Dynamical typicality for initial states with a preset measurement statistics of several commuting observables

We consider all pure or mixed states of a quantum many-body system which exhibit the same, arbitrary but fixed measurement outcome statistics for several commuting observables. Taking those states as initial conditions, which are then propagated by the pertinent Schr\"odinger or von Neumann equation up to some later time point, and invoking a few additional, fairly weak and realistic assumptions, we show that most of them still entail very similar expectation values for any given observable. This so-called dynamical typicality property thus corroborates the widespread observation that a few macroscopic features are sufficient to ensure the reproducibility of experimental measurements despite many unknown and uncontrollable microscopic details of the system. We also discuss and exemplify the usefulness of our general analytical result as a powerful numerical tool

Size distribution of sputtered particles from Au nanoislands due to MeV self-ion bombardment

Nanoisland gold films, deposited by vacuum evaporation of gold onto Si(100) substrates, were irradiated with 1.5 MeV Au$^{2+}$ ions up to a fluence of $5\times 10^{14}$ ions cm$^{-2}$ and at incidence angles up to $60^{\circ}$ with respect to the surface normal. The sputtered particles were collected on carbon coated grids (catcher grid) during ion irradiation and were analyzed with transmission electron microscopy and Rutherford backscattering spectrometry. The average sputtered particle size and the areal coverage are determined from transmission electron microscopy measurements, whereas the amount of gold on the substrate is found by Rutherford backscattering spectrometry. The size distributions of larger particles (number of atoms/particle, $n$ $\ge$ 1,000) show an inverse power-law with an exponent of $\sim$ -1 in broad agreement with a molecular dynamics simulation of ion impact on cluster targets.Comment: 13 pages, 8 figures, Submitted for publication in JA

Spinally projecting preproglucagon axons preferentially innervate sympathetic preganglionic neurons

Glucagon-like peptide-1 (GLP-1) affects central autonomic neurons, including those controlling the cardiovascular system, thermogenesis, and energy balance. Preproglucagon (PPG) neurons, located mainly in the nucleus tractus solitarius (NTS) and medullary reticular formation, produce GLP-1. In transgenic mice expressing glucagon promoter-driven yellow fluorescent protein (YFP), these brainstem PPG neurons project to many central autonomic regions where GLP-1 receptors are expressed. The spinal cord also contains GLP-1 receptor mRNA but the distribution of spinal PPG axons is unknown. Here, we used two-color immunoperoxidase labeling to examine PPG innervation of spinal segments T1–S4 in YFP-PPG mice. Immunoreactivity for YFP identified spinal PPG axons and perikarya. We classified spinal neurons receiving PPG input by immunoreactivity for choline acetyltransferase (ChAT), nitric oxide synthase (NOS) and/or Fluorogold (FG) retrogradely transported from the peritoneal cavity. FG microinjected at T9 defined cell bodies that supplied spinal PPG innervation. The deep dorsal horn of lower lumbar cord contained YFP-immunoreactive neurons. Non-varicose, YFP-immunoreactive axons were prominent in the lateral funiculus, ventral white commissure and around the ventral median fissure. In T1–L2, varicose, YFP-containing axons closely apposed many ChAT-immunoreactive sympathetic preganglionic neurons (SPN) in the intermediolateral cell column (IML) and dorsal lamina X. In the sacral parasympathetic nucleus, about 10% of ChAT-immunoreactive preganglionic neurons received YFP appositions, as did occasional ChAT-positive motor neurons throughout the rostrocaudal extent of the ventral horn. YFP appositions also occurred on NOS-immunoreactive spinal interneurons and on spinal YFP-immunoreactive neurons. Injecting FG at T9 retrogradely labeled many YFP-PPG cell bodies in the medulla but none of the spinal YFP-immunoreactive neurons. These results show that brainstem PPG neurons innervate spinal autonomic and somatic motor neurons. The distributions of spinal PPG axons and spinal GLP-1 receptors correlate well. SPN receive the densest PPG innervation. Brainstem PPG neurons could directly modulate sympathetic outflow through their spinal inputs to SPN or interneurons

Equilibration of quantum systems and subsystems

We unify two recent results concerning equilibration in quantum theory. We first generalise a proof of Reimann [PRL 101,190403 (2008)], that the expectation value of 'realistic' quantum observables will equilibrate under very general conditions, and discuss its implications for the equilibration of quantum systems. We then use this to re-derive an independent result of Linden et. al. [PRE 79, 061103 (2009)], showing that small subsystems generically evolve to an approximately static equilibrium state. Finally, we consider subspaces in which all initial states effectively equilibrate to the same state.Comment: 5 page

Learning to assess in higher education: exploring the interplay of ‘formal’ and ‘informal’ learning in the academic workplace.

While there has been considerable research into HE teachers’ development and their conceptions of teaching more generally (see, for instance, Trigwell et al., 1994, Trigwell et al., 1999, Kane et al., 2002, Eley, 2006), the way in which HE staff become assessors and learn about assessment has not been widely researched. Researchers such as Knight et al. (2006) have highlighted the importance of non-/informal learning in the academic workplace, and there is conflicting evidence for the impact of more formal academic development. Research on workplace learning stresses that formal and informal dimensions of learning are intertwined, but also that formal learning opportunities have a role to play in the development of expertise (Tynjälä, 2008). This paper explores the ways in which academics learn to assess in HE. It draws on empirical data from two research projects undertaken at one UK university. The ‘assessment cultures’ project explores interrelationships between socio-cultural context and academic assessment practices. 37 interviews were conducted; 24 initial and 13 follow-up interviews. In the initial interviews lecturers described how they assess in specific modules they teach. In the follow-up interviews themes such as disciplinary and occupational background and the ways lecturers learned to assess were explored. The ‘staff learning’ project examines learning about assessment through different types of academic development: a compulsory course on assessment for new academics, a module on assessment for learning for experienced staff and a university assessment for learning network providing support and activities to its members. 31 semi-structured interviews were conducted: 17 with network members, 8 with participants of the course for new academics and 6 with participants of the course on assessment for learning. Interviewees were asked about the benefits or otherwise of the type of academic development experienced, changes in their understanding of assessment and changes in their assessment practices. Taken together, the data generated by the two projects provide insight into the complex process whereby formal and informal interact. Using data from the two projects has enabled us to extend the scope of each individual project in order to address questions which could otherwise not have been answered. It has also provided an opportunity to examine the data through the lenses of different theoretical frameworks, thus heightening our awareness for the ‘the theoretical frames of reference and methodological approaches which shape (…) [our] knowledge claims’ (Shay, p.1). The collaboration has increased our awareness for our own approaches and backgrounds and has required us to engage with each others’ theoretical and professional perspectives: that of Foucauldian post-structuralism and a focus on the discursive construction of identity with that of academic development and a focus on scholarship of teaching and learning and evaluation of academic development initiatives. It has also meant drawing on each others’ respective research backgrounds in student and teacher learning in higher education research on the one hand and workplace learning research on the other. In the critical review of student learning research which Shay refers to in her ‘think piece’, Haggis (2009) explicitly highlights the potential of the workplace learning literatures and debates for ‘thinking differently’ about higher education research. By challenging ways of knowing through collaboration, the paper intends to make a contribution to the ‘region’ (Shay, p.2). Our focus in the analysis is on the interrelationships between theoretical concepts and assessment practices. In some interviews encounters with a concept such as ‘assessment for learning’ were described as a lens which sheds light onto existing assessment practices as well as a tool which enables the development of practice. This draws attention to the importance of the discursive resources that are available to academics for talking about their practice. In addition, certain ways of learning appear to be significant both in everyday workplace practice and in academic development activities, for instance learning from and through others. Interaction with other members of staff, particularly from other disciplines, enables staff to question the taken for granted and re-think their assessment practices. The data suggest that the simple distinction between formal and informal learning in the academic workplace may be too crude and that it may therefore be difficult to evaluate or ‘prove’ the impact of formal learning opportunities. However, the findings have implications for academic development since they draws attention to the potential of specific types of ideas and learning activities to transform understandings of assessment and assessment practices. The paper will discuss the main findings as well as the benefits and challenges of working with data across two research projects with different theoretical underpinnings

Slice Stretching at the Event Horizon when Geodesically Slicing the Schwarzschild Spacetime with Excision

Slice-stretching effects are discussed as they arise at the event horizon when geodesically slicing the extended Schwarzschild black-hole spacetime while using singularity excision. In particular, for Novikov and isotropic spatial coordinates the outward movement of the event horizon (``slice sucking'') and the unbounded growth there of the radial metric component (``slice wrapping'') are analyzed. For the overall slice stretching, very similar late time behavior is found when comparing with maximal slicing. Thus, the intuitive argument that attributes slice stretching to singularity avoidance is incorrect.Comment: 5 pages, 2 figures, published version including minor amendments suggested by the refere

Equilibration of isolated macroscopic quantum systems

We investigate the equilibration of an isolated macroscopic quantum system in the sense that deviations from a steady state become unmeasurably small for the overwhelming majority of times within any sufficiently large time interval. The main requirements are that the initial state, possibly far from equilibrium, exhibits a macroscopic population of at most one energy level and that degeneracies of energy eigenvalues and of energy gaps (differences of energy eigenvalues) are not of exceedingly large multiplicities. Our approach closely follows and extends recent works by Short and Farrelly [2012 New J. Phys. 14 013063], in particular going beyond the realm of finite-dimensional systems and large effective dimensions.Comment: 19 page

Energy Flow Puzzle of Soliton Ratchets

We study the mechanism of directed energy transport for soliton ratchets. The energy flow appears due to the progressive motion of a soliton (kink) which is an energy carrier. However, the energy current formed by internal system deformations (the total field momentum) is zero. We solve the underlying puzzle by showing that the energy flow is realized via an {\it inhomogeneous} energy exchange between the system and the external ac driving. Internal kink modes are unambiguously shown to be crucial for that transport process to take place. We also discuss effects of spatial discretization and combination of ac and dc external drivings.Comment: 4 pages, 3 figures, submitted to PR

Characteristics of the polymer transport in ratchet systems

Molecules with complex internal structure in time-dependent periodic potentials are studied by using short Rubinstein-Duke model polymers as an example. We extend our earlier work on transport in stochastically varying potentials to cover also deterministic potential switching mechanisms, energetic efficiency and non-uniform charge distributions. We also use currents in the non-equilibrium steady state to identify the dominating mechanisms that lead to polymer transportation and analyze the evolution of the macroscopic state (e.g., total and head-to-head lengths) of the polymers. Several numerical methods are used to solve the master equations and nonlinear optimization problems. The dominating transport mechanisms are found via graph optimization methods. The results show that small changes in the molecule structure and the environment variables can lead to large increases of the drift. The drift and the coherence can be amplified by using deterministic flashing potentials and customized polymer charge distributions. Identifying the dominating transport mechanism by graph analysis tools is found to give insight in how the molecule is transported by the ratchet effect.Comment: 35 pages, 17 figures, to appear in Phys. Rev.

Quantum equilibration in finite time

It has recently been shown that small quantum subsystems generically equilibrate, in the sense that they spend most of the time close to a fixed equilibrium state. This relies on just two assumptions: that the state is spread over many different energies, and that the Hamiltonian has non-degenerate energy gaps. Given the same assumptions, it has also been shown that closed systems equilibrate with respect to realistic measurements. We extend these results in two important ways. First, we prove equilibration over a finite (rather than infinite) time-interval, allowing us to bound the equilibration time. Second, we weaken the non degenerate energy gaps condition, showing that equilibration occurs provided that no energy gap is hugely degenerate.Comment: 7 page