Boson Pairs in a One-dimensional Split Trap

We describe the properties of a pair of ultracold bosonic atoms in a one-dimensional harmonic trapping potential with a tunable zero-ranged barrier at the trap centre. The full characterisation of the ground state is done by calculating the reduced single-particle density, the momentum distribution and the two-particle entanglement. We derive several analytical expressions in the limit of infinite repulsion (Tonks-Girardeau limit) and extend the treatment to finite interparticle interactions by numerical solution. As pair interactions in double wells form a fundamental building block for many-body systems in periodic potentials, our results have implications for a wide range of problems.Comment: 9 pages, 8 figure

Features of the Acoustic Mechanism of Core-Collapse Supernova Explosions

In the context of 2D, axisymmetric, multi-group, radiation/hydrodynamic simulations of core-collapse supernovae over the full 180$^{\circ}$ domain, we present an exploration of the progenitor dependence of the acoustic mechanism of explosion. All progenitor models we have tested with our Newtonian code explode. We investigate the roles of the Standing-Accretion-Shock-Instability (SASI), the excitation of core g-modes, the generation of core acoustic power, the ejection of matter with r-process potential, the wind-like character of the explosion, and the fundamental anisotropy of the blasts. We find that the breaking of spherical symmetry is central to the supernova phenomenon and the blasts, when top-bottom asymmetric, are self-collimating. We see indications that the initial explosion energies are larger for the more massive progenitors, and smaller for the less massive progenitors, and that the neutrino contribution to the explosion energy may be an increasing function of progenitor mass. The degree of explosion asymmetry we obtain is completely consistent with that inferred from the polarization measurements of Type Ic supernovae. Furthermore, we calculate for the first time the magnitude and sign of the net impulse on the core due to anisotropic neutrino emission and suggest that hydrodynamic and neutrino recoils in the context of our asymmetric explosions afford a natural mechanism for observed pulsar proper motions. [abridged]Comment: Accepted to the Astrophysical Journal, 23 pages in emulateapj format, including 12 figure

Fragility of a class of highly entangled states of many quantum-bits

We consider a Quantum Computer with n quantum-bits (`qubits'), where each qubit is coupled independently to an environment affecting the state in a dephasing or depolarizing way. For mixed states we suggest a quantification for the property of showing {\it quantum} uncertainty on the macroscopic level. We illustrate in which sense a large parameter can be seen as an indicator for large entanglement and give hypersurfaces enclosing the set of separable states. Using methods of the classical theory of maximum likelihood estimation we prove that this parameter is decreasing with 1/\sqrt{n} for all those states which have been exposed to the environment. Furthermore we consider a Quantum Computer with perfect 1-qubit gates and 2-qubit gates with depolarizing error and show that any state which can be obtained from a separable initial state lies inbetween a family of pairs of certain hypersurfaces parallel to those enclosing the separable ones.Comment: 9 Pages, RevTe

Photofission of heavy nuclei at energies up to 4 GeV

Total photofission cross sections for 238U, 235U, 233U, 237Np, 232Th, and natPb have been measured simultaneously, using tagged photons in the energy range Egamma=0.17-3.84 GeV. This was the first experiment performed using the Photon Tagging Facility in Hall B at Jefferson Lab. Our results show that the photofission cross section for 238U relative to that for 237Np is about 80%, implying the presence of important processes that compete with fission. We also observe that the relative photofission cross sections do not depend strongly on the incident photon energy over this entire energy range. If we assume that for 237Np the photofission probability is equal to unity, we observe a significant shadowing effect starting below 1.5 GeV.Comment: 4 pages of RevTex, 6 postscript figures, Submitted to Phys. Rev. Let

Multidimensional supernova simulations with approximative neutrino transport. II. Convection and the advective-acoustic cycle in the supernova core

By 2D hydrodynamic simulations including a detailed equation of state and neutrino transport, we investigate the interplay between different non-radial hydrodynamic instabilities that play a role during the postbounce accretion phase of collapsing stellar cores. The convective mode of instability, which is driven by negative entropy gradients caused by neutrino heating or by time variations of the shock strength, can be identified clearly by the development of typical Rayleigh-Taylor mushrooms. However, in cases where the gas in the postshock region is rapidly advected towards the gain radius, the growth of such a buoyancy instability can be suppressed. In such a situation the shocked flow nevertheless can develop non-radial asymmetry with an oscillatory growth of the amplitude. This phenomenon has been termed ``standing accretion shock instability'' (SASI). It is shown here that the SASI oscillations can trigger convective instability and like the latter they lead to an increase of the average shock radius and of the mass in the gain layer. Both hydrodynamic instabilities in combination stretch the advection time of matter through the neutrino-heating layer and thus enhance the neutrino energy deposition in support of the neutrino-driven explosion mechanism. A rapidly contracting and more compact nascent NS turns out to be favorable for explosions, because the accretion luminosity and neutrino heating are larger and the growth rate of the SASI is higher. Moreover, we show that the oscillation period of the SASI and a variety of other features in our simulations agree with estimates for the advective-acoustic cycle (AAC), in which perturbations are carried by the accretion flow from the shock to the neutron star and pressure waves close an amplifying global feedback loop. (abridged)Comment: 23 pages, 20 figures; revised version with extended Sect.5, accepted by Astronomy & Astrophysics; high-resolution images can be obtained upon reques

High-precision wavelength calibration of astronomical spectrographs with laser frequency combs

We describe a possible new technique for precise wavelength calibration of high-resolution astronomical spectrographs using femtosecond-pulsed mode-locked lasers controlled by stable oscillators such as atomic clocks. Such `frequency combs' provide a series of narrow modes which are uniformly spaced according to the laser's pulse repetition rate and whose absolute frequencies are known a priori with relative precision better than 10^{-12}. Simulations of frequency comb spectra show that the photon-limited wavelength calibration precision achievable with existing echelle spectrographs should be ~1 cm/s when integrated over a 4000A range. Moreover, comb spectra may be used to accurately characterise distortions of the wavelength scale introduced by the spectrograph and detector system. The simulations show that frequency combs with pulse repetition rates of 5-30GHz are required, given the typical resolving power of existing and possible future echelle spectrographs. Achieving such high repetition rates, together with the desire to produce all comb modes with uniform intensity over the entire optical range, represent the only significant challenges in the design of a practical system. Frequency comb systems may remove wavelength calibration uncertainties from all practical spectroscopic experiments, even those combining data from different telescopes over many decades.Comment: 10 pages, 6 figures, 1 table. Accepted by MNRAS. v2: Fig. 3 augmented and minor changes to text (including extended title

Phenotypic covariance of longevity, immunity and stress resistance in the Caenorhabditis nematodes

Background \ud Ageing, immunity and stresstolerance are inherent characteristics of all organisms. In animals, these traits are regulated, at least in part, by forkhead transcription factors in response to upstream signals from the Insulin/Insulin– like growth factor signalling (IIS) pathway. In the nematode Caenorhabditis elegans, these phenotypes are molecularly linked such that activation of the forkhead transcription factor DAF-16 both extends lifespan and simultaneously increases immunity and stress resistance. It is known that lifespan varies significantly among the Caenorhabditis species but, although DAF-16 signalling is highly conserved, it is unclear whether this phenotypic linkage occurs in other species. Here we investigate this phenotypic covariance by comparing longevity, stress resistance and immunity in four \ud Caenorhabditis species. \ud \ud Methodology/Principal Findings \ud We show using phenotypic analysis of DAF-16 influenced phenotypes that among four closely related Caenorhabditis nematodes, the gonochoristic species (Caenorhabditis remanei and Caenorhabditis brenneri) have diverged \ud significantly with a longer lifespan, improved stress resistance and higher immunity than the hermaphroditic species (C. elegans and Caenorhabditis briggsae). Interestingly, we also observe significant differences in expression levels between the daf-16 homologues in these species using Real-Time PCR, which positively correlate with the observed phenotypes. Finally, we provide additional evidence in support of a role for DAF-16 in regulating phenotypic coupling by using a combination of wildtype isolates, constitutively active daf-16 mutants and bioinformatic analysis. \ud \ud Conclusions \ud The gonochoristic species display a significantly longer lifespan (p < 0.0001)and more robust immune and stress response (p<0.0001, thermal stress; p<0.01, heavy metal stress; p<0.0001, pathogenic stress) than the hermaphroditic species. Our data suggests that divergence in DAF-16 mediated phenotypes may underlie many of the differences observed between these four species of Caenorhabditis nematodes. These findings are further supported by the correlative higher daf-16 expression levels among the gonochoristic species and significantly higher lifespan, immunity and stress tolerance in the constitutively active daf-16 hermaphroditic mutants