1,555 research outputs found
Quantum measurements without macroscopic superpositions
We study a class of quantum measurement models. A microscopic object is
entangled with a macroscopic pointer such that each eigenvalue of the measured
object observable is tied up with a specific pointer deflection. Different
pointer positions mutually decohere under the influence of a bath.
Object-pointer entanglement and decoherence of distinct pointer readouts
proceed simultaneously. Mixtures of macroscopically distinct object-pointer
states may then arise without intervening macroscopic superpositions.
Initially, object and apparatus are statistically independent while the latter
has pointer and bath correlated according to a metastable local thermal
equilibrium. We obtain explicit results for the object-pointer dynamics with
temporal coherence decay in general neither exponential nor Gaussian. The
decoherence time does not depend on details of the pointer-bath coupling if it
is smaller than the bath correlation time, whereas in the opposite Markov
regime the decay depends strongly on whether that coupling is Ohmic or
super-Ohmic.Comment: 50 pages, 5 figures, changed conten
Curie-Weiss model of the quantum measurement process
A hamiltonian model is solved, which satisfies all requirements for a
realistic ideal quantum measurement. The system S is a spin-\half, whose
-component is measured through coupling with an apparatus A=M+B, consisting
of a magnet \RM formed by a set of spins with quartic infinite-range
Ising interactions, and a phonon bath \RB at temperature . Initially A is
in a metastable paramagnetic phase. The process involves several time-scales.
Without being much affected, A first acts on S, whose state collapses in a very
brief time. The mechanism differs from the usual decoherence. Soon after its
irreversibility is achieved. Finally the field induced by S on M, which may
take two opposite values with probabilities given by Born's rule, drives A into
its up or down ferromagnetic phase. The overall final state involves the
expected correlations between the result registered in M and the state of S.
The measurement is thus accounted for by standard quantum statistical mechanics
and its specific features arise from the macroscopic size of the apparatus.Comment: 5 pages Revte
Public engagement in local government: the voice and influence of citizens in online communicative spaces
The communications and engagement strategies of local councils play an important role in contributing to the public's understanding of local democracies, and their engagement with local issues. Based on a study of the local authority in the third largest city in the UK, Leeds, this article presents an empirically based analysis of the impact of new opportunities for public engagement afforded by digital media on the Council's communication with citizens. Drawing on over 20 face-to-face semi-structured interviews with elected politicians, Council strategists, Council communications specialists, mainstream journalists, and citizen journalists, the article explores perceptions of the Council's engagement and communication with citizens from the perspective of a range of actors involved in the engagement process. The research asks what the differing motivations behind the Council's communications and engagement strategies mean for the way that digital media are and might be used in the future to enhance the role of citizens in local governance. The research suggests that while there are no grounds for expecting digital media to displace existing channels of public engagement, digital media are beginning to play an important role in defining and reconfiguring the role of citizens within local governance
Lower Spectral Branches of a Particle Coupled to a Bose Field
The structure of the lower part (i.e. -away below the two-boson
threshold) spectrum of Fr\"ohlich's polaron Hamiltonian in the weak coupling
regime is obtained in spatial dimension . It contains a single polaron
branch defined for total momentum , where is a bounded domain, and, for any , a
manifold of polaron + one-boson states with boson momentum in a bounded
domain depending on . The polaron becomes unstable and dissolves into the
one boson manifold at the boundary of . The dispersion laws and
generalized eigenfunctions are calculated
A Dicke Type Model for Equilibrium BEC Superradiance
We study the effect of electromagnetic radiation on the condensate of a Bose
gas. In an earlier paper we considered the problem for two simple models
showing the cooperative effect between Bose-Einstein condensation and
superradiance. In this paper we formalise the model suggested by Ketterle et al
in which the Bose condensate particles have a two level structure. We present a
soluble microscopic Dicke type model describing a thermodynamically stable
system. We find the equilibrium states of the system and compute the
thermodynamic functions giving explicit formulae expressing the cooperative
effect between Bose-Einstein condensation and superradiance
Cultural and Media Identity Among Latvian Migrants in Germany
This chapter explores how transnational media and culture impacts on the identity formation of recent Latvian migrants in Germany. In the context of the EU, Germany opened its labour market to the new EU countries rather late, when compared to other ‘old’ EU countries. This has had an effect on the composition of the group of Latvian migrants going to Germany, and their identities. In the light of this, this chapter examines how Latvian migrants in Germany feel and experience their belonging to Latvia and its culture. It analyses the social and communicative practices crucial for the development of belonging, including the rootedness in the country where they live and the cultural references that are important for them. The evidence for the analysis in this chapter comes from in-depth interviews, open media diaries and network maps of Latvian migrants in Germany. The chapter situates the description of evidence in the framework of cultural identity concepts and discusses the role of culture and media in the process of building migrant identity. The chapter argues that culture is shaping the transnational self-perception of Latvian migrants in Germany – as it provides collective narratives of imagined common frames of references, and confirms feelings of belonging and distinction
Master integrals for massive two-loop Bhabha scattering in QED
We present a set of scalar master integrals (MIs) needed for a complete
treatment of massive two-loop corrections to Bhabha scattering in QED,
including integrals with arbitrary fermionic loops. The status of analytical
solutions for the MIs is reviewed and examples of some methods to solve MIs
analytically are worked out in more detail. Analytical results for the pole
terms in epsilon of so far unknown box MIs with five internal lines are given.Comment: 23 pages, 5 tables, 12 figures, references added, appendix B enlarge
A comparative study of the neutrino-nucleon cross section at ultra high energies
The high energy neutrino cross section is a crucial ingredient in the
calculation of the event rate in high energy neutrino telescopes. Currently
there are several approaches which predict different behaviours for its
magnitude for ultrahigh energies. In this paper we present a comparison between
the predictions based on linear DGLAP dynamics, non-linear QCD and in the
imposition of a Froissart-like behaviour at high energies. In particular, we
update the predictions based on the Color Glass Condensate, presenting for the
first time the results for using the solution of the running
coupling Balitsky-Kovchegov equation. Our results demonstrate that the current
theoretical uncertainty for the neutrino-nucleon cross section reaches a factor
three for neutrinos energies around GeV and increases to a factor
five for GeV.Comment: 6 pages, 3 figure
Using the Hopf Algebra Structure of QFT in Calculations
We employ the recently discovered Hopf algebra structure underlying
perturbative Quantum Field Theory to derive iterated integral representations
for Feynman diagrams. We give two applications: to massless Yukawa theory and
quantum electrodynamics in four dimensions.Comment: 28 p, Revtex, epsf for figures, minor changes, to appear in
Phys.Rev.
Mesoscopic mean-field theory for spin-boson chains in quantum optical systems
We present a theoretical description of a system of many spins strongly coupled to a bosonic chain. We rely on the use of a spin-wave theory describing the Gaussian fluctuations around the mean-field solution, and focus on spin-boson chains arising as a generalization of the Dicke Hamiltonian. Our model is motivated by experimental setups such as trapped ions, or atoms/qubits coupled to cavity arrays. This situation corresponds to the cooperative (E⊗β) Jahn-Teller distortion studied in solid-state physics. However, the ability to tune the parameters of the model in quantum optical setups opens up a variety of novel intriguing situations. The main focus of this paper is to review the spin-wave theoretical description of this problem as well as to test the validity of mean-field theory. Our main result is that deviations from mean-field effects are determined by the interplay between magnetic order and mesoscopic cooperativity effects, being the latter strongly size-dependent
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