7,185 research outputs found
Optically driven ultra-stable nanomechanical rotor
Nanomechanical devices have attracted the interest of a growing
interdisciplinary research community, since they can be used as highly
sensitive transducers for various physical quantities. Exquisite control over
these systems facilitates experiments on the foundations of physics. Here, we
demonstrate that an optically trapped silicon nanorod, set into rotation at MHz
frequencies, can be locked to an external clock, transducing the properties of
the time standard to the rod's motion with the remarkable frequency stability
of . While the dynamics of
this periodically driven rotor generally can be chaotic, we derive and verify
that stable limit cycles exist over a surprisingly wide parameter range. This
robustness should enable, in principle, measurements of external torques with
sensitivities better than 0.25zNm, even at room temperature. We show that in a
dilute gas, real-time phase measurements on the locked nanorod transduce
pressure values with a sensitivity of 0.3%.Comment: 5 pages, 4 figure
Progress in Atomic Fountains at LNE-SYRTE
We give an overview of the work done with the Laboratoire National de
M\'etrologie et d'Essais-Syst\`emes de R\'ef\'erence Temps-Espace (LNE-SYRTE)
fountain ensemble during the last five years. After a description of the clock
ensemble, comprising three fountains, FO1, FO2, and FOM, and the newest
developments, we review recent studies of several systematic frequency shifts.
This includes the distributed cavity phase shift, which we evaluate for the FO1
and FOM fountains, applying the techniques of our recent work on FO2. We also
report calculations of the microwave lensing frequency shift for the three
fountains, review the status of the blackbody radiation shift, and summarize
recent experimental work to control microwave leakage and spurious phase
perturbations. We give current accuracy budgets. We also describe several
applications in time and frequency metrology: fountain comparisons,
calibrations of the international atomic time, secondary representation of the
SI second based on the 87Rb hyperfine frequency, absolute measurements of
optical frequencies, tests of the T2L2 satellite laser link, and review
fundamental physics applications of the LNE-SYRTE fountain ensemble. Finally,
we give a summary of the tests of the PHARAO cold atom space clock performed
using the FOM transportable fountain.Comment: 19 pages, 12 figures, 5 tables, 126 reference
Modelling delta-notch perturbations during zebrafish somitogenesis
The discovery over the last 15 years of molecular clocks and gradients in the pre-somitic mesoderm of numerous vertebrate species has added significant weight to Cooke and Zeeman's âclock and wavefrontâ model of somitogenesis, in which a travelling wavefront determines the spatial position of somite formation and the somitogenesis clock controls periodicity (Cooke and Zeeman, 1976). However, recent high-throughput measurements of spatiotemporal patterns of gene expression in different zebrafish mutant backgrounds allow further quantitative evaluation of the clock and wavefront hypothesis. In this study we describe how our recently proposed model, in which oscillator coupling drives the propagation of an emergent wavefront, can be used to provide mechanistic and testable explanations for the following observed phenomena in zebrafish embryos: (a) the variation in somite measurements across a number of zebrafish mutants; (b) the delayed formation of somites and the formation of âsalt and pepperâ patterns of gene expression upon disruption of oscillator coupling; and (c) spatial correlations in the âsalt and pepperâ patterns in Delta-Notch mutants. In light of our results, we propose a number of plausible experiments that could be used to further test the model
From segment to somite: segmentation to epithelialization analyzed within quantitative frameworks
One of the most visually striking patterns in the early developing embryo is somite segmentation. Somites form as repeated, periodic structures in pairs along nearly the entire caudal vertebrate axis. The morphological process involves short- and long-range signals that drive cell rearrangements and cell shaping to create discrete, epithelialized segments. Key to developing novel strategies to prevent somite birth defects that involve axial bone and skeletal muscle development is understanding how the molecular choreography is coordinated across multiple spatial scales and in a repeating temporal manner. Mathematical models have emerged as useful tools to integrate spatiotemporal data and simulate model mechanisms to provide unique insights into somite pattern formation. In this short review, we present two quantitative frameworks that address the morphogenesis from segment to somite and discuss recent data of segmentation and epithelialization
The Time-Energy Uncertainty Relation
The time energy uncertainty relation has been a controversial issue since the
advent of quantum theory, with respect to appropriate formalisation, validity
and possible meanings. A comprehensive account of the development of this
subject up to the 1980s is provided by a combination of the reviews of Jammer
(1974), Bauer and Mello (1978), and Busch (1990). More recent reviews are
concerned with different specific aspects of the subject. The purpose of this
chapter is to show that different types of time energy uncertainty relation can
indeed be deduced in specific contexts, but that there is no unique universal
relation that could stand on equal footing with the position-momentum
uncertainty relation. To this end, we will survey the various formulations of a
time energy uncertainty relation, with a brief assessment of their validity,
and along the way we will indicate some new developments that emerged since the
1990s.Comment: 33 pages, Latex. This expanded version (prepared for the 2nd edition
of "Time in quantum mechanics") contains minor corrections, new examples and
pointers to some additional relevant literatur
A noise-immune cavity-assisted non-destructive detection for an optical lattice clock in the quantum regime
We present and implement a non-destructive detection scheme for the
transition probability readout of an optical lattice clock. The scheme relies
on a differential heterodyne measurement of the dispersive properties of
lattice-trapped atoms enhanced by a high finesse cavity. By design, this scheme
offers a 1st order rejection of the technical noise sources, an enhanced
signal-to-noise ratio, and an homogeneous atom-cavity coupling. We
theoretically show that this scheme is optimal with respect to the photon shot
noise limit. We experimentally realize this detection scheme in an operational
strontium optical lattice clock. The resolution is on the order of a few atoms
with a photon scattering rate low enough to keep the atoms trapped after
detection. This scheme opens the door to various different interrogations
protocols, which reduce the frequency instability, including atom recycling,
zero-dead time clocks with a fast repetition rate, and sub quantum projection
noise frequency stability
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