Exact relaxation in a class of non-equilibrium quantum lattice systems

A reasonable physical intuition in the study of interacting quantum systems says that, independent of the initial state, the system will tend to equilibrate. In this work we study a setting where relaxation to a steady state is exact, namely for the Bose-Hubbard model where the system is quenched from a Mott quantum phase to the strong superfluid regime. We find that the evolving state locally relaxes to a steady state with maximum entropy constrained by second moments, maximizing the entanglement, to a state which is different from the thermal state of the new Hamiltonian. Remarkably, in the infinite system limit this relaxation is true for all large times, and no time average is necessary. For large but finite system size we give a time interval for which the system locally "looks relaxed" up to a prescribed error. Our argument includes a central limit theorem for harmonic systems and exploits the finite speed of sound. Additionally, we show that for all periodic initial configurations, reminiscent of charge density waves, the system relaxes locally. We sketch experimentally accessible signatures in optical lattices as well as implications for the foundations of quantum statistical mechanics.Comment: 8 pages, 3 figures, replaced with final versio

Black hole hunting in the Andromeda Galaxy

We present a new technique for identifying stellar mass black holes in low mass X-ray binaries (LMXBs), and apply it to XMM-Newton observations of M31. We examine X-ray time series variability seeking power density spectra (PDS) typical of LMXBs accreting at a low accretion rate (which we refer to as Type A PDS); these are very similar for black hole and neutron star LMXBs. Galactic neutron star LMXBs exhibit Type A PDS at low luminosities (~10^36--10^37 erg/s) while black hole LMXBs can exhibit them at luminosities >10^38 erg/s. We propose that Type A PDS are confined to luminosities below a critical fraction of the Eddington limit, $l_c$ that is constant for all LMXBs; we have examined asample of black hole and neutron star LMXBs and find they are all consistent with $l_c$ = 0.10+/-0.04 in the 0.3--10 keV band. We present luminosity and PDS data from 167 observations of X-ray binaries in M31 that provide strong support for our hypothesis. Since the theoretical maximum mass for a neutron star is \~3.1 M_Sun, we therefore assert that any LMXB that exhibits a Type A PDS at a 0.3--10 keV luminosity greater than 4 x 10^37 erg/s is likely to contain a black hole primary. We have found eleven new black hole candidates in M31 using this method. We focus on XMM-Newton observations of RX J0042.4+4112, an X-ray source in M31 and find the mass of the primary to be 7+/-2 M_Sun, if our assumptions are correct. Furthermore, RX J0042.4+4112 is consistently bright in \~40 observations made over 23 years, and is likely to be a persistently bright LMXB; by contrast all known Galactic black hole LMXBs are transient. Hence our method may be used to find black holes in known, persistently bright Galactic LMXBs and also in LMXBs in other galaxies.Comment: 6 Pages, 6 figures. To appear in the conference proceedings of "Interacting Binaries: Accretion, Evolution and Outcomes" (Cefalu, July 4-10 2004

Time and dose-dependent effects of phenobarbital on the rat liver miRNAome.

In a previous study we had shown that treatment of male Fischer rats with exogenous chemicals for three months resulted in prominent, mode-of-action dependent effects on liver microRNA (miRNA) (Koufaris et al., 2012). Here we investigated how the effects of chemicals on liver miRNA in male Fischer rats relate to the length and dose of exposure to phenobarbital (PB), a drug with multiple established hepatic effects. Importantly, although acute PB treatment (1-7 days) had significant effects on liver mRNA and the expected effects on the liver phenotype (transient hyperplasia, hepatomegaly, cytochrome P450 induction), limited effects on liver miRNA were observed. However, at 14 days of PB treatment clear dose-dependent effects on miRNA were observed. The main effect of PB treatment from days 1 to 90 on liver miRNA was found to be the persistent, progressive, and highly correlated induction of the miR-200a/200b/429 and miR-96/182 clusters, occurring after the termination of the xenobiotic-induced transient hyperplasia. Moreover, in agreement with their reported functions in the literature we found associations between perturbations of miR-29b and miR-200a/200b by PB with global DNA methylation and zeb1/zeb2 proteins respectively. Our data suggest that miRNA are unlikely to play an important role in the acute responses of the adult rodent liver to PB treatment. However, the miRNA responses to longer PB exposures suggest a potential role for maintaining liver homeostasis in response to sub-chronic and chronic xenobiotic-induced perturbations. Similar studies for more chemicals are needed to clarify whether the temporal and dose pattern of miRNA-toxicant interaction identified here for PB are widely applicable to other xenobiotics. © 2013 Elsevier Ireland Ltd

Exploring Interacting Quantum Many-Body Systems by Experimentally Creating Continuous Matrix Product States in Superconducting Circuits

Improving the understanding of strongly correlated quantum many body systems such as gases of interacting atoms or electrons is one of the most important challenges in modern condensed matter physics, materials research and chemistry. Enormous progress has been made in the past decades in developing both classical and quantum approaches to calculate, simulate and experimentally probe the properties of such systems. In this work we use a combination of classical and quantum methods to experimentally explore the properties of an interacting quantum gas by creating experimental realizations of continuous matrix product states - a class of states which has proven extremely powerful as a variational ansatz for numerical simulations. By systematically preparing and probing these states using a circuit quantum electrodynamics (cQED) system we experimentally determine a good approximation to the ground-state wave function of the Lieb-Liniger Hamiltonian, which describes an interacting Bose gas in one dimension. Since the simulated Hamiltonian is encoded in the measurement observable rather than the controlled quantum system, this approach has the potential to apply to exotic models involving multicomponent interacting fields. Our findings also hint at the possibility of experimentally exploring general properties of matrix product states and entanglement theory. The scheme presented here is applicable to a broad range of systems exploiting strong and tunable light-matter interactions.Comment: 11 pages, 9 figure

Cumulative luminosity functions of the X-ray point source population in M31

We present preliminary results from a detailed analysis of the X-ray point sources in the XMM-Newton survey of M31. These sources are expected to be mostly X-ray binaries. We have so far studied 225 of the 535 sources found by automated source detection. Only sources which were present in all three EPIC images were considered. X-ray binaries are identified by their energy spectrum and power density spectrum. Unlike in other surveys we have obtained source luminosities from freely fit emission models. We present uncorrected luminosity functions of the sources analysed so far.Comment: 2 pages, 1 figure. To appear in proceedings of IAUS23

Sparse and stable Markowitz portfolios

We consider the problem of portfolio selection within the classical Markowitz mean-variance framework, reformulated as a constrained least-squares regression problem. We propose to add to the objective function a penalty proportional to the sum of the absolute values of the portfolio weights. This penalty regularizes (stabilizes) the optimization problem, encourages sparse portfolios (i.e. portfolios with only few active positions), and allows to account for transaction costs. Our approach recovers as special cases the no-short-positions portfolios, but does allow for short positions in limited number. We implement this methodology on two benchmark data sets constructed by Fama and French. Using only a modest amount of training data, we construct portfolios whose out-of-sample performance, as measured by Sharpe ratio, is consistently and significantly better than that of the naive evenly-weighted portfolio which constitutes, as shown in recent literature, a very tough benchmark.Comment: Better emphasis of main result, new abstract, new examples and figures. New appendix with full details of algorithm. 17 pages, 6 figure

Exploring scholarly data with Rexplore.

Despite the large number and variety of tools and services available today for exploring scholarly data, current support is still very limited in the context of sensemaking tasks, which go beyond standard search and ranking of authors and publications, and focus instead on i) understanding the dynamics of research areas, ii) relating authors ‘semantically’ (e.g., in terms of common interests or shared academic trajectories), or iii) performing fine-grained academic expert search along multiple dimensions. To address this gap we have developed a novel tool, Rexplore, which integrates statistical analysis, semantic technologies, and visual analytics to provide effective support for exploring and making sense of scholarly data. Here, we describe the main innovative elements of the tool and we present the results from a task-centric empirical evaluation, which shows that Rexplore is highly effective at providing support for the aforementioned sensemaking tasks. In addition, these results are robust both with respect to the background of the users (i.e., expert analysts vs. ‘ordinary’ users) and also with respect to whether the tasks are selected by the evaluators or proposed by the users themselves

Discovery of disc precession in the M31 dipping X-ray binary Bo 158

We present results from three XMM-Newton observations of the M31 low mass X-ray binary XMMU J004314.4+410726.3 (Bo 158), spaced over 3 days in 2004, July. Bo 158 was the first dipping LMXB to be discovered in M31. Periodic intensity dips were previously seen to occur on a 2.78-hr period, due to absorption in material that is raised out of the plane of the accretion disc. The report of these observations stated that the dip depth was anti-correlated with source intensity. However, our new observations do not favour a strict intensity dependance, but rather suggest that the dip variation is due to precession of the accretion disc. This is to be expected in LMXBs with a mass ratio <~ 0.3 (period <~ 4 hr), as the disc reaches the 3:1 resonance with the binary companion, causing elongation and precession of the disc. A smoothed particle hydrodynamics simulation of the disc in this system shows retrograde rotation of a disc warp on a period of ~11 P_orb, and prograde disc precession on a period of ~29 P_orb. This is consistent with the observed variation in the depth of the dips. We find that the dipping behaviour is most likely to be modified by the disc precession, hence we predict that the dipping behaviour repeats on a 81+/-3 hr cycle.Comment: 9 pages, 6 figures, accepted for publication by MNRAS, changed conten

XMM-Newton reveals ~100 new LMXBs in M31 from variability studies

We have conducted a survey of X-ray sources in XMM-Newton observations of M31, examining their power density spectra (PDS) and spectral energy distributions (SEDs). Our automated source detection yielded 535 good X-ray sources; to date, we have studied 225 of them. In particular, we examined the PDS because low mass X-ray binaries (LMXBs) exhibit two distinctive types of PDS. At low accretion rates, the PDS is characterised by a broken power law, with the spectral index changing from ~0 to ~1 at some frequency in the range \~0.01--1 Hz; we refer to such PDS as Type A. At higher accretion rates, the PDS is described by a simple power law; we call these PDS Type B. Of the 225 sources studied to date, 75 exhibit Type A variability, and are almost certainly LMXBs, while 6 show Type B but not Type A, and are likely LMXBs. Of these 81 candidate LMXBs, 71 are newly identified in this survey; furthermore, they are mostly found near the centre of M31. Furthermore, most of the X-ray population in the disc are associated with the spiral arms, making them likely high mass X-ray binaries (HMXBs). In general these HMXBs do not exhibit Type A variability, while many central X-ray sources (LMXBs) in the same luminosity range do. Hence the PDS may distinguish between LMXBs and HMXBs in this luminosity range.Comment: 4 pages, 2 figures. To appear in proceedings of IAUS230: "Populations of High Energy Sources in Galaxies", 14-19 August 2005, Dublin, Eds E.J.A. Meurs and G. Fabbian

Space tug propulsion system failure mode, effects and criticality analysis

For purposes of the study, the propulsion system was considered as consisting of the following: (1) main engine system, (2) auxiliary propulsion system, (3) pneumatic system, (4) hydrogen feed, fill, drain and vent system, (5) oxygen feed, fill, drain and vent system, and (6) helium reentry purge system. Each component was critically examined to identify possible failure modes and the subsequent effect on mission success. Each space tug mission consists of three phases: launch to separation from shuttle, separation to redocking, and redocking to landing. The analysis considered the results of failure of a component during each phase of the mission. After the failure modes of each component were tabulated, those components whose failure would result in possible or certain loss of mission or inability to return the Tug to ground were identified as critical components and a criticality number determined for each. The criticality number of a component denotes the number of mission failures in one million missions due to the loss of that component. A total of 68 components were identified as critical with criticality numbers ranging from 1 to 2990