16,606 research outputs found
Dynamics of a Raman coupled model: entanglement and quantum computation
The evolution of a Raman coupled three-level lambda atom with two quantized
cavity modes is studied in the large detuning case; i.e. when the upper atomic
level can be adiabatically eliminated. Particularly the situation when the two
modes are prepared in initial coherent or squeezed states, with a large average
number of photons, is investigated. It is found that the atom, after specific
interaction times, disentangles from the two modes, leaving them, in certain
cases, in entangled Schrodinger cat states. These disentanglement times can be
controlled by adjusting the ratio between average numbers of photons in the two
modes. It is also shown how this effective model may be used for implementing
quantum information processing. Especially it is demonstrated how to generate
various entangled states, such as EPR- and GHZ-states, and quantum logic
operations, such as the control-not and the phase-gate.Comment: 8 pages, 6 figure
Travelling to exotic places with cavity QED systems
Recent theoretical schemes for utilizing cavity QED models as quantum
simulators are reviewed. By considering a quadrature representation for the
fields, it is shown how Jahn-Teller models, effective Abelian or non-Abelian
gauge potentials, transverse Hall currents, and relativistic effects naturally
arise in these systems. Some of the analytical predictions are verified
numerically using realistic experimental parameters taking into account for
system losses. Thereby demonstrating their feasibility with current
experimental setups.Comment: 5 pages, 3 figure
Hands-on Gravitational Wave Astronomy: Extracting astrophysical information from simulated signals
In this paper we introduce a hands-on activity in which introductory
astronomy students act as gravitational wave astronomers by extracting
information from simulated gravitational wave signals. The process mimics the
way true gravitational wave analysis will be handled by using plots of a pure
gravitational wave signal. The students directly measure the properties of the
simulated signal, and use these measurements to evaluate standard formulae for
astrophysical source parameters. An exercise based on the discussion in this
paper has been written and made publicly available online for use in
introductory laboratory courses.Comment: 5 pages, 4 figures; submitted to Am. J. Phy
Science Icebreaker Activities: An Example from Gravitational Wave Astronomy
At the beginning of a class or meeting an icebreaker activity is often used
to help loosen the group and get everyone talking. Our motivation is to develop
activities that serve the purpose of an icebreaker, but are designed to enhance
and supplement a science-oriented agenda. The subject of this article is an
icebreaker activity related to gravitational wave astronomy. We first describe
the unique gravitational wave signals from three distinct sources:
monochromatic binaries, merging compact objects, and extreme mass ratio
encounters. These signals form the basis of the activity where participants
work to match an ideal gravitational wave signal with noisy detector output for
each type of source.Comment: Accepted to The Physics Teacher. Original manuscript divided into two
papers at the request of the referee. For a related paper on gravitational
wave observatories see physics/050920
Dissipative transformation of non-nucleated dwarf galaxies into nucleated systems
Recent photometric observations by the {\it Hubble Space Telescope (HST)}
have revealed the physical properties of stellar galactic nuclei in nucleated
dwarf galaxies in the Virgo cluster of galaxies. In order to elucidate the
formation processes of nucleated dwarfs, we numerically investigate gas
dynamics, star formation, and chemical evolution within the central 1 kpc of
gas disks embedded within the galactic stellar components of non-nucleated
dwarfs. We find that high density, compact stellar systems can be formed in the
central regions of dwarfs as a result of dissipative, repeated merging of
massive stellar and gaseous clumps developed from nuclear gaseous spiral arms
as a result of local gravitational instability. The central stellar components
are found to have stellar masses which are typically 5% of their host dwarfs
and show very flattened shapes, rotational kinematics, and central velocity
dispersions significantly smaller than those of their host dwarfs. We also find
that more massive dwarfs can develop more massive, more metal-rich, and higher
density stellar systems in their central regions, because star formation and
chemical enrichment proceed more efficiently owing to the less dramatic
suppression of star formation by supernovae feedback effects in more massive
dwarfs. Based on these results, we suggest that gas-rich, non-nucleated dwarfs
can be transformed into nucleated ones as a result of dissipative gas dynamics
in their central regions. We discuss the origin of the observed correlations
between physical properties of stellar galactic nuclei and those of their host
galaxies.Comment: 13 pages, 4 figures (1 color), ApJL in pres
Determination of lunar ilmentite abundances from remotely sensed data
The mapping of ilmenite on the surface of the moon is a necessary precursor to the investigation of prospective lunar base sites. Telescopic observations of the moon using a variety of narrow bandpass optical interference filters are being performed as a preliminary means of achieving this goal. Specifically, ratios of images obtained using filters centered at 0.40 and 0.56 microns provide quantitative estimates of TiO2 abundances. Analysis of preliminary distribution maps of TiO2 concentrations allows identification of specific high-Ti areas. Investigations of these areas using slit spectra in the range 0.03 to 0.85 microns are underway to search for discrete spectral signatures attributable to ilmenite
UV Interstellar Absorption Lines towards the Starburst Dwarf Galaxy NGC 1705
Archival Goddard High Resolution Spectrograph low-resolution spectra of NGC
1705, with wavelength ranges 1170.3 to 1461.7 A and 1453.5 to 1740.1 A and a
velocity resolution of about 100 km\s, have been used to derive the velocity
structure and equivalent widths of the absorption lines of Si II 1190.42,
1260.42, 1304.37 and 1526.71 A, S II 1253 , Al II 1670.79 Aand Fe II 1608.45 A
in this sightline. Three relatively narrow absorption components are seen at
LSR velocities --20 km/s, 260 km/sand 540 km/s. Arguments are presented to show
these absorption features are interstellar rather than stellar in origin based
on a comparison with the C III 1175.7 A absorption feature. We identify the
--20 km/s component with Milky Way disk/halo gas and the 260 km/s component
with an isolated high-velocity cloud HVC 487. This small HVC is located about
10 degrees from the H I gas which envelops the Magellanic Clouds and the
Magellanic Stream (MS). The (Si/H) ratio for this HVC is > 0.6 (Si/H)solar
which together with velocity agreement, suggests association with the
Magellanic Cloud and MS gas. H-alpha emission line kinematics of NGC 1705 show
the presence of a kpc-scale expanding supershell of ionized gas centered on the
central nucleus with a blue-shifted emission component at 540 km/s (Meurer et
al. 1992). We identify the 540 km/s absorption component seen in the GHRS
spectra with the front side of this expanding, ionized supershell. The most
striking feature of this component is strong Si II and Al II absorption but
weak Fe II 1608 A absorption. The low Fe II column density derived is most
likely intrinsic since it cannot be accounted for by ionization corrections or
dust depletion. Due to their shallow gravitational potential wells, dwarf
galaxies have small gravitational binding energies and are vulnerable to largeComment: 15 pages, LaTEX, 1 figure. Accepted for publication in Astrophysical
Journal Letter
Modeling a high mass turn down in the stellar initial mass function
Statistical sampling from the stellar initial mass function (IMF) for all
star-forming regions in the Galaxy would lead to the prediction of ~1000 Msun
stars unless there is a rapid turn-down in the IMF beyond several hundred solar
masses. Such a turndown is not necessary for dense clusters because the number
of stars sampled is always too small. Here we explore several mechanisms for an
upper mass cutoff, including an exponential decline of the star formation
probability after a turbulent crossing time. The results are in good agreement
with the observed IMF over the entire stellar mass range, and they give a
gradual turn down compared to the Salpeter function above ~100 Msun for normal
thermal Jeans mass, M_J. The upper mass turn down should scale with M_J in
different environments. A problem with the models is that they cannot give both
the observed power-law IMF out to the high-mass sampling limit in dense
clusters, as well as the observed lack of supermassive stars in whole galaxy
disks. Either there is a sharper upper-mass cutoff in the IMF, perhaps from
self-limitation, or the IMF is different for dense clusters than for the
majority of star formation that occurs at lower density. Dense clusters seem to
have an overabundance of massive stars relative to the average IMF in a galaxy.Comment: 19 pages, 2 figures, Astrophysical Journal, Vol 539, August 10, 200
Flows, Fragmentation, and Star Formation. I. Low-mass Stars in Taurus
The remarkably filamentary spatial distribution of young stars in the Taurus
molecular cloud has significant implications for understanding low-mass star
formation in relatively quiescent conditions. The large scale and regular
spacing of the filaments suggests that small-scale turbulence is of limited
importance, which could be consistent with driving on large scales by flows
which produced the cloud. The small spatial dispersion of stars from gaseous
filaments indicates that the low-mass stars are generally born with small
velocity dispersions relative to their natal gas, of order the sound speed or
less. The spatial distribution of the stars exhibits a mean separation of about
0.25 pc, comparable to the estimated Jeans length in the densest gaseous
filaments, and is consistent with roughly uniform density along the filaments.
The efficiency of star formation in filaments is much higher than elsewhere,
with an associated higher frequency of protostars and accreting T Tauri stars.
The protostellar cores generally are aligned with the filaments, suggesting
that they are produced by gravitational fragmentation, resulting in initially
quasi-prolate cores. Given the absence of massive stars which could strongly
dominate cloud dynamics, Taurus provides important tests of theories of
dispersed low-mass star formation and numerical simulations of molecular cloud
structure and evolution.Comment: 32 pages, 9 figures: to appear in Ap
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