation
Pre-print available at: http://arxiv.org/abs/1109.1922Wind energy plays an increasing role in the supply of energy world wide. The energy output of a wind farm is highly dependent on the weather conditions present at its site. If the output can be predicted more accurately, energy suppliers can coordinate the collaborative production of different energy sources more efficiently to avoid costly overproduction. In this paper, we take a computer science perspective on energy prediction based on weather data and analyze the important parameters as well as their correlation on the energy output. To deal with the interaction of the different parameters, we use symbolic regression based on the genetic programming tool DataModeler. Our studies are carried out on publicly available weather and energy data for a wind farm in Australia. We report on the correlation of the different variables for the energy output. The model obtained for energy prediction gives a very reliable prediction of the energy output for newly supplied weather data. © 2012 Elsevier Ltd.Ekaterina Vladislavleva, Tobias Friedrich, Frank Neumann, Markus Wagne
The boundaries and twist defects of the color code and their applications to topological quantum computation
The color code is both an interesting example of an exactly solved topologically ordered phase of matter and also among the most promising candidate models to realize fault-tolerant quantum computation with minimal resource overhead. The contributions of this work are threefold. First of all, we build upon the abstract theory of boundaries and domain walls of topological phases of matter to comprehensively catalog the objects realizable in color codes. Together with our classification we also provide lattice representations of these objects which include three new types of boundaries as well as a generating set for all 72 color code twist defects. Our work thus provides an explicit toy model that will help to better understand the abstract theory of domain walls. Secondly, we discover a number of interesting new applications of the cataloged objects for quantum information protocols. These include improved methods for performing quantum computations by code deformation, a new four-qubit error-detecting code, as well as families of new quantum error-correcting codes we call stellated color codes, which encode logical qubits at the same distance as the next best color code, but using approximately half the number of physical qubits. To the best of our knowledge, our new topological codes have the highest encoding rate of local stabilizer codes with bounded-weight stabilizers in two dimensions. Finally, we show how the boundaries and twist defects of the color code are represented by multiple copies of other phases. Indeed, in addition to the well studied comparison between the color code and two copies of the surface code, we also compare the color code to two copies of the three-fermion model. In particular, we find that this analogy offers a very clear lens through which we can view the symmetries of the color code
which gives rise to its multitude of domain walls
Poking Holes and Cutting Corners to Achieve Clifford Gates with the Surface Code
The surface code is currently the leading proposal to achieve fault-tolerant quantum computation. Among its strengths are the plethora of known ways in which fault-tolerant Clifford operations can be performed, namely, by deforming the topology of the surface, by the fusion and splitting of codes, and even by braiding engineered Majorana modes using twist defects. Here, we present a unified framework to describe these methods, which can be used to better compare different schemes and to facilitate the design of hybrid schemes. Our unification includes the identification of twist defects with the corners of the planar code. This identification enables us to perform single-qubit Clifford gates by exchanging the corners of the planar code via code deformation. We analyze ways in which different schemes can be combined and propose a new logical encoding. We also show how all of the Clifford gates can be implemented with the planar code, without loss of distance, using code deformations, thus offering an attractive alternative to ancilla-mediated schemes to complete the Clifford group with lattice surgery
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