170 research outputs found
Trapped ions beyond carrier and sideband interactions
Trapped ions driven by electromagnetic radiation constitute one of the most
developed quantum technologies to date. The scenarios range from
proof-of-principle experiments to on-chip integration for quantum information
units. In most cases, these systems have operated in a regime where the
magnitude of the ion-radiation coupling constant is much smaller than the trap
and electronic transition frequencies. This regime allows the use of simple
effective Hamiltonians based on the validity of the rotating wave
approximation. However, novel trap and cavity designs now permit regimes in
which the trap frequency and the ion-radiation coupling constant are
commensurate. This opens up new venues for faster quantum gates and state
transfers from the ion to a photon, and other quantum operations. From the
theoretical side, however, there is not yet much known in terms of models and
applications that go beyond the weak driving scenario. In this work, we will
present two main results in the scenario of stronger drivings. First, we
revisit a known protocol to reconstruct the motional Wigner function and expand
it to stronger driving lasers. This extension is not trivial because the
original protocol makes use of effective Hamiltonians valid only for weak
drivings. The use of stronger fields or faster operations is desirable since
experimental reconstruction methods of that kind are usually hindered by
decoherence. We then present a model that allows the analytical treatment of
stronger drivings and that works well for non-resonant interactions, which are
generally out of the reach of the previous models.Comment: 9 pages, 6 figure
Protostar Formation in Magnetic Molecular Clouds beyond Ion Detachment: I. Formulation of the Problem and Method of Solution
We formulate the problem of the formation of magnetically supercritical cores
in magnetically subcritical parent molecular clouds, and the subsequent
collapse of the cores to high densities, past the detachment of ions from
magnetic field lines and into the opaque regime. We employ the six-fluid MHD
equations, accounting for the effects of grains (negative, positive and
neutral) including their inelastic collisions with other species. We do not
assume that the magnetic flux is frozen in any of the charged species. We
derive a generalized Ohm's law that explicitly distinguishes between flux
advection (and the associated process of ambipolar diffusion) and Ohmic
dissipation, in order to assess the contribution of each mechanism to the
increase of the mass-to-flux ratio of the central parts of a collapsing core
and possibly to the resolution of the magnetic flux problem of star formation.
We show how our formulation is related to and can be transformed into the
traditional, directional formulation of the generalized Ohm's law, and we
derive formulae for the perpendicular, parallel and Hall conductivities
entering the latter, which include, for the first time, the effect of inelastic
collisions between grains. In addition, we present a general (valid in any
geometry) solution for the velocities of charged species as functions of the
velocity of the neutrals and of the effective flux velocity (which can in turn
be calculated from the dynamics of the system and Faraday's law). The last two
sets of formulae can be adapted for use in any general non-ideal MHD code to
study phenomena beyond star formation in magnetic clouds. The results,
including a detailed parameter study, are presented in two accompanying papers.Comment: 17 pages, emulateapj; accepted for publication in the Astrophysical
Journa
Observational Constraints on the Ages of Molecular Clouds and the Star-Formation Timescale: Ambipolar-Diffusion--Controlled or Turbulence-Induced Star Formation?
We revisit the problem of the star formation timescale and the ages of
molecular clouds. The apparent overabundance of star-forming molecular clouds
over clouds without active star formation has been thought to indicate that
molecular clouds are "short-lived" and that star formation is "rapid". We show
that this statistical argument lacks self-consistency and, even within the
rapid star-formation scenario, implies cloud lifetimes of approximately 10 Myr.
We discuss additional observational evidence from external galaxies that
indicate lifetimes of molecular clouds and a timescale of star formation of
approximately 10 Myr . These long cloud lifetimes in conjunction with the rapid
(approximately 1 Myr) decay of supersonic turbulence present severe
difficulties for the scenario of turbulence-controlled star formation. By
contrast, we show that all 31 existing observations of objects for which the
linewidth, the size, and the magnetic field strength have been reliably
measured are in excellent quantitative agreement with the predictions of the
ambipolar-diffusion theory. Within the ambipolar-diffusion-controlled star
formation theory the linewidths may be attributed to large-scale non-radial
cloud oscillations (essentially standing large-amplitude, long-wavelength
Alfven waves), and the predicted relation between the linewidth, the size, and
the magnetic field is a natural consequence of magnetic support of
self-gravitating clouds.Comment: 7 pages, 2 figures, uses emulateapj; accepted for publication in Ap
Numerical Tests of Fast Reconnection in Weakly Stochastic Magnetic Fields
We study the effects of turbulence on magnetic reconnection using 3D
numerical simulations. This is the first attempt to test a model of fast
magnetic reconnection in the presence of weak turbulence proposed by Lazarian &
Vishniac (1999). This model predicts that weak turbulence, generically present
in most of astrophysical systems, enhances the rate of reconnection by reducing
the transverse scale for reconnection events and by allowing many independent
flux reconnection events to occur simultaneously. As a result the reconnection
speed becomes independent of Ohmic resistivity and is determined by the
magnetic field wandering induced by turbulence. To quantify the reconnection
speed we use both an intuitive definition, i.e. the speed of the reconnected
flux inflow, as well as a more sophisticated definition based on a formally
derived analytical expression. Our results confirm the predictions of the
Lazarian & Vishniac model. In particular, we find that Vrec Pinj^(1/2), as
predicted by the model. The dependence on the injection scale for some of our
models is a bit weaker than expected, i.e. l^(3/4), compared to the predicted
linear dependence on the injection scale, which may require some refinement of
the model or may be due to the effects like finite size of the excitation
region. The reconnection speed was found to depend on the expected rate of
magnetic field wandering and not on the magnitude of the guide field. In our
models, we see no dependence on the guide field when its strength is comparable
to the reconnected component. More importantly, while in the absence of
turbulence we successfully reproduce the Sweet-Parker scaling of reconnection,
in the presence of turbulence we do not observe any dependence on Ohmic
resistivity, confirming that our reconnection is fast.Comment: 22 pages, 20 figure
Do Lognormal Column-Density Distributions in Molecular Clouds Imply Supersonic Turbulence?
Recent observations of column densities in molecular clouds find lognormal
distributions with power-law high-density tails. These results are often
interpreted as indications that supersonic turbulence dominates the dynamics of
the observed clouds. We calculate and present the column-density distributions
of three clouds, modeled with very different techniques, none of which is
dominated by supersonic turbulence. The first star-forming cloud is simulated
using smoothed particle hydrodynamics (SPH); in this case gravity, opposed only
by thermal-pressure forces, drives the evolution. The second cloud is
magnetically subcritical with subsonic turbulence, simulated using nonideal
MHD; in this case the evolution is due to gravitationally-driven ambipolar
diffusion. The third cloud is isothermal, self-gravitating, and has a smooth
density distribution analytically approximated with a uniform inner region and
an r^-2 profile at larger radii. We show that in all three cases the
column-density distributions are lognormal. Power-law tails develop only at
late times (or, in the case of the smooth analytic profile, for strongly
centrally concentrated configurations), when gravity dominates all opposing
forces. It therefore follows that lognormal column-density distributions are
generic features of diverse model clouds, and should not be interpreted as
being a consequence of supersonic turbulence.Comment: 6 pages, 6 figures, accepted for publication in MNRA
Thermal instability in ionized plasma
We study magnetothermal instability in the ionized plasmas including the
effects of Ohmic, ambipolar and Hall diffusion. Magnetic field in the single
fluid approximation does not allow transverse thermal condensations, however,
non-ideal effects highly diminish the stabilizing role of the magnetic field in
thermally unstable plasmas. Therefore, enhanced growth rate of thermal
condensation modes in the presence of the diffusion mechanisms speed up the
rate of structure formation.Comment: Accepted for publication in Astrophysics & Space Scienc
RoboPol: Connection between optical polarization plane rotations and gamma-ray flares in blazars
We use results of our 3 year polarimetric monitoring program to investigate
the previously suggested connection between rotations of the polarization plane
in the optical emission of blazars and their gamma-ray flares in the GeV band.
The homogeneous set of 40 rotation events in 24 sources detected by {\em
RoboPol} is analysed together with the gamma-ray data provided by {\em
Fermi}-LAT. We confirm that polarization plane rotations are indeed related to
the closest gamma-ray flares in blazars and the time lags between these events
are consistent with zero. Amplitudes of the rotations are anticorrelated with
amplitudes of the gamma-ray flares. This is presumably caused by higher
relativistic boosting (higher Doppler factors) in blazars that exhibit smaller
amplitude polarization plane rotations. Moreover, the time scales of rotations
and flares are marginally correlated.Comment: 12 pages, 16 figures, accepted to MNRA
RoboPol: First season rotations of optical polarization plane in blazars
We present first results on polarization swings in optical emission of
blazars obtained by RoboPol, a monitoring program of an unbiased sample of
gamma-ray bright blazars specially designed for effective detection of such
events. A possible connection of polarization swing events with periods of high
activity in gamma rays is investigated using the dataset obtained during the
first season of operation. It was found that the brightest gamma-ray flares
tend to be located closer in time to rotation events, which may be an
indication of two separate mechanisms responsible for the rotations. Blazars
with detected rotations have significantly larger amplitude and faster
variations of polarization angle in optical than blazars without rotations. Our
simulations show that the full set of observed rotations is not a likely
outcome (probability ) of a random walk of the
polarization vector simulated by a multicell model. Furthermore, it is highly
unlikely () that none of our rotations is physically
connected with an increase in gamma-ray activity.Comment: 16 pages, 9 figure
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