21,894 research outputs found
Structure formation by cosmic strings with a cosmological constant
Final published version.Comment: 4 Page
Some Remarks on Quantum Coherence
There are many striking phenomena which are attributed to
``quantum coherence''. It is natural to wonder if there are new quantum
coherence effects waiting to be discovered which could lead to interesting
results and perhaps even practical applications. A useful starting point for
such discussions is a definition of ``quantum coherence''. In this article I
give a definition of quantum coherence and use a number of illustrations to
explore the implications of this definition. I point to topics of current
interest in the fields of cosmology and quantum computation where questions of
quantum coherence arise, and I emphasize the impact that interactions with the
environment can have on quantum coherence.Comment: 25 pages plain LaTeX, no figures. More references have been added and
typos have been corrected. Journal of Modern Optics, in press.
Imperial/TP/93-94/1
A master relation defines the nonlinear viscoelasticity of single fibroblasts
Cell mechanical functions like locomotion, contraction and division are
controlled by the cytoskeleton, a dynamic biopolymer network whose mechanical
properties remain poorly understood. We perform single-cell uniaxial stretching
experiments on 3T3 fibroblasts. By superimposing small amplitude oscillations
on a mechanically prestressed cell, we find a transition from linear
viscoelastic behavior to power-law stress stiffening. Data from different cells
over several stress decades can be uniquely scaled to obtain a master-relation
between the viscoelastic moduli and the average force. Remarkably, this
relation holds independently of deformation history, adhesion biochemistry, and
intensity of active contraction. In particular, it is irrelevant whether force
is actively generated by the cell or externally imposed by stretching. We
propose that the master-relation reflects the mechanical behavior of the force
bearing actin cytoskeleton, in agreement with stress stiffening known from
semiflexible filament networks.Comment: 12 pages, 11 figures. Accepted for publication in Biophysical
Journal, scheduled to appear in May 200
Large-scale variability in marine stratocumulus clouds defined from simultaneous aircraft and satellite measurements
Satellite images often show significant variations in the structure of marine stratocumulus clouds on scales ranging from 10 to 1000 km. This is illustrated where a GOES West satellite image shows a well-defined variation in cloud structure near 32 N, 122 W on 30 June 1987. Aircraft measurements were made with the UK C-130 and the NCAR Electra on this day as part of the FIRE Marine Stratocumulus Intensive Field Observations (IFO). The mean, turbulent, and the microphysical structure of the clouds sampled in these two areas are compared an an attempt is made to explain the differences in cloud structure. In an attempt to identify any systematic differences between the measurements made with the two aircraft, data were analyzed that were collected on 14 July 1987 with the C-130 and the Electra flying in close formation at an altitude of 250 m. The microphysical and turbulence data are being compared in an attempt to explain the differences in the cloud liquid water content obtained with the two aircraft and the differences in cloud structure shown by the GOES image. In addition, data are being analyzed for three other days during the experiment when coordinated downstream flights were made with the Electra and the C-130
Offset frequency dynamics and phase noise properties of a self-referenced 10 GHz Ti:sapphire frequency comb
This paper shows the experimental details of the stabilization scheme that
allows full control of the repetition rate and the carrier-envelope offset
frequency of a 10 GHz frequency comb based on a femtosecond Ti:sapphire laser.
Octave-spanning spectra are produced in nonlinear microstructured optical
fiber, in spite of the reduced peak power associated with the 10 GHz repetition
rate. Improved stability of the broadened spectrum is obtained by
temperature-stabilization of the nonlinear optical fiber. The carrier-envelope
offset frequency and the repetition rate are simultaneously frequency
stabilized, and their short- and long-term stabilities are characterized. We
also measure the transfer of amplitude noise of the pump source to phase noise
on the offset frequency and verify an increased sensitivity of the offset
frequency to pump power modulation compared to systems with lower repetition
rate. Finally, we discuss merits of this 10 GHz system for the generation of
low-phase-noise microwaves
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