1,042 research outputs found
Dynamos with weakly convecting outer layers: implications for core-mantle boundary interaction
Convection in the Earth's core is driven much harder at the bottom than the top. This is partly because the adiabatic gradient steepens towards the top, partly because the spherical geometry means the area involved increases towards the top, and partly because compositional convection is driven by light material released at the lower boundary and remixed uniformly throughout the outer core, providing a volumetric sink of buoyancy. We have therefore investigated dynamo action of thermal convection in a Boussinesq fluid contained within a rotating spherical shell driven by a combination of bottom and internal heating or cooling. We first apply a homogeneous temperature on the outer boundary in order to explore the effects of heat sinks on dynamo action; we then impose an inhomogeneous temperature proportional to a single spherical harmonic Y2² in order to explore core-mantle interactions. With homogeneous boundary conditions and moderate Rayleigh numbers, a heat sink reduces the generated magnetic field appreciably; the magnetic Reynolds number remains high because the dominant toroidal component of flow is not reduced significantly. The dipolar structure of the field becomes more pronounced as found by other authors. Increasing the Rayleigh number yields a regime in which convection inside the tangent cylinder is strongly affected by the magnetic field. With inhomogeneous boundary conditions, a heat sink promotes boundary effects and locking of the magnetic field to boundary anomalies. We show that boundary locking is inhibited by advection of heat in the outer regions. With uniform heating, the boundary effects are only significant at low Rayleigh numbers, when dynamo action is only possible for artificially low magnetic diffusivity. With heat sinks, the boundary effects remain significant at higher Rayleigh numbers provided the convection remains weak or the fluid is stably stratified at the top. Dynamo action is driven by vigorous convection at depth while boundary thermal anomalies dominate in the upper regions. This is a likely regime for the Earth's core
Phase diffusion pattern in quantum nondemolition systems
We quantitatively analyze the dynamics of the quantum phase distribution
associated with the reduced density matrix of a system, as the system evolves
under the influence of its environment with an energy-preserving quantum
nondemolition (QND) type of coupling. We take the system to be either an
oscillator (harmonic or anharmonic) or a two-level atom (or equivalently, a
spin-1/2 system), and model the environment as a bath of harmonic oscillators,
initially in a general squeezed thermal state. The impact of the different
environmental parameters is explicitly brought out as the system starts out in
various initial states. The results are applicable to a variety of physical
systems now studied experimentally with QND measurements.Comment: 18 pages, REVTeX, 8 figure
Dual-Resonator Speed Meter for a Free Test Mass
A description and analysis are given of a ``speed meter'' for monitoring a
classical force that acts on a test mass. This speed meter is based on two
microwave resonators (``dual resonators''), one of which couples evanescently
to the position of the test mass. The sloshing of the resulting signal between
the resonators, and a wise choice of where to place the resonators' output
waveguide, produce a signal in the waveguide that (for sufficiently low
frequencies) is proportional to the test-mass velocity (speed) rather than its
position. This permits the speed meter to achieve force-measurement
sensitivities better than the standard quantum limit (SQL), both when operating
in a narrow-band mode and a wide-band mode. A scrutiny of experimental issues
shows that it is feasible, with current technology, to construct a
demonstration speed meter that beats the wide-band SQL by a factor 2. A concept
is sketched for an adaptation of this speed meter to optical frequencies; this
adaptation forms the basis for a possible LIGO-III interferometer that could
beat the gravitational-wave standard quantum limit h_SQL, but perhaps only by a
factor 1/xi = h_SQL/h ~ 3 (constrained by losses in the optics) and at the
price of a very high circulating optical power --- larger by 1/xi^2 than that
required to reach the SQL.Comment: RevTex: 13 pages with 4 embedded figures (two .eps format and two
drawn in TeX); Submitted to Physical Review
Schroedinger Cat States of a Nanomechanical Resonator
We present a scheme of generating large-amplitude Schr\"{o}dinger cat states
and entanglement in a coupled system of nanomechanical resonator and single
Cooper pair box (SCPB), without being limited by the magnitude of the coupling.
It is shown that the entanglement between the resonator and the SCPB can be
detected by a spectroscopic method.Comment: 1 figur
Persistent Rabi oscillations probed via low-frequency noise correlation
The qubit Rabi oscillations are known to be non-decaying (though with a
fluctuating phase) if the qubit is continuously monitored in the weak-coupling
regime. In this paper we propose an experiment to demonstrate these persistent
Rabi oscillations via low-frequency noise correlation. The idea is to measure a
qubit by two detectors, biased stroboscopically at the Rabi frequency. The
low-frequency noise depends on the relative phase between the two combs of
biasing pulses, with a strong increase of telegraph noise in both detectors for
the in-phase or anti-phase combs. This happens because of self-synchronization
between the persistent Rabi oscillations and measurement pulses. Almost perfect
correlation of the noise in the two detectors for the in-phase regime and
almost perfect anticorrelation for the anti-phase regime indicates a presence
of synchronized persistent Rabi oscillations. The experiment can be realized
with semiconductor or superconductor qubits.Comment: 5 page
Superconducting re-entrant cavity transducer for a resonant bar gravitational radiation antenna
Copyright @ American Institute of PhysicsA 10‐GHz superconducting niobium re‐entrant cavity parametric transducer was developed for use in a cryogenic 1.5‐tonne Nb resonant bar gravitational radiation antenna. The transducer has a very high electrical Q (6×105 at 4.2 K), and was operated at high cavity fields without degrading the Q. A very high electromechanical coupling between the antenna and the transducer was therefore achieved. The highest coupling attained, constrained by the available pump power, was 0.11. If the transducer were to be operated in conjunction with a wideband impedance matching element, an antenna bandwidth comparable to the frequency of the antenna would be attained. The temperature dependence of the Q of the transducer was in good agreement with theory. At temperatures above about 6 K the Q was degraded by the increase in the BCS surface resistance, while at lower temperatures the Q was limited by radiative losses
High-sensitivity force measurement using entangled probes
We show the possibility to improve the measurement sensitivity of a weak
force by using two meters in an entangled state. This latter can be achieved by
exploiting radiation pressure effects.Comment: ReVTeX file, 11 pages, 2 eps figure
Increasing future gravitational-wave detectors sensitivity by means of amplitude filter cavities and quantum entanglement
The future laser interferometric gravitational-wave detectors sensitivity can
be improved using squeezed light. In particular, recently a scheme which uses
the optical field with frequency dependent squeeze factor, prepared by means of
a relatively short (~30 m) amplitude filter cavity, was proposed
\cite{Corbitt2004-3}. Here we consider an improved version of this scheme,
which allows to further reduce the quantum noise by exploiting the quantum
entanglement between the optical fields at the filter cavity two ports.Comment: 10 pages, 7 figure
Minimum Length from Quantum Mechanics and Classical General Relativity
We derive fundamental limits on measurements of position, arising from
quantum mechanics and classical general relativity. First, we show that any
primitive probe or target used in an experiment must be larger than the Planck
length, . This suggests a Planck-size {\it minimum ball} of uncertainty in
any measurement. Next, we study interferometers (such as LIGO) whose precision
is much finer than the size of any individual components and hence are not
obviously limited by the minimum ball. Nevertheless, we deduce a fundamental
limit on their accuracy of order . Our results imply a {\it device
independent} limit on possible position measurements.Comment: 8 pages, latex, to appear in the Physical Review Letter
Pendulum Mode Thermal Noise in Advanced Interferometers: A comparison of Fused Silica Fibers and Ribbons in the Presence of Surface Loss
The use of fused-silica ribbons as suspensions in gravitational wave
interferometers can result in significant improvements in pendulum mode thermal
noise. Surface loss sets a lower bound to the level of noise achievable, at
what level depends on the dissipation depth and other physical parameters. For
LIGO II, the high breaking strength of pristine fused silica filaments, the
correct choice of ribbon aspect ratio (to minimize thermoelastic damping), and
low dissipation depth combined with the other achievable parameters can reduce
the pendulum mode thermal noise in a ribbon suspension well below the radiation
pressure noise. Despite producing higher levels of pendulum mode thermal noise,
cylindrical fiber suspensions provide an acceptable alternative for LIGO II,
should unforeseen problems with ribbon suspensions arise.Comment: Submitted to Physics Letters A (Dec. 14, 1999). Resubmitted to
Physics Letters A (Apr. 3, 2000) after internal (LSC) review process. PACS -
04.80.Nn, 95.55.Ym, 05.40.C
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