3,263 research outputs found
Einselection without pointer states
We consider small subsystems of large, closed quantum systems that evolve
according to the von Neumann equation. Without approximations and without
making any special assumptions on the form of the interaction we prove that,
for almost all initial states and almost all times, the off-diagonal elements
of the density matrix of the subsystem in the eigenbasis of its local
Hamiltonian must be small, whenever the energy difference of the corresponding
eigenstates is larger than the interaction energy. This proves that decoherence
with respect to the local energy eigenbasis is a natural property of weakly
interacting quantum systems.Comment: published in Phys. Rev. E, 4 pages, no figures, revised introduction
and conclusions, references revised and new references added, editorial
change
The basis problem in many-worlds theories
It is emphasized that a many-worlds interpretation of quantum theory exists
only to the extent that the associated basis problem is solved. The core basis
problem is that the robust enduring states specified by environmental
decoherence effects are essentially Gaussian wave packets that form continua of
non-orthogonal states. Hence they are not a discrete set of orthogonal basis
states to which finite probabilities can be assigned by the usual rules. The
natural way to get an orthogonal basis without going outside the Schroedinger
dynamics is to use the eigenstates of the reduced density matrix, and this idea
is the basis of some recent attempts by many-worlds proponents to solve the
basis problem. But these eigenstates do not enjoy the locality and
quasi-classicality properties of the states defined by environmental
decoherence effects, and hence are not satisfactory preferred basis states. The
basis problem needs to be addressed and resolved before a many-worlds-type
interpretation can be said to exist.Comment: This extended version is to be published in The Canadian Journal of
Physic
Angular momentum transport and large eddy simulations in magnetorotational turbulence: the small Pm limit
Angular momentum transport in accretion discs is often believed to be due to
magnetohydrodynamic turbulence mediated by the magnetorotational instability.
Despite an abundant literature on the MRI, the parameters governing the
saturation amplitude of the turbulence are poorly understood and the existence
of an asymptotic behavior in the Ohmic diffusion regime is not clearly
established. We investigate the properties of the turbulent state in the small
magnetic Prandtl number limit. Since this is extremely computationally
expensive, we also study the relevance and range of applicability of the most
common subgrid scale models for this problem. Unstratified shearing boxes
simulations are performed both in the compressible and incompressible limits,
with a resolution up to 800 cells per disc scale height. The latter constitutes
the largest resolution ever attained for a simulation of MRI turbulence. In the
presence of a mean magnetic field threading the domain, angular momentum
transport converges to a finite value in the small Pm limit. When the mean
vertical field amplitude is such that {\beta}, the ratio between the thermal
and magnetic pressure, equals 1000, we find {\alpha}~0.032 when Pm approaches
zero. In the case of a mean toroidal field for which {\beta}=100, we find
{\alpha}~0.018 in the same limit. Both implicit LES and Chollet-Lesieur closure
model reproduces these results for the {\alpha} parameter and the power
spectra. A reduction in computational cost of a factor at least 16 (and up to
256) is achieved when using such methods. MRI turbulence operates efficiently
in the small Pm limit provided there is a mean magnetic field. Implicit LES
offers a practical and efficient mean of investigation of this regime but
should be used with care, particularly in the case of a vertical field.
Chollet-Lesieur closure model is perfectly suited for simulations done with a
spectral code.Comment: Accepted for publication in A&
Disc formation in turbulent cloud cores: Circumventing the magnetic braking catastrophe
We present collapse simulations of strongly magnetised, 100 M_sun, turbulent
cloud cores. Around the protostars formed during the collapse Keplerian discs
with typical sizes of up to 100 AU build up in contrast to previous simulations
neglecting turbulence. Analysing the condensations in which the discs form, we
show that the magnetic flux loss is not sufficient to explain the build-up of
Keplerian discs. The average magnetic field is strongly inclined to the disc
which might reduce the magnetic braking efficiency. However, the main reason
for the reduced magnetic braking efficiency is the highly disordered magnetic
field in the surroundings of the discs. Furthermore, due to the lack of a
coherently rotating structure in the turbulent environment of the disc no
toroidal magnetic field necessary for angular momentum extraction can build up.
Simultaneously the angular momentum inflow remains high due to local shear
flows created by the turbulent motions. We suggest that the "magnetic braking
catastrophe" is an artefact of the idealised non-turbulent initial conditions
and that turbulence provides a natural mechanism to circumvent this problem.Comment: 4 pages, 2 figures. To appear in the proceedings of 'The Labyrinth of
Star Formation' (18-22 June 2012, Chania, Greece), published by Springe
Reduced coherence in double-slit diffraction of neutrons
In diffraction experiments with particle beams, several effects lead to a
fringe visibility reduction of the interference pattern. We theoretically
describe the intensity one can measure in a double-slit setup and compare the
results with the experimental data obtained with cold neutrons. Our conclusion
is that for cold neutrons the fringe visibility reduction is due not to
decoherence, but to initial incoherence.Comment: 4 pages LaTeX, 2 figure
Non-Markovianity, Loschmidt echo and criticality: a unified picture
A simple relationship between recently proposed measures of non-Markovianity
and the Loschmidt echo is established, holding for a two-level system (qubit)
undergoing pure dephasing due to a coupling with a many-body environment. We
show that the Loschmidt echo is intimately related to the information flowing
out from and occasionally back into the system. This, in turn, determines the
non-Markovianity of the reduced dynamics. In particular, we consider a central
qubit coupled to a quantum Ising ring in the transverse field. In this context,
the information flux between system and environment is strongly affected by the
environmental criticality; the qubit dynamics is shown to be Markovian exactly
and only at the critical point. Therefore non-Markovianity is an indicator of
criticality in the model considered here
Thermal limitation of far-field matter-wave interference
We assess the effect of the heat radiation emitted by mesoscopic particles on
their ability to show interference in a double slit arrangement. The analysis
is based on a stationary, phase-space based description of matter wave
interference in the presence of momentum-exchange mediated decoherence.Comment: 8 pages, 2 figures; published versio
A facility and a web application for real-time monitoring of the TTC backbone status
The Timing Trigger and Control (TTC) system distributes timing signals from the LHC Radio Frequency (RF) source to the four experiments (ATLAS, ALICE, CMS and LHCb). A copy of these signals is also transmitted to a monitoring system, installed in the CERN Control Centre, which provides continuous measurement of selected parameters. A web application has been designed to ensure real time remote monitoring and post-mortem analysis of these data. The implemented system is aimed at providing a tool for a fast detection of TTC signal abnormality and unavailability which results in reliability improvement of the whole TTC dependent infrastructure. The paper discusses the architecture of the monitoring system including measurement setup as well as various concerns of data acquisition, storage and visualization
Orographic cirrus in a future climate
A cloud resolving model (CRM) is used to investigate the formation of orographic cirrus clouds in the current and future climate. The formation of cirrus clouds depends on a variety of dynamical and thermodynamical processes, which act on different scales. First, the capability of the CRM in realistically simulating orographic cirrus clouds has been tested by comparing the simulated results to aircraft measurements of an orographic cirrus cloud. The influence of a warmer climate on the microphysical and optical properties of cirrus clouds has been investigated by initializing the CRM with vertical profiles of horizontal wind, potential temperature and equivalent potential temperature, respectively. The vertical profiles are extracted from IPCC A1B simulations for the current climate and for the period 2090–2099 for two regions representative for North and South America. The influence of additional moisture in a future climate on the propagation of gravity waves and the formation of orographic cirrus could be estimated. In a future climate, the increase in moisture dampens the vertical propagation of gravity waves and the occurring vertical velocities in the moist simulations. Together with higher temperatures fewer ice crystals nucleate homogeneously. Assuming that the relative humidity does not change in a warmer climate the specific humidity in the model is increased. This increase in specific humidity in a warmer climate results in a higher ice water content. The net effect of a reduced ice crystal number concentration and a higher ice water content is an increased optical depth. However, in some moist simulations dynamical changes contribute to changes in the ice water content, ice crystal number concentration and optical depth. For the corresponding dry simulations dynamical changes are more pronounced leading to a decreased optical depth in a future climate in some cases
Influence of molecular temperature on the coherence of fullerenes in a near-field interferometer
We study C70 fullerene matter waves in a Talbot-Lau interferometer as a
function of their temperature. While the ideal fringe visibility is observed at
moderate molecular temperatures, we find a gradual degradation of the
interference contrast if the molecules are heated before entering the
interferometer. A method is developed to assess the distribution of the
micro-canonical temperatures of the molecules in free flight. This way the
heating-dependent reduction of interference contrast can be compared with the
predictions of quantum theory. We find that the observed loss of coherence
agrees quantitatively with the expected decoherence rate due to the thermal
radiation emitted by the hot molecules.Comment: 11 pages, 9 figure
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