733 research outputs found
Narrowing the filter cavity bandwidth via optomechanical interaction
We propose using optomechanical interaction to narrow the bandwidth of filter
cavities for achieving frequency-dependent squeezing in advanced
gravitational-wave detectors, inspired by the idea of optomechanically induced
transparency. This not only allows us to achieve narrow bandwidth, comparable
to the detection band of few hundred Hz, with tabletop optical cavities, but
also to tune the bandwidth over a wide range, which is ideal for optimizing
sensitivity for different gravitational-wave sources. The experimental
challenge for its implementation is the stringent requirement on low thermal
noise, which would need superb mechanical quality factor that is quite
difficult to achieve by using currently-available low-loss mechanical
oscillators; one possible solution is to use optical dilution of the mechanical
damping, which can considerably relax the requirement on the mechanics.Comment: 5 pages + 3 appendix. 4 figures and 2 tables Accepted by Physical
Review Letter
Passive, free-space heterodyne laser gyroscope
Laser gyroscopes making use of the Sagnac effect have been used as highly accurate rotation sensors for many years. First used in aerospace and defense applications, these devices have more recently been used for precision seismology and in other research settings. In particular, mid-sized (~1 m-scale) laser gyros have been under development as tilt sensors to augment the adaptive active seismic isolation systems in terrestrial interferometric gravitational wave detectors. The most prevalent design is the 'active' gyroscope, in which the optical ring cavity used to measure the Sagnac degeneracy breaking is itself a laser resonator. In this article, we describe another topology: a 'passive' gyroscope, in which the sensing cavity is not itself a laser but is instead tracked using external laser beams. While subject to its own limitations, this design is free from the deleterious lock-in effects observed in active systems, and has the advantage that it can be constructed using commercially available components. We demonstrate that our device achieves comparable sensitivity to those of similarly sized active laser gyroscopes
Noncovalent Interactions by QMC: Speedup by One-Particle Basis-Set Size Reduction
While it is empirically accepted that the fixed-node diffusion Monte-Carlo
(FN-DMC) depends only weakly on the size of the one-particle basis sets used to
expand its guiding functions, limits of this observation are not settled yet.
Our recent work indicates that under the FN error cancellation conditions,
augmented triple zeta basis sets are sufficient to achieve a benchmark level of
0.1 kcal/mol in a number of small noncovalent complexes. Here we report on a
possibility of truncation of the one-particle basis sets used in FN-DMC guiding
functions that has no visible effect on the accuracy of the production FN-DMC
energy differences. The proposed scheme leads to no significant increase in the
local energy variance, indicating that the total CPU cost of large-scale
benchmark noncovalent interaction energy FN-DMC calculations may be reduced.Comment: ACS book chapter, accepte
A Mercury Lander Mission Concept Study for the Next Decadal Survey
Mariner 10 provided our first closeup reconnaissance of Mercury during its three flybys in 1974 and 1975. MESSENGERs 20112015 orbital investigation enabled numerous discoveries, several of which led to substantial or complete changes in our fundamental understanding of the planet. Among these were the unanticipated, widespread presence of volatile elements (e.g., Na, K, S); a surface with extremely low Fe abundance whose darkening agent is likely C; a previously unknown landformhollows that may form by volatile sublimation from within rocks exposed to the harsh conditions on the surface; a history of expansive effusive and explosive volcanism; substantial radial contraction of the planet from interior cooling; offset of the dipole moment of the internal magnetic field northward from the geographic equator by ~20% of the planets radius; crustal magnetization, attributed at least in part to an ancient field; unexpected seasonal variability and relationships among exospheric species and processes; and the presence in permanently shadowed polar terrain of water ice and other volatile materials, likely to include complex organic compounds. Mercurys highly chemically reduced and unexpectedly volatile-rich composition is unique among the terrestrial planets and was not predicted by earlier hypotheses for the planets origin. As an end-member of terrestrial planet formation, Mercury holds unique clues about the original distribution of elements in the earliest stages of the Solar System and how planets (and exoplanets) form and evolve in close proximity to their host stars. The BepiColombo mission promises to expand our knowledge of this planet and to shed light on some of the mysteries revealed by the MESSENGER mission. However, several fundamental science questions raised by MESSENGERs pioneering exploration of Mercury can only be answered with in situ measurements from the planets surface
In situ evidence for the structure of the magnetic null in a 3D reconnection event in the Earth's magnetotail
Magnetic reconnection is one of the most important processes in
astrophysical, space and laboratory plasmas. Identifying the structure around
the point at which the magnetic field lines break and subsequently reform,
known as the magnetic null point, is crucial to improving our understanding
reconnection. But owing to the inherently three-dimensional nature of this
process, magnetic nulls are only detectable through measurements obtained
simultaneously from at least four points in space. Using data collected by the
four spacecraft of the Cluster constellation as they traversed a diffusion
region in the Earth's magnetotail on 15 September, 2001, we report here the
first in situ evidence for the structure of an isolated magnetic null. The
results indicate that it has a positive-spiral structure whose spatial extent
is of the same order as the local ion inertial length scale, suggesting that
the Hall effect could play an important role in 3D reconnection dynamics.Comment: 14 pages, 4 figure
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