3,977 research outputs found
Multiplexed communication over a high-speed quantum channel
In quantum information systems it is of particular interest to consider the
best way in which to use the non-classical resources consumed by that system.
Quantum communication protocols are integral to quantum information systems and
are amongst the most promising near-term applications of quantum information
science. Here we show that a multiplexed, digital quantum communications system
supported by comb of vacuum squeezing has a greater channel capacity per photon
than a source of broadband squeezing with the same analogue bandwidth. We
report on the time-resolved, simultaneous observation of the first dozen teeth
in a 2.4 GHz comb of vacuum squeezing produced by a sub-threshold OPO, as
required for such a quantum communications channel. We also demonstrate
multiplexed communication on that channel
Review of SIS Experimental Results on Strangeness
>A review of meson emission in heavy ion collisions at incident energies
around 1 -- 2 GeV is presented. It is shown how the shape of the
spectra and the various particle yields vary with system size, with centrality
and with incident energy. A statistical model assuming thermal and chemical
equilibrium and exact strangeness conservation (i.e. strangeness conservation
per collision) explains most of the observed features.
Emphasis is put onto the study of and emission. In the framework
of this statistical model it is shown that the experimentally observed equality
of and rates at threshold corrected energies is due to a crossing of two excitation functions. Furthermore,
the independence of the to ratio on the number of participating
nucleons observed between 1 and 10 GeV is consistent with this model.
The observed flow effects are beyond the scope of this model.Comment: 10 pages, 9 figures, Strangeness 2000, V International Conference on
Strangeness in Quark Matter, July, 2000, Berkeley, Californi
The Global Star Formation Rate from the 1.4 GHz Luminosity Function
The decimetric luminosity of many galaxies appears to be dominated by
synchrotron emission excited by supernova explosions. Simple models suggest
that the luminosity is directly proportional to the rate of supernova
explosions of massive stars averaged over the past 30 Myr. The proportionality
may be used together with models of the evolving 1.4 GHz luminosity function to
estimate the global star formation rate density in the era z < 1. The local
value is estimated to be 0.026 solar masses per year per cubic megaparsec, some
50% larger than the value inferred from the Halpha luminosity density. The
value at z ~ 1 is found to be 0.30 solar masses per year per cubic megaparsec.
The 10-fold increase in star formation rate density is consistent with the
increase inferred from mm-wave, far-infrared, ultra-violet and Halpha
observations.Comment: 10 pages, 2 figures, Astrophysical Journal Letters (in press); new PS
version has improved figure placemen
T-PHOT version 2.0: improved algorithms for background subtraction, local convolution, kernel registration, and new options
We present the new release v2.0 of T-PHOT, a publicly available software
package developed to perform PSF-matched, prior-based, multiwavelength
deconfusion photometry of extragalactic fields. New features included in the
code are presented and discussed: background estimation, fitting using position
dependent kernels, flux prioring, diagnostical statistics on the residual
image, exclusion of selected sources from the model and residual images,
individual registration of fitted objects. These new options improve on the
performance of the code, allowing for more accurate results and providing
useful aids for diagnostics.Comment: 7 pages, 8 figure
Magnetic configurations of the tilted current sheets in magnetotail
In this research, the geometrical structures of tilted current sheet and tail flapping waves have been analysed based on multiple spacecraft measurements and some features of the tilted current sheets have been made clear for the first time. The geometrical features of the tilted current sheet revealed in this investigation are as follows: (1) The magnetic field lines (MFLs) in the tilted current sheet are generally plane curves and the osculating planes in which the MFLs lie are about vertical to the equatorial plane, while the normal of the tilted current sheet leans severely to the dawn or dusk side. (2) The tilted current sheet may become very thin, the half thickness of its neutral sheet is generally much less than the minimum radius of the curvature of the MFLs. (3) In the neutral sheet, the field-aligned current density becomes very large and has a maximum value at the center of the current sheet. (4) In some cases, the current density is a bifurcated one, and the two humps of the current density often superpose two peaks in the gradient of magnetic strength, indicating that the magnetic gradient drift current is possibly responsible for the formation of the two humps of the current density in some tilted current sheets. Tilted current sheets often appear along with tail current sheet flapping waves. It is found that, in the tail flapping current sheets, the minimum curvature radius of the MFLs in the current sheet is rather large with values around 1 <I>R<sub>E</sub></I>, while the neutral sheet may be very thin, with its half thickness being several tenths of <I>R<sub>E</sub></I>. During the flapping waves, the current sheet is tilted substantially, and the maximum tilt angle is generally larger than 45&deg;. The phase velocities of these flapping waves are several tens km/s, while their periods and wavelengths are several tens of minutes, and several earth radii, respectively. These tail flapping events generally last several hours and occur during quiet periods or periods of weak magnetospheric activity
Pulsed flows at the high-altitude cusp poleward boundary, and associated ionospheric convection and particle signatures, during a cluster - FAST - SuperDARN - sondrestrom conjunction under a southwest
Particle and magnetic field observations during a magnetic conjunction Cluster 1-FAST-Søndrestrøm within the field of view of SuperDARN radars on 21 January 2001 allow us to draw a detailed, comprehensive and self-consistent picture at three heights of signatures associated with transient reconnection under a steady south-westerly IMF (clock angle ≈130◦). Cluster 1 was outbound through the high altitude (∼12RE ) exterior northern cusp tailward of the bifurcation line (geomagnetic Bx>0) when a solar wind dynamic pressure release shifted the spacecraft into a boundary layer downstream of the cusp. The centerpiece of the investigation is a series of flow bursts observed there by the spacecraft, which were accompanied by strong field pertur- bations and tailward flow deflections. Analysis shows these to be Alfven waves. We interpret these flow events as being due to a sequence of reconnected flux tubes, with field-aligned currents in the associated Alfven waves carrying stresses to the underlying ionosphere, a view strengthened by the other observations. At the magnetic footprint of the region of Cluster flow bursts, FAST observed an ion energy- latitude disperison of the stepped cusp type, with individual cusp ion steps corresponding to individual flow bursts. Simultaneously, the SuperDARN Stokkseyri radar observed very strong poleward-moving radar auroral forms (PMRAFs) which were conjugate to the flow bursts at Cluster. FAST was traversing these PMRAFs when it observed the cusp ion steps. The Søndrestrøm radar observed pulsed ionospheric flows (PIFs) just poleward of the convection reversal boundary. As at Cluster, the flow was eastward (tailward), implying a coherent eastward (tailward) motion of the hypothesized open flux tubes. The joint Søndrestrøm and FAST observations indicate that the open/closed field line boundary was equatorward of the convection reversal boundary by ∼2 deg. The unprecedented accuracy of the conjunction argues strongly for the validity of the interpretation of the various signatures as resulting from transient reconnection. In particular, the cusp ion steps arise on this pass from this origin, in consonance with the original pulsating cusp model. The observations point to the need of extending current ideas on the response of the ionosphere to transient reconnection. Specifically, it argues in favor of re-establishing the high-latitude boundary layer downstream of the cusp as an active site of momentum transfer
Breaking the Redshift Deadlock - I: Constraining the star formation history of galaxies with sub-millimetre photometric redshifts
Future extragalactic sub-millimetre and millimetre surveys have the potential
to provide a sensitive census of the level of obscured star formation in
galaxies at all redshifts. While in general there is good agreement between the
source counts from existing SCUBA (850um) and MAMBO (1.25mm) surveys of
different depths and areas, it remains difficult to determine the redshift
distribution and bolometric luminosities of the sub-millimetre and millimetre
galaxy population. This is principally due to the ambiguity in identifying an
individual sub-millimetre source with its optical, IR or radio counterpart
which, in turn, prevents a confident measurement of the spectroscopic redshift.
Additionally, the lack of data measuring the rest-frame FIR spectral peak of
the sub-millimetre galaxies gives rise to poor constraints on their rest-frame
FIR luminosities and star formation rates. In this paper we describe
Monte-Carlo simulations of ground-based, balloon-borne and satellite
sub-millimetre surveys that demonstrate how the rest-frame FIR-sub-millimetre
spectral energy distributions (250-850um) can be used to derive photometric
redshifts with an r.m.s accuracy of +/- 0.4 over the range 0 < z < 6. This
opportunity to break the redshift deadlock will provide an estimate of the
global star formation history for luminous optically-obscured galaxies [L(FIR)
> 3 x 10^12 Lsun] with an accuracy of 20 per cent.Comment: 14 pages, 22 figures, submitted to MNRAS, replaced with accepted
versio
On the location of dayside magnetic reconnection during an interval of duskward oriented IMF
We present space- and ground-based observations of the signatures of magnetic reconnection during an interval of duskward-oriented interplanetary magnetic field on 25 March 2004. In situ field and plasma measurements are drawn from the Double Star and Cluster satellites during traversals of the pre-noon sector dayside magnetopause at low and high latitudes, respectively. These reveal the typical signatures of flux transfer events (FTEs), namely bipolar perturbations in the magnetic field component normal to the local magnetopause, enhancements in the local magnetic field strength and mixing of magnetospheric and magnetosheath plasmas. Further evidence of magnetic reconnection is inferred from the ground-based signatures of pulsed ionospheric flow observed over an extended interval. In order to ascertain the location of the reconnection site responsible for the FTEs, a simple model of open flux tube motion over the surface of the magnetopause is employed. A comparison of the modelled and observed motion of open flux tubes (i.e. FTEs) and plasma flow in the magnetopause boundary layer indicates that the FTEs observed at both low and high latitudes were consistence with the existence of a tilted X-line passing through the sub-solar region, as suggested by the component reconnection paradigm. While a high latitude X-line (as predicted by the anti-parallel description of reconnection) may have been present, we find it unlikely that it could have been responsible for the FTEs observed in the pre-noon sector under the observed IMF conditions. Finally, we note that throughout the interval, the magnetosphere was bathed in ULF oscillations within the solar wind electric field. While no one-to-one correspondence with the pulsed reconnection rate suggested by the ground-based observation of pulsed ionospheric flow has been demonstrated, we note that similar periodicity oscillations were observed throughout the solar wind-magnetosphere-ionosphere system. These findings are consistent with previously proposed mechanisms of solar wind modulation of the dayside reconnection rate
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