30,688 research outputs found
A bimodal search strategy for SETI
The search strategy and resultant observational plan which was developed to carry out a comprehensive Search for Extraterrestrial Intelligence (SETI) over that portion of the electromagnetic spectrum known as the terrestrial microwave window is described. The limiting sensitivity achieved was parameterized and calculated for Deep Space Network antennas as well as several radio astronomy observatories. A brief description of the instrumentation to be employed in the search and the classes of signals to be looked for is given. One observational goal is to survey the entire sky over a wide range of frequency to a relatively constant flux level. This survey ensures that all potential life sites are observed to some limiting equivalent isotropic radiated power depending upon their distance. A second goal is to survey a set of potential transmission sites selected a priori to be especially promising, achieving very high sensitivity over a smaller range of frequency
Generating controllable atom-light entanglement with a Raman atom laser system
We introduce a scheme for creating continuous variable entanglement between
an atomic beam and an optical field, by using squeezed light to outcouple atoms
from a BEC via a Raman transition. We model the full multimode dynamics of the
atom laser beam and the squeezed optical field, and show that with appropriate
two-photon detuning and two-photon Rabi frequency, the transmitted light is
entangled in amplitude and phase with the outcoupled atom laser beam. The
degree of entanglement is controllable via changes in the two-photon Rabi
frequency of the outcoupling process.Comment: 4 pages, 4 figure
Asymmetric Gaussian steering: when Alice and Bob disagree
Asymmetric steering is an effect whereby an inseparable bipartite system can
be found to be described by either quantum mechanics or local hidden variable
theories depending on which one of Alice or Bob makes the required
measurements. We show that, even with an inseparable bipartite system,
situations can arise where Gaussian measurements on one half are not sufficient
to answer the fundamental question of which theory gives an adequate
description and the whole system must be considered. This phenomenon is
possible because of an asymmetry in the definition of the original
Einstein-Podolsky-Rosen paradox and in this article we show theoretically that
it may be demonstrated, at least in the case where Alice and Bob can only make
Gaussian measurements, using the intracavity nonlinear coupler.Comment: 5 Pages, 4 Figure
First principles structures and circular dichroism spectra for the close-packed and the 7/2 motif of collagen
The recently proposed close-packed motif for collagen is investigated using
first principles semi-empirical wave function theory and Kohn-Sham density
functional theory. Under these refinements the close-packed motif is shown to
be stable. For the case of the 7/2 motif a similar stability exists. The
electronic circular dichroism of the close-packed model has a significant
negative bias and a large signal. An interesting feature of the close-packed
structure is the existence of a central channel. Simulations show that, if
hydrogen atoms are placed in the cavity, a chain of molecular hydrogens is
formed suggesting a possible biological function for molecular hydrogen.Comment: 12 pages, 3 figures; 3(PPG)_6 xyz file attached; v2: minor
modification
Pairing mean-field theory for the dynamics of dissociation of molecular Bose-Einstein condensates
We develop a pairing mean-field theory to describe the quantum dynamics of
the dissociation of molecular Bose-Einstein condensates into their constituent
bosonic or fermionic atoms. We apply the theory to one, two, and
three-dimensional geometries and analyze the role of dimensionality on the atom
production rate as a function of the dissociation energy. As well as
determining the populations and coherences of the atoms, we calculate the
correlations that exist between atoms of opposite momenta, including the column
density correlations in 3D systems. We compare the results with those of the
undepleted molecular field approximation and argue that the latter is most
reliable in fermionic systems and in lower dimensions. In the bosonic case we
compare the pairing mean-field results with exact calculations using the
positive- stochastic method and estimate the range of validity of the
pairing mean-field theory. Comparisons with similar first-principle simulations
in the fermionic case are currently not available, however, we argue that the
range of validity of the present approach should be broader for fermions than
for bosons in the regime where Pauli blocking prevents complete depletion of
the molecular condensate.Comment: 16 pages, 10 figure
Heat and extension at mid- and lower crustal levels of the Rio Grande rift
The process by which large amounts (50 to 200 percent) of crustal extension are produced was concisely described by W. Hamilton in 1982 and 1983. More recently, England, Sawyer, P. Morgan and others have moved toward quantifying models of lithospheric thinning by incorporating laboratory and theoretical data on rock rheology as a function of composition, temperature, and strain rate. Hamilton's description identifies three main crustal layers, each with a distinctive mechanical behavior; brittle fracturing and rotation in the upper crust, discontinuous ductile flow in the middle crust and laminar ductile flow in the lower crust. The temperature and composition dependent brittle-ductile transition essentially defines the diffuse boundary between upper and middle crust. It was concluded that the heat responsible for the highly ductile nature of the lower crust and the lensoidal and magma body structures at mid-crustal depths in the rift was infused into the crust by relatively modest ( 10 percent by mass) magmatic upwelling (feeder dikes) from Moho levels. Seismic velocity-versus-depth data, supported by gravity modeling and the fact that volumes of rift related volcanics are relatively modest ( 6000 cubic km) for the Rio Grande system, all imply velocities and densities too small to be consistent with a massive, composite, mafic intrusion in the lower crust
vbyCaHbeta CCD Photometry of Clusters. VI. The Metal-Deficient Open Cluster NGC 2420
CCD photometry on the intermediate-band vbyCaHbeta system is presented for
the metal-deficient open cluster, NGC 2420. Restricting the data to probable
single members of the cluster using the CMD and the photometric indices alone
generates a sample of 106 stars at the cluster turnoff. The average E(b-y) =
0.03 +/- 0.003 (s.e.m.) or E(B-V) = 0.050 +/- 0.004 (s.e.m.), where the errors
refer to internal errors alone. With this reddening, [Fe/H] is derived from
both m1 and hk, using b-y and Hbeta as the temperature index. The agreement
among the four approaches is reasonable, leading to a final weighted average of
[Fe/H] = -0.37 +/- 0.05 (s.e.m.) for the cluster, on a scale where the Hyades
has [Fe/H] = +0.12. When combined with the abundances from DDO photometry and
from recalibrated low-resolution spectroscopy, the mean metallicity becomes
[Fe/H] = -0.32 +/- 0.03. It is also demonstrated that the average cluster
abundances based upon either DDO data or low-resolution spectroscopy are
consistently reliable to 0.05 dex or better, contrary to published attempts to
establish an open cluster metallicity scale using simplistic offset corrections
among different surveys.Comment: scheduled for Jan. 2006 AJ; 33 pages, latex, includes 7 figures and 2
table
Observation of Heteronuclear Feshbach Resonances in a Bose-Fermi Mixture
Three magnetic-field induced heteronuclear Feshbach resonances were
identified in collisions between bosonic 87Rb and fermionic 40K atoms in their
absolute ground states. Strong inelastic loss from an optically trapped mixture
was observed at the resonance positions of 492, 512, and 543 +/- 2 G. The
magnetic-field locations of these resonances place a tight constraint on the
triplet and singlet cross-species scattering lengths, yielding -281 +/- 15 Bohr
and -54 +/- 12 Bohr, respectively. The width of the loss feature at 543 G is
3.7 +/- 1.5 G wide; this broad Feshbach resonance should enable experimental
control of the interspecies interactions.Comment: revtex4 + 5 EPS figure
Measurement of the interaction strength in a Bose-Fermi mixture with 87Rb and 40K
A quantum degenerate, dilute gas mixture of bosonic and fermionic atoms was
produced using 87Rb and 40K. The onset of degeneracy was confirmed by observing
the spatial distribution of the gases after time-of-flight expansion. Further,
the magnitude of the interspecies scattering length between the doubly spin
polarized states of 87Rb and 40K, |a_RbK|, was determined from
cross-dimensional thermal relaxation. The uncertainty in this collision
measurement was greatly reduced by taking the ratio of interspecies and
intraspecies relaxation rates, yielding |a_RbK| = 250 +/- 30 a_0, which is a
lower value than what was reported in [M. Modugno et al., Phys. Rev. A 68,
043626 (2003)]. Using the value for |a_RbK| reported here, current T=0 theory
would predict a threshold for mechanical instability that is inconsistent with
the experimentally observed onset for sudden loss of fermions in [G. Modugno et
al., Science 297, 2240 (2002)].Comment: RevTeX4 + 4 eps figures; Replaced with published versio
Cross-Dimensional relaxation in Bose-Fermi mixtures
We consider the equilibration rate for fermions in Bose-Fermi mixtures
undergoing cross-dimensional rethermalization. Classical Monte Carlo
simulations of the relaxation process are performed over a wide range of
parameters, focusing on the effects of the mass difference between species and
the degree of initial departure from equilibrium. A simple analysis based on
Enskog's equation is developed and shown to be accurate over a variety of
different parameter regimes. This allows predictions for mixtures of commonly
used alkali atoms.Comment: 7 pages, 4 figures, uses Revtex 4. This is a companion paper to [PRA
70, 021601(R) (2004)] (cond-mat/0405419
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