28,213 research outputs found
Supersonic quantum communication
When locally exciting a quantum lattice model, the excitation will propagate
through the lattice. The effect is responsible for a wealth of non-equilibrium
phenomena, and has been exploited to transmit quantum information through spin
chains. It is a commonly expressed belief that for local Hamiltonians, any such
propagation happens at a finite "speed of sound". Indeed, the Lieb-Robinson
theorem states that in spin models, all effects caused by a perturbation are
limited to a causal cone defined by a constant speed, up to exponentially small
corrections. In this work we show that for translationally invariant bosonic
models with nearest-neighbor interactions, this belief is incorrect: We prove
that one can encounter excitations which accelerate under the natural dynamics
of the lattice and allow for reliable transmission of information faster than
any finite speed of sound. The effect is only limited by the model's range of
validity (eventually by relativity). It also implies that in non-equilibrium
dynamics of strongly correlated bosonic models far-away regions may become
quickly entangled, suggesting that their simulation may be much harder than
that of spin chains even in the low energy sector.Comment: 4+3 pages, 1 figure, some material added, typographic error fixe
Interaction of a Modulated Electron Beam with a Plasma
The results of a theoretical and experimental investigation of the high-frequency interaction of an electron beam with a plasma are reported. An electron beam, modulated at a microwave frequency, passes through a uniform region of a mercury arc discharge after which it is demodulated. Exponentially growing wave amplification along the electron beam was experimentally observed for the first time at a microwave frequency equal to the plasma frequency. Approximate theories of the effects of 1) plasma-electron collision frequencies, 2) plasma-electron thermal velocities and 3) finite beam diameter, are given. In a second experiment the interaction between a modulated electron beam and a slow electrostatic wave on a plasma column has been studied. A strong interaction occurs when the velocity of the electron beam is approximately equal to the velocity of the wave and the interaction is essentially the same as that which occurs in traveling-wave amplifiers, except that here the plasma colum replaces the usual helical slow-wave circuit. The theory predicting rates of growth is presented and compared with the experimental results
Scalable reconstruction of density matrices
Recent contributions in the field of quantum state tomography have shown
that, despite the exponential growth of Hilbert space with the number of
subsystems, tomography of one-dimensional quantum systems may still be
performed efficiently by tailored reconstruction schemes. Here, we discuss a
scalable method to reconstruct mixed states that are well approximated by
matrix product operators. The reconstruction scheme only requires local
information about the state, giving rise to a reconstruction technique that is
scalable in the system size. It is based on a constructive proof that generic
matrix product operators are fully determined by their local reductions. We
discuss applications of this scheme for simulated data and experimental data
obtained in an ion trap experiment.Comment: 9 pages, 5 figures, replaced with published versio
Discussion quality diffuses in the digital public square
Studies of online social influence have demonstrated that friends have
important effects on many types of behavior in a wide variety of settings.
However, we know much less about how influence works among relative strangers
in digital public squares, despite important conversations happening in such
spaces. We present the results of a study on large public Facebook pages where
we randomly used two different methods--most recent and social feedback--to
order comments on posts. We find that the social feedback condition results in
higher quality viewed comments and response comments. After measuring the
average quality of comments written by users before the study, we find that
social feedback has a positive effect on response quality for both low and high
quality commenters. We draw on a theoretical framework of social norms to
explain this empirical result. In order to examine the influence mechanism
further, we measure the similarity between comments viewed and written during
the study, finding that similarity increases for the highest quality
contributors under the social feedback condition. This suggests that, in
addition to norms, some individuals may respond with increased relevance to
high-quality comments.Comment: 10 pages, 6 figures, 2 table
Bubble Growth in Superfluid 3-He: The Dynamics of the Curved A-B Interface
We study the hydrodynamics of the A-B interface with finite curvature. The
interface tension is shown to enhance both the transition velocity and the
amplitudes of second sound. In addition, the magnetic signals emitted by the
growing bubble are calculated, and the interaction between many growing bubbles
is considered.Comment: 20 pages, 3 figures, LaTeX, ITP-UH 11/9
Post Big Bang Processing of the Primordial Elements
We explore the Gnedin-Ostriker suggestion that a post-Big-Bang
photodissociation process may modify the primordial abundances of the light
elements. We consider several specific models and discuss the general features
that are necessary (but not necessarily sufficient) to make the model work. We
find that with any significant processing, the final D and He abundances,
which are independent of their initial standard big bang nucleosynthesis (SBBN)
values, rise quickly to a level several orders of magnitude above the
observationally inferred primordial values. Solutions for specific models show
that the only initial abundances that can be photoprocessed into agreement with
observations are those that undergo virtually no processing and are already in
agreement with observation. Thus it is unlikely that this model can work for
any non-trivial case unless an artificial density and/or photon distribution is
invoked.Comment: 12 page Latex file (AASTEX style). Tarred, gzipped, and uuencoded
postscript files of seven figures. Also available (with ps file of paper) at
ftp://www-physics.mps.ohio-state.edu/pub/nucex/phot
Redefining design in construction
Several theoretical framings have been proposed regarding the nature and process of
designing but these mainly see design as a creative activity. However, designing in
construction is a collective undertaking that involves not only 'creating' discipline-specific
parts of the design but also 'organising' them to ensure consistency and coherence.
Nevertheless, organising design in construction is under-theorised, and there are no clear
explanations of what constitutes design collaboration and how it is different from
designing. These gaps imply not only a shortcoming for managing design in construction,
but also a difficulty for developing technology that effectively supports it. Therefore, this
paper adopts a practice-focused approach to explore the interdisciplinary design
interactions in a project from an organisational point of view. This develops further
insight into the natures and processes of designing and design collaboration in
construction, thus informing the management of design. When seen from a practice-based
perspective, multidisciplinary design development becomes an ongoing process of reestablishing 'a shared sense of purposefulness' that enables both autonomy of, and
consistency between, different design disciplines. This provides an explanation of the
interdependency between specialist knowledge and interdisciplinary interactions. Thus,
the paper develops definitions of design and design collaboration in construction that are
centred on 'organisation' rather than 'creativity'. Implications for technology development
and management are outlined
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