1,773 research outputs found
Response of Bose gases in time-dependent optical superlattices
The dynamic response of ultracold Bose gases in one-dimensional optical
lattices and superlattices is investigated based on exact numerical time
evolutions in the framework of the Bose-Hubbard model. The system is excited by
a temporal amplitude modulation of the lattice potential, as it was done in
recent experiments. For regular lattice potentials, the dynamic signatures of
the superfluid to Mott-insulator transition are studied and the position and
the fine-structure of the resonances is explained by a linear response
analysis. Using direct simulations and the perturbative analysis it is shown
that in the presence of a two-colour superlattice the excitation spectrum
changes significantly when going from the homogeneous Mott-insulator the quasi
Bose-glass phase. A characteristic and experimentally accessible signature for
the quasi Bose-glass is the appearance of low-lying resonances and a
suppression of the dominant resonance of the Mott-insulator phase.Comment: 20 pages, 9 figures; added references and corrected typo
DC-readout of a signal-recycled gravitational wave detector
All first-generation large-scale gravitational wave detectors are operated at
the dark fringe and use a heterodyne readout employing radio frequency (RF)
modulation-demodulation techniques. However, the experience in the currently
running interferometers reveals several problems connected with a heterodyne
readout, of which phase noise of the RF modulation is the most serious one. A
homodyne detection scheme (DC-readout), using the highly stabilized and
filtered carrier light as local oscillator for the readout, is considered to be
a favourable alternative. Recently a DC-readout scheme was implemented on the
GEO 600 detector. We describe the results of first measurements and give a
comparison of the performance achieved with homodyne and heterodyne readout.
The implications of the combined use of DC-readout and signal-recycling are
considered.Comment: 11 page
Photon pressure induced test mass deformation in gravitational-wave detectors
A widely used assumption within the gravitational-wave community has so far
been that a test mass acts like a rigid body for frequencies in the detection
band, i.e. for frequencies far below the first internal resonance. In this
article we demonstrate that localized forces, applied for example by a photon
pressure actuator, can result in a non-negligible elastic deformation of the
test masses. For a photon pressure actuator setup used in the gravitational
wave detector GEO600 we measured that this effect modifies the standard
response function by 10% at 1 kHz and about 100% at 2.5 kHz
Bose-Fermi mixtures in 1D optical superlattices
The zero temperature phase diagram of binary boson-fermion mixtures in
two-colour superlattices is investigated. The eigenvalue problem associated
with the Bose-Fermi-Hubbard Hamiltonian is solved using an exact numerical
diagonalization technique, supplemented by an adaptive basis truncation scheme.
The physically motivated basis truncation allows to access larger systems in a
fully controlled and very flexible framework. Several experimentally relevant
observables, such as the matter-wave interference pattern and the
condensatefraction, are investigated in order to explore the rich phase
diagram. At symmetric half filling a phase similar to the Mott-insulating phase
in a commensurate purely bosonic system is identified and an analogy to recent
experiments is pointed out. Furthermore a phase of complete localization of the
bosonic species generated by the repulsive boson-fermion interaction is
identified. These localized condensates are of a different nature than the
genuine Bose-Einstein condensates in optical lattices.Comment: 18 pages, 9 figure
Triple Michelson Interferometer for a Third-Generation Gravitational Wave Detector
The upcoming European design study `Einstein gravitational-wave Telescope'
represents the first step towards a substantial, international effort for the
design of a third-generation interferometric gravitational wave detector. It is
generally believed that third-generation instruments might not be installed
into existing infrastructures but will provoke a new search for optimal
detector sites. Consequently, the detector design could be subject to fewer
constraints than the on-going design of the second generation instruments. In
particular, it will be prudent to investigate alternatives to the traditional
L-shaped Michelson interferometer. In this article, we review an old proposal
to use three Michelson interferometers in a triangular configuration. We use
this example of a triple Michelson interferometer to clarify the terminology
and will put this idea into the context of more recent research on
interferometer technologies. Furthermore the benefits of a triangular detector
will be used to motivate this design as a good starting point for a more
detailed research effort towards a third-generation gravitational wave
detector.Comment: Minor corrections to the main text and two additional appendices. 14
pages, 6 figure
The upgrade of GEO600
The German / British gravitational wave detector GEO 600 is in the process of
being upgraded. The upgrading process of GEO 600, called GEO-HF, will
concentrate on the improvement of the sensitivity for high frequency signals
and the demonstration of advanced technologies. In the years 2009 to 2011 the
detector will undergo a series of upgrade steps, which are described in this
paper.Comment: 9 pages, Amaldi 8 conference contributio
Using the etalon effect for in-situ balancing of the Advanced Virgo arm cavities
Several large-scale interferometric gravitational-wave detectors use resonant
arm cavities to enhance the light power in the interferometer arms. These
cavities are based on different optical designs: One design uses wedged input
mirrors to create additional optical pick-off ports for deriving control
signals. The second design employs input mirrors without wedge and thus offers
the possibility to use the etalon effect inside the input mirrors for tuning
the finesse of the arm cavities. In this article we introduce a concept of
maximized flexibility that combines both of these options, by featuring wedges
at the input mirrors and using the etalon effect instead in the end mirrors. We
present a design for the arm cavities of Advanced Virgo. We have used numerical
simulations to derive requirements for the manufacturing accuracy of an end
mirror etalon for Advanced Virgo. Furthermore, we give analytical
approximations for the achievable tuning range of the etalon in dependence on
the reflectance, the curvature and the orientation of the etalon back surface.Comment: 12 pages, 6 Figure
Thermal noise of folding mirrors
Current gravitational wave detectors rely on the use of Michelson interferometers. One crucial limitation of their sensitivity is the thermal noise of their optical components. Thus, for example fluctuational deformations of the mirror surface are probed by a laser beam being reflected from the mirrors at normal incidence. Thermal noise models are well evolved for that case but mainly restricted to single reflections. In this work we present the effect of two consecutive reflections under a non-normal incidence onto mirror thermal noise. This situation is inherent to detectors using a geometrical folding scheme such as GEO\,600. We revise in detail the conventional direct noise analysis scheme to the situation of non-normal incidence allowing for a modified weighting funtion of mirror fluctuations. An application of these results to the GEO\,600 folding mirror for Brownian, thermoelastic and thermorefractive noise yields an increase of displacement noise amplitude by 20\% for most noise processes. The amplitude of thermoelastic substrate noise is increased by a factor 4 due to the modified weighting function. Thus the consideration of the correct weighting scheme can drastically alter the noise predictions and demands special care in any thermal noise design process
Physical instrumental vetoes for gravitational-wave burst triggers
We present a robust strategy to \emph{veto} certain classes of instrumental
glitches that appear at the output of interferometric gravitational-wave (GW)
detectors.This veto method is `physical' in the sense that, in order to veto a
burst trigger, we make use of our knowledge of the coupling of different
detector subsystems to the main detector output. The main idea behind this
method is that the noise in an instrumental channel X can be \emph{transferred}
to the detector output (channel H) using the \emph{transfer function} from X to
H, provided the noise coupling is \emph{linear} and the transfer function is
\emph{unique}. If a non-stationarity in channel H is causally related to one in
channel X, the two have to be consistent with the transfer function. We
formulate two methods for testing the consistency between the burst triggers in
channel X and channel H. One method makes use of the \emph{null-stream}
constructed from channel H and the \emph{transferred} channel X, and the second
involves cross-correlating the two. We demonstrate the efficiency of the veto
by `injecting' instrumental glitches in the hardware of the GEO 600 detector.
The \emph{veto safety} is demonstrated by performing GW-like hardware
injections. We also show an example application of this method using 5 days of
data from the fifth science run of GEO 600. The method is found to have very
high veto efficiency with a very low accidental veto rate.Comment: Minor changes, To appear in Phys. Rev.
Self-similar chain conformations in polymer gels
We use molecular dynamics simulations to study the swelling of randomly
end-cross-linked polymer networks in good solvent conditions. We find that the
equilibrium degree of swelling saturates at Q_eq = N_e**(3/5) for mean strand
lengths N_s exceeding the melt entanglement length N_e. The internal structure
of the network strands in the swollen state is characterized by a new exponent
nu=0.72. Our findings are in contradiction to de Gennes' c*-theorem, which
predicts Q_eq proportional N_s**(4/5) and nu=0.588. We present a simple Flory
argument for a self-similar structure of mutually interpenetrating network
strands, which yields nu=7/10 and otherwise recovers the classical Flory-Rehner
theory. In particular, Q_eq = N_e**(3/5), if N_e is used as effective strand
length.Comment: 4 pages, RevTex, 3 Figure
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