1,691 research outputs found
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
Review of the Laguerre-Gauss mode technology research program at Birmingham
Gravitational wave detectors from the advanced generation onwards are
expected to be limited in sensitivity by thermal noise of the optics, making
the reduction of this noise a key factor in the success of such detectors. A
proposed method for reducing the impact of this noise is to use higher-order
Laguerre-Gauss (LG) modes for the readout beam, as opposed to the currently
used fundamental mode. We present here a synopsis of the research program
undertaken by the University of Birmingham into the suitability of LG mode
technology for future gravitational wave detectors. This will cover our
previous and current work on this topic, from initial simulations and table-top
LG mode experiments up to implementation in a prototype scale suspended cavity
and high-power laser bench
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
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
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
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
Stochastic Gravitational Wave Background from Coalescing Binary Black Holes
We estimate the stochastic gravitational wave (GW) background signal from the
field population of coalescing binary stellar mass black holes (BHs) throughout
the Universe. This study is motivated by recent observations of BH-Wolf-Rayet
star systems and by new estimates in the metallicity abundances of star forming
galaxies that imply BH-BH systems are more common than previously assumed.
Using recent analytical results of the inspiral-merger-ringdown waveforms for
coalescing binary BH systems, we estimate the resulting stochastic GW
background signal. Assuming average quantities for the single source energy
emissions, we explore the parameter space of chirp mass and local rate density
required for detection by advanced and third generation interferometric GW
detectors. For an average chirp mass of 8.7, we find that detection
through 3 years of cross-correlation by two advanced detectors will require a
rate density, . Combining data from
multiple pairs of detectors can reduce this limit by up to 40%. Investigating
the full parameter space we find that detection could be achieved at rates for populations of coalescing binary BH
systems with average chirp masses of which are predicted by
recent studies of BH-Wolf-Rayet star systems. While this scenario is at the
high end of theoretical estimates, cross-correlation of data by two Einstein
Telescopes could detect this signal under the condition . Such a signal could potentially mask a primordial
GW background signal of dimensionless energy density, , around the (1--500) Hz frequency range.Comment: 22 pages, 5 figures, 2 tables, Accepted for publication by Ap
Microelectromechanical system gravimeters as a new tool for gravity imaging
A microelectromechanical system (MEMS) gravimeter has been manufactured with a sensitivity of 40 ppb in an integration time of 1 s. This sensor has been used to measure the Earth tides: the elastic deformation of the globe due to tidal forces. No such measurement has been demonstrated before now with a MEMS gravimeter. Since this measurement, the gravimeter has been miniaturized and tested in the field. Measurements of the free-air and Bouguer effects have been demonstrated by monitoring the change in gravitational acceleration measured while going up and down a lift shaft of 20.7 m, and up and down a local hill of 275 m. These tests demonstrate that the device has the potential to be a useful field-portable instrument. The development of an even smaller device is underway, with a total package size similar to that of a smartphone
Quantum noise of non-ideal Sagnac speed meter interferometer with asymmetries
The speed meter concept has been identified as a technique that can
potentially provide laser-interferometric measurements at a sensitivity level
which surpasses the Standard Quantum Limit (SQL) over a broad frequency range.
As with other sub-SQL measurement techniques, losses play a central role in
speed meter interferometers and they ultimately determine the quantum noise
limited sensitivity that can be achieved. So far in the literature, the quantum
noise limited sensitivity has only been derived for lossless or lossy cases
using certain approximations (for instance that the arm cavity round trip loss
is small compared to the arm cavity mirror transmission). In this article we
present a generalised, analytical treatment of losses in speed meters that
allows accurate calculation of the quantum noise limited sensitivity of Sagnac
speed meters with arm cavities. In addition, our analysis allows us to take
into account potential imperfections in the interferometer such as an
asymmetric beam splitter or differences of the reflectivities of the two arm
cavity input mirrors. Finally,we use the examples of the proof-of-concept
Sagnac speed meter currently under construction in Glasgow and a potential
implementation of a Sagnac speed meter in the Einstein Telescope (ET) to
illustrate how our findings affect Sagnac speed meters with meter- and
kilometre-long baselines.Comment: 22 pages, 8 figures, 1 table, (minor corrections and changes made to
text and figures in version 2
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
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