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Search Results: 1 - 10 of 192403 matches for " Spencer D. Stirling "
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Counterexamples in Levin-Wen string-net models, group categories, and Turaev unimodality
Spencer D. Stirling
Mathematics , 2010,
Abstract: We remark on the claim that the string-net model of Levin and Wen is a microscopic Hamiltonian formulation of the Turaev-Viro topological quantum field theory. Using simple counterexamples we indicate where interesting extra structure may be needed in the Levin-Wen model for this to hold (however we believe that some form of the correspondence is true). In order to be accessible to the condensed matter community we provide a brief and gentle introduction to the relevant concepts in category theory (relying heavily on analogy with ordinary group representation theory). Likewise, some physical ideas are briefly surveyed for the benefit of the more mathematical reader. The main feature of group categories under consideration is Turaev's unimodality. We pinpoint where unimodality should fit into the Levin-Wen construction, and show that the simplest example fails to be unimodal. Unimodality is straightforward to compute for group categories, and we provide a complete classification at the end of the paper.
Abelian Chern-Simons theory with toral gauge group, modular tensor categories, and group categories
Spencer D. Stirling
Mathematics , 2008,
Abstract: Classical and quantum Chern-Simons with gauge group $\text{U}(1)^N$ were classified by Belov and Moore in \cite{belov_moore}. They studied both ordinary topological quantum field theories as well as spin theories. On the other hand a correspondence is well known between ordinary $(2+1)$-dimensional TQFTs and modular tensor categories. We study group categories and extend them slightly to produce modular tensor categories that correspond to toral Chern-Simons. Group categories have been widely studied in other contexts in the literature \cite{frolich_kerler},\cite{quinn},\cite{joyal_street},\cite{eno},\cite{dgno}. The main result is a proof that the associated projective representation of the mapping class group is isomorphic to the one from toral Chern-Simons. We also remark on an algebraic theorem of Nikulin that is used in this paper.
Big Bang Nucleosynthesis with Gaussian Inhomogeneous Neutrino Degeneracy
Spencer D. Stirling,Robert J. Scherrer
Physics , 2002, DOI: 10.1103/PhysRevD.66.043531
Abstract: We consider the effect of inhomogeneous neutrino degeneracy on Big Bang nucleosynthesis for the case where the distribution of neutrino chemical potentials is given by a Gaussian. The chemical potential fluctuations are taken to be isocurvature, so that only inhomogeneities in the electron chemical potential are relevant. Then the final element abundances are a function only of the baryon-photon ratio $\eta$, the effective number of additional neutrinos $\Delta N_\nu$, the mean electron neutrino degeneracy parameter $\bar \xi$, and the rms fluctuation of the degeneracy parameter, $\sigma_\xi$. We find that for fixed $\eta$, $\Delta N_\nu$, and $\bar \xi$, the abundances of helium-4, deuterium, and lithium-7 are, in general, increasing functions of $\sigma_\xi$. Hence, the effect of adding a Gaussian distribution for the electron neutrino degeneracy parameter is to decrease the allowed range for $\eta$. We show that this result can be generalized to a wide variety of distributions for $\xi$.
Braided Categorical Quantum Mechanics I
Spencer D. Stirling,Yong-Shi Wu
Mathematics , 2009,
Abstract: This is the first paper in a series where we generalize the Categorical Quantum Mechanics program (due to Abramsky, Coecke, et al) to braided systems. In our view a uniform description of quantum information for braided systems has not yet emerged. The picture is complicated by a diversity of examples that lacks a unifying framework for proving theorems and discovering new protocols. We use category theory to construct a high-level language that abstracts the quantum mechanical properties of braided systems. We exploit this framework to propose an axiomatic description of braided quantum information intended for topological quantum computation. In this installment we first generalize the primordial Abramsky-Coecke "quantum information flow" paradigm from compact closed categories to right-rigid strict monoidal categories. We then study dagger structures for rigid and/or braided categories and formulate a graphical dagger calculus. We then propose two generalizations of strongly compact closed categories. Finally we study partial traces in the context of dagger categories.
Ground State Degeneracy in the Levin-Wen Model for Topological Phases
Yuting Hu,Spencer D. Stirling,Yong-Shi Wu
Physics , 2011, DOI: 10.1103/PhysRevB.85.075107
Abstract: We study properties of topological phases by calculating the ground state degeneracy (GSD) of the 2d Levin-Wen (LW) model. Here it is explicitly shown that the GSD depends only on the spatial topology of the system. Then we show that the ground state on a sphere is always non-degenerate. Moreover, we study an example associated with a quantum group, and show that the GSD on a torus agrees with that of the doubled Chern-Simons theory, consistent with the conjectured equivalence between the LW model associated with a quantum group and the doubled Chern-Simons theory.
Emergent Exclusion Statistics of Fibonacci Anyons in 2D Topological Phases
Yuting Hu,Spencer D. Stirling,Yong-Shi Wu
Physics , 2013, DOI: 10.1103/PhysRevB.89.115133
Abstract: We demonstrate how the generalized Pauli exclusion principle emerges for quasiparticle excitations in 2d topological phases. As an example, we examine the Levin-Wen model with the Fibonacci data (specified in the text), and construct the number operator for fluxons living on plaquettes. By numerically counting the many-body states with fluxon number fixed, the matrix of exclusion statistics parameters is identified and is shown to depend on the spatial topology (sphere or torus) of the system. Our work reveals the structure of the (many-body) Hilbert space and some general features of thermodynamics for quasiparticle excitations in topological matter.
MERLIN observations of relativistic ejections from GRS 1915+105
R. P. Fender,S. T. Garrington,D. J. McKay,T. W. B. Muxlow,G. G. Pooley,R. E. Spencer,A. M. Stirling,E. B. Waltman
Physics , 1998, DOI: 10.1046/j.1365-8711.1999.02364.x
Abstract: We present high resolution MERLIN radio images of multiple relativistic ejections from GRS 1915+105 in 1997 October / November. The observations were made at a time of complex radio behaviour, corresponding to multiple optically-thin outbursts and several days of rapid radio flux oscillations. The radio imaging resolved four major ejection events from the system. As previously reported from earlier VLA observations of the source, we observe apparent superluminal motions resulting from intrinsically relativistic motions of the ejecta. However, our measured proper motions are significantly greater than those observed on larger angular scales with the VLA. Under the assumption of an intrinsically symmetric ejection, we can place an upper limit on the distance to GRS 1915+105 of 11.2 +/- 0.8 kpc. Solutions for the velocities unambiguously require a higher intrinsic speed by about 0.1c than that derived from the earlier VLA observations, whilst the angle to the line-of-sight is not found to be significantly different. At a distance of 11 kpc, we obtain solutions of v = 0.98 (-0.05,+0.02)c and theta = 66 +/- 2 degrees. The jet also appears to be curved on a scale which corresponds to a period of around 7 days. We observe significant evolution of the linear polarisation of the approaching component, with large rotations in position angle and a general decrease in fractional polarisation. The power input into the formation of the jet is very large, >10^38 erg/s at 11 kpc for a pair plasma. If the plasma contains a cold proton for each electron, then the mass outflow rate, >10^18 g/sec is comparable to inflow rates previously derived from X-ray spectral fits.
MERLIN observations of GRS 1915+105 : a progress report
R. P. Fender,S. T. Garrington,D. J. McKay,T. W. B. Muxlow,G. G. Pooley,R. E. Spencer,A. M. Stirling,E. B. Waltman
Physics , 1998, DOI: 10.1016/S1387-6473(98)00078-5
Abstract: We present a progress report on MERLIN radio imaging of a radio outburst from GRS 1915+105. The major ejection occurred at the end of an approximately 20-day `plateau' state, characterised by low/hard X-ray fluxes and a relatively strong flat-spectrum radio component. Apparent superluminal motions have been mapped with unprecedented resolution, and imply higher velocities in the jet than previously derived.
A review of the expanded use of co-trimoxazole in HIV-infected Africans
D Spencer
Southern African Journal of HIV Medicine , 2006,
Abstract: The HIV/AIDS epidemic affects large numbers of people in sub-Saharan Africa. Most are unaware of their HIV status. Despite two decades of scientific advance, the education of communities and the provision of antiretroviral medication to some, many still succumb to the virus. Can this situation be changed? Antiretroviral (ARV) drugs have been shown to be effective in both poor and rich communities.1,2 But for the majority, these agents remain unaffordable and difficult to access. After 2 years, the public-sector ARV rollout programme in South Africa remains under-subscribed and under-utilised.3 Diets, vitamins, micronutrients and herbal concoctions have been advocated.4 But none has provided the survival benefit, freedom from opportunistic disease, and completeness of recovery, of antiretroviral therapy. In recent years, research in developing cou ntries has suggested that the daily use of the sulfonamide combination antibiotic, co-trimoxazole (CTX, trimethoprim-sulfamethoxazole, TMP/SMX), enhances the survival of infected adults and children.6-8 Cotrimoxazole (CTX) use in patients with advanced HIV infection became widespread in the 1980s when efficacy against Pneumocystis jiroveci pneumonia (PJP) was demonstrated. In this context, prophylactic CTX was commenced at CD4 levels of 200 cells/μl or less or following an AIDS-defining condition including PJP itself, prolonged and unexplained fever and weight loss. It was discontinued once the CD4 count rose to and remained above 200 cells/μl for at least 6 months.9 But the landscape for prophylactic CTX use in Africa appears to be changing. Recent World Health Organization (WHO) Guidelines have recommended expanding CTX use to all HIV-infected persons – where CD4 levels are unknown – with symptomatic WHO stage 2, 3 or 4 disease and where CD4 counts are available, to all with counts below 350 cells/μl. All HIVinfected persons with TB - pulmonary and non-pulmonary – are to be placed on CTX prophylaxis irrespective of their CD4 cell count. A ‘universal option\' of ‘CTX to all\' is offered to those who live in regions of high HIV prevalence and inadequate health care support.10 When is CTX prophylaxis stopped? ‘The general view is to continue CTX prophylaxis in adults – in resource-poor settings – indefinitely.\' Southern African Journal of HIV Medicine Vol. 7 (3) 2006: pp. 28-32
Ethics - Disclosure of doctors with HIV/AIDS on antiretroviral therapy
M Richter, D Spencer
Southern African Journal of HIV Medicine , 2007,
Abstract: The Southern African HIV Clinicians Society initiated an online discussion forum on ‘HIV Ethics and Policy\' in 2007. The first case study concerned the ethical question of whether a surgeon with HIV/AIDS on antiretroviral therapy should have disclosed her HIV status to her patient when she discovered blood on the inside of the first of her double gloves after surgery. The case study, and some responses submitted to the forum, follow below. Southern African Journal of HIV Medicine Vol. 8 (1) 2007: pp. 44-47
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