391 research outputs found
User-System Dialogues and the Notion of Focus
In recent years, the capabilities of knowledge-based systems to communicate with their users have evolved from simple interactions to complex dialogues. With this evolution comes a need to understand what makes a good dialogue. In this paper, we are concerned with dialogue coherence. We review the notion of focus, which partly explains this property, and its use for user-system communication. First, we examine the major theories dealing with this notion. We describe what their contribution is and how they differ. Then, we illustrate the benefits of using the notion of focus and especially the improvement in text coherence. We pay particular attention to how the notion can concretely be implemented. Its integration with other techniques and theories is described. We conclude the paper by pointing out remaining issues in the understanding of the notion of focus. The contribution of this paper is to provide a classification of the theories of focus and to show the improvements they offer in elaborate user-system dialogues.</jats:p
Research activities arising from the University of Kent
In this paper I describe research activities in the field of optical fiber sensing undertaken by me after leaving the Applied Optics Group at the University of Kent. The main topics covered are long period gratings, neural network based signal processing, plasmonic sensors, and polymer fiber gratings. I also give a summary of my two periods of research at the University of Kent, covering 1985–1988 and 1991–2001
Point absorbers in Advanced LIGO
Small, highly absorbing points are randomly present on the surfaces of the main interferometer optics in Advanced LIGO. The resulting nano-meter scale thermo-elastic deformations and substrate lenses from these micron-scale absorbers significantly reduces the sensitivity of the interferometer directly though a reduction in the power-recycling gain and indirect interactions with the feedback control system. We review the expected surface deformation from point absorbers and provide a pedagogical description of the impact on power build-up in second generation gravitational wave detectors (dual-recycled Fabry-Perot Michelson interferometers). This analysis predicts that the power-dependent reduction in interferometer performance will significantly degrade maximum stored power by up to 50% and hence, limit GW sensitivity, but suggests system wide corrections that can be implemented in current and future GW detectors. This is particularly pressing given that future GW detectors call for an order of magnitude more stored power than currently used in Advanced LIGO in Observing Run 3. We briefly review strategies to mitigate the effects of point absorbers in current and future GW wave detectors to maximize the success of these enterprises
LIGO’s quantum response to squeezed states
Gravitational Wave interferometers achieve their profound sensitivity by combining a Michelson interferometer with optical cavities, suspended masses, and now, squeezed quantum states of light. These states modify the measurement process of the LIGO, VIRGO and GEO600 interferometers to reduce the quantum noise that masks astrophysical signals; thus, improvements to squeezing are essential to further expand our gravitational view of the universe. Further reducing quantum noise will require both lowering decoherence from losses as well more sophisticated manipulations to counter the quantum back-action from radiation pressure. Both tasks require fully understanding the physical interactions between squeezed light and the many components of km-scale interferometers. To this end, data from both LIGO observatories in observing run three are expressed using frequency-dependent metrics to analyze each detector's quantum response to squeezed states. The response metrics are derived and used to concisely describe physical mechanisms behind squeezing's simultaneous interaction with transverse-mode selective optical cavities and the quantum radiation pressure noise of suspended mirrors. These metrics and related analysis are broadly applicable for cavity-enhanced optomechanics experiments that incorporate external squeezing, and -- for the first time -- give physical descriptions of every feature so far observed in the quantum noise of the LIGO detectors
Point absorbers in Advanced LIGO
Small, highly absorbing points are randomly present on the surfaces of the main interferometer optics in Advanced LIGO. The resulting nanometer scale thermo-elastic deformations and substrate lenses from these micron-scale absorbers significantly reduce the sensitivity of the interferometer directly though a reduction in the power-recycling gain and indirect interactions with the feedback control system. We review the expected surface deformation from point absorbers and provide a pedagogical description of the impact on power buildup in second generation gravitational wave detectors (dual-recycled Fabry–Perot Michelson interferometers). This analysis predicts that the power-dependent reduction in interferometer performance will significantly degrade maximum stored power by up to 50% and, hence, limit GW sensitivity, but it suggests system wide corrections that can be implemented in current and future GW detectors. This is particularly pressing given that future GW detectors call for an order of magnitude more stored power than currently used in Advanced LIGO in Observing Run 3. We briefly review strategies to mitigate the effects of point absorbers in current and future GW wave detectors to maximize the success of these enterprises
Sensitivity and performance of the Advanced LIGO detectors in the third observing run
On April 1st, 2019, the Advanced Laser Interferometer Gravitational-Wave Observatory (aLIGO), joined by the Advanced Virgo detector, began the third observing run, a year-long dedicated search for gravitational radiation. The LIGO detectors have achieved a higher duty cycle and greater sensitivity to gravitational waves than ever before, with LIGO Hanford achieving angle-averaged sensitivity to binary neutron star coalescences to a distance of 111 Mpc, and LIGO Livingston to 134 Mpc with duty factors of 74.6% and 77.0% respectively. The improvement in sensitivity and stability is a result of several upgrades to the detectors, including doubled intracavity power, the addition of an in-vacuum optical parametric oscillator for squeezed-light injection, replacement of core optics and end reaction masses, and installation of acoustic mode dampers. This paper explores the purposes behind these upgrades, and explains to the best of our knowledge the noise currently limiting the sensitivity of each detector
Approaching the motional ground state of a 10 kg object
The motion of a mechanical object -- even a human-sized object -- should be
governed by the rules of quantum mechanics. Coaxing them into a quantum state
is, however, difficult: the thermal environment masks any quantum signature of
the object's motion. Indeed, the thermal environment also masks effects of
proposed modifications of quantum mechanics at large mass scales. We prepare
the center-of-mass motion of a 10 kg mechanical oscillator in a state with an
average phonon occupation of 10.8. The reduction in temperature, from room
temperature to 77 nK, is commensurate with an 11 orders-of-magnitude
suppression of quantum back-action by feedback -- and a 13 orders-of-magnitude
increase in the mass of an object prepared close to its motional ground state.
This begets the possibility of probing gravity on massive quantum systems.Comment: published version containing minor change
Quantum correlations between the light and kilogram-mass mirrors of LIGO
Measurement of minuscule forces and displacements with ever greater precision
encounters a limit imposed by a pillar of quantum mechanics: the Heisenberg
uncertainty principle. A limit to the precision with which the position of an
object can be measured continuously is known as the standard quantum limit
(SQL). When light is used as the probe, the SQL arises from the balance between
the uncertainties of photon radiation pressure imposed on the object and of the
photon number in the photoelectric detection. The only possibility surpassing
the SQL is via correlations within the position/momentum uncertainty of the
object and the photon number/phase uncertainty of the light it reflects. Here,
we experimentally prove the theoretical prediction that this type of quantum
correlation is naturally produced in the Laser Interferometer
Gravitational-wave Observatory (LIGO). Our measurements show that the quantum
mechanical uncertainties in the phases of the 200 kW laser beams and in the
positions of the 40 kg mirrors of the Advanced LIGO detectors yield a joint
quantum uncertainty a factor of 1.4 (3dB) below the SQL. We anticipate that
quantum correlations will not only improve gravitational wave (GW)
observatories but all types of measurements in future
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