Non-perturbative gadget for topological quantum codes

Many-body entangled systems, in particular topologically ordered spin systems proposed as resources for quantum information processing tasks, often involve highly non-local interaction terms. While one may approximate such systems through two-body interactions perturbatively, these approaches have a number of drawbacks in practice. Here, we propose a scheme to simulate many-body spin Hamiltonians with two-body Hamiltonians non-perturbatively. Unlike previous approaches, our Hamiltonians are not only exactly solvable with exact ground state degeneracy, but also support completely localized quasi-particle excitations, which are ideal for quantum information processing tasks. Our construction is limited to simulating the toric code and quantum double models, but generalizations to other non-local spin Hamiltonians may be possible.Comment: 13 pages, 8 figures, PRL Accepte

Super-poissonian noise, negative differential conductance, and relaxation effects in transport through molecules, quantum dots and nanotubes

We consider charge transport through a nanoscopic object, e.g. single molecules, short nanotubes, or quantum dots, that is weakly coupled to metallic electrodes. We account for several levels of the molecule/quantum dot with level-dependent coupling strengths, and allow for relaxation of the excited states. The current-voltage characteristics as well as the current noise are calculated within first-order perturbation expansion in the coupling strengths. For the case of asymmetric coupling to the leads we predict negative-differential-conductance accompanied with super-poissonian noise. Both effects are destroyed by fast relaxation processes. The non-monotonic behavior of the shot noise as a function of bias and relaxation rate reflects the details of the electronic structure and level-dependent coupling strengths.Comment: 8 pages, 7 figures, submitted to Phys. Rev. B, added reference

A search for water maser emission in nearby low-luminosity QSO host galaxies

A sample of nearby galaxies hosting low-luminosity type 1 quasi-stellar objects (QSOs), previously studied in CO and HI emission lines, has been searched for maser emission. Using the Effelsberg 100-m telescope, we observed 17 sources with redshifts of z-30{\deg} for emission in the 22 GHz water vapor maser transition. The sample objects have been drawn from a wide-angle survey for optically bright QSOs (Hamburg /ESO survey, HES). No host galaxies from the sample have been detected in the water maser emission line. In this paper we review the discussion on the reasons for water megamasers being rarely found in Seyfert 1 galaxies. Eight of them are bulge dominated and probably of elliptical type (E/S0), whereas 6 have spiral geometry. Three of the objects seem to be in a phase of merging/interaction. We found 3{\sigma} upper limits for the flux density of 27 to 60 mJy at spectral resolutions of ~0.43 km/s. We furthermore find that the viewing angle to the line of sight to the galaxy, under which the probability to detect megamaser emission is highest, is about 6{\deg}.Comment: accepted for publication in MNRAS, 9 pages, 6 figure

English article usage as a window on the meanings of same, identical and similar

We propose an explanation for a traditional puzzle in English linguistics involving the use of articles with the nominal modifiers same, identical and similar. Same can only take the definite article the, whereas identical and similar take either the or a. We argue that there is a fundamental difference in the manner in which a comparison is made with these modifiers. Identical and similar involve direct comparisons between at least two entities and an assertion of either full property matching (identical), or partial property matching (similar). The comparison with same proceeds differently: what is compared is not linguistic entities directly, but definite descriptions of these entities that can be derived through logical entailments. John and Mary live in the same house entails the house that John lives in is the (same) house that Mary lives in. There must be a pragmatic equivalence between these entailed definite descriptions, ranging from full referential equivalence to a possibly quite minimal overlap in semantic and real-world properties shared by distinct referents. These differences in meaning and article cooccurrence reveal the sensitivity of syntax to semantic and pragmatic properties, without which all and only the grammatical sentences of a language cannot be predicted

Influence of non-local exchange on RKKY interactions in III-V diluted magnetic semiconductors

The RKKY interaction between substitutional Mn local moments in GaAs is both spin-direction-dependent and spatially anisotropic. In this Letter we address the strength of these anisotropies using a semi-phenomenological tight-binding model which treats the hybridization between Mn d-orbitals and As p-orbitals perturbatively and accounts realistically for the non-local exchange interaction between their spins. We show that exchange non-locality, valence-band spin-orbit coupling, and band-structure anisotropy all play a role in determining the strength of both effects. We use these results to estimate the degree of ground-state magnetization suppression due to frustrating interactions between randomly located Mn ions.Comment: 4 pages RevTeX, 2 figures included, v2: replacement because of font proble

Magnetic domains in III-V magnetic semiconductors

Recent progress in theoretical understanding of magnetic anisotropy and stiffness in III-V magnetic semiconductors is exploited for predictions of magnetic domain characteristics and methods of their tuning. We evaluate the width and the energy of domain walls as well as the period of stripe domains in perpendicular films. The computed stripe width d = 1.1 um for Ga_0.957Mn_0.043As/In_0.16Ga_0.84As compares favorably to the experimental value 1.5 um, as determined by Shono et al. [Appl. Phys. Lett. 77, 1363 (2000)].Comment: 4 RevTex pages, 2 figures spelling of author's names corrected in abstract pag

Income and distance elasticities of values of travel time savings: New Swiss results

This paper presents the findings of a study looking into the valuation of travel time savings (VTTS) in Switzerland, across modes as well as across purpose groups. The study makes several departures from the usual practice in VTTS studies, with the main one being a direct representation of the income and distance elasticity of the VTTS measures. Here, important gains in model performance and significantly different results are obtained through this approach. Additionally, the analysis shows that the estimation of robust coefficients for congested car travel time is hampered by the low share of congested time in the overall travel time, and the use of an additional rate-of-congestion coefficient, in addition to a generic car travel time coefficient, is preferable. Finally, the analysis demonstrates that the population mean of the indicators calculated is quite different from the sample means and presents methods to calculate those, along with the associated variances. These variances are of great interest as they allow the generation of confidence intervals, which can be extremely useful in cost-benefit analyses