Quantum Stirring in low dimensional devices

A circulating current can be induced in the Fermi sea by displacing a scatterer, or more generally by integrating a quantum pump into a closed circuit. The induced current may have either the same or the opposite sense with respect to the "pushing" direction of the pump. We work out explicit expressions for the associated geometric conductance using the Kubo-Dirac monopoles picture, and illuminate the connection with the theory of adiabatic passage in multiple path geometry.Comment: 6 pages, 5 figures, improved versio

Towards realistic implementations of a Majorana surface code

Surface codes have emerged as promising candidates for quantum information processing. Building on the previous idea to realize the physical qubits of such systems in terms of Majorana bound states supported by topological semiconductor nanowires, we show that the basic code operations, namely projective stabilizer measurements and qubit manipulations, can be implemented by conventional tunnel conductance probes and charge pumping via single-electron transistors, respectively. The simplicity of the access scheme suggests that a functional code might be in close experimental reach.Comment: 5 pages, 1 p. suppl.mat, PRL in pres

The Averaged Periodogram Estimator for a Power Law in Coherency

We prove the consistency of the averaged periodogram estimator (APE) in two new cases. First, we prove that the APE is consistent for negative memory parameters, after suitable tapering. Second, we prove that the APE is consistent for a power law in the cross-spectrum and therefore for a power law in the coherency, provided that sufficiently many frequencies are used in estimation. Simulation evidence suggests that the lower bound on the number of frequencies is a necessary condition for consistency. For a Taylor series approximation to the estimator of the power law in the cross-spectrum, we consider the rate of convergence, and obtain a central limit theorem under suitable regularity conditions.J.P. Morgan Chase and Co. and New York UniversityStatistics Working Papers Serie

Computationally Efficient Gaussian Maximum Likelihood Methods for Vector ARFIMA Models

In this paper, we discuss two distinct multivariate time series models that extend the univariate ARFIMA model. We describe algorithms for computing the covariances of each model, for computing the quadratic form and approximating the determinant for maximum likelihood estimation, and for simulating from each model. We compare the speed and accuracy of each algorithm to existing methods and measure the performance of the maximum likelihood estimator compared to existing methods. We also fit models to data on unemployment and inflation in the United States, to data on goods and services inflation in the United States, and to data about precipitation in the Great Lakes.Statistics Working Papers Serie

Local Magnetization in the Boundary Ising Chain at Finite Temperature

We study the local magnetization in the 2-D Ising model at its critical temperature on a semi-infinite cylinder geometry, and with a nonzero magnetic field $h$ applied at the circular boundary of circumference $\beta$. This model is equivalent to the semi-infinite quantum critical 1-D transverse field Ising model at temperature $T \propto \beta^{-1}$, with a symmetry-breaking field $\propto h$ applied at the point boundary. Using conformal field theory methods we obtain the full scaling function for the local magnetization analytically in the continuum limit, thereby refining the previous results of Leclair, Lesage and Saleur in Ref. \onlinecite{Leclair}. The validity of our result as the continuum limit of the 1-D lattice model is confirmed numerically, exploiting a modified Jordan-Wigner representation. Applications of the result are discussed.Comment: 9 pages, 3 figure

CONTRIBUTION OF DIFFERENT CELL TYPES TO THE GENETIC CONTROL OF IMMUNE RESPONSES AS A FUNCTION OF THE CHEMICAL NATURE OF THE POLYMERIC SIDE CHAINS (POLY-L-PROLYL AND POLY-DL-ALANYL) OF SYNTHETIC IMMUNOGENS

Genetic regulation of immunological responsiveness was studied at the cellular level by comparing the limiting dilutions of immunocompetent cells from spleen, thymus, and bone marrow of high and low responders as a function of the poly-L-prolyl and poly-DL-alanyl side chains of two synthetic polypeptide immunogens. The spleens of immunized and unimmunized high responder DBA/1 mice were found to contain respectively, 18- and 7-fold more limiting precursor cells specific for (Phe, G)-A--L than the spleens of SJL low responder donors. These results, using a synthetic polypeptide built on multichain poly-DL-alanine, confirm the findings reported for polypeptides built on multichain poly-L-proline (1, 2), that there is a direct correlation between immune response potential and the relative number of immunocompetent precursors stimulated. Cell cooperation between thymocytes and bone marrow cells was demonstrated for both (T, G)-Pro--L and (Phe, G)-A--L. Limiting dilutions of thymus and bone marrow cells in the presence of an excess amount of the complementary cell type indicated an eightfold lower number of detected (T, G)-Pro--L-specific precursors in DBA/1 (low responder) marrow when compared with SJL (high responder) marrow. No differences were observed in the frequency of relevant high and low responder thymocytes for the (T, G)-Pro--L immunogen. These results are similar to those reported for the (Phe, G)-Pro--L (3). In contrast to the cellular studies reported for the Pro--L series of immunogens, the marrow and thymus cell dilution experiments for (Phe, G)-A--L revealed genetically associated differences in both the marrow and thymus populations of immunocytes from high (DBA/1) and low (SJL) responders. In addition to a fivefold difference in limiting marrow cell precursors (similar to that seen in the Pro--L studies), a striking difference was observed between the helper cell activity of high responder DBA/1 and low responder SJL thymocytes. This difference was indicated by the observation that low responder thymocyte dilutions followed the predictions of the Poisson model, whereas dilutions of high responder thymocytes did not conform to Poisson statistics. Transfers of allogeneic thymus and marrow cell mixtures from DBA/1 and SJL donors confirmed the syngeneic dilution studies showing that the genetic defect of immune responsiveness to (Phe, G)-A--L is expressed at both the thymus and marrow immunocompetent cell level. The parameters presently known for genetic control of immune responses specific for (Phe, G) (Ir-1 gene) and for Pro--L (Ir-3 gene) have been compared. The Ir-1 and Ir-3 genes are not only distinct by genetic linkage tests (to H-2) (5, 6, 9), but they are also seen to be different by cellular studies. Furthermore, expression of low responsiveness within a given cell population was shown to depend on the chemical structure of the whole immunogenic macromolecule