434 research outputs found
Photoionization Broadening of the 1S-2S Transition in a Beam of Atomic Hydrogen
We consider the excitation dynamics of the two-photon \sts transition in a
beam of atomic hydrogen by 243 nm laser radiation. Specifically, we study the
impact of ionization damping on the transition line shape, caused by the
possibility of ionization of the 2S level by the same laser field. Using a
Monte-Carlo simulation, we calculate the line shape of the \sts transition for
the experimental geometry used in the two latest absolute frequency
measurements (M. Niering {\it et al.}, PRL 84, 5496 (2000) and M. Fischer {\it
et al.}, PRL 92, 230802 (2004)). The calculated line shift and line width are
in excellent agreement with the experimentally observed values. From this
comparison we can verify the values of the dynamic Stark shift coefficient for
the \sts transition for the first time on a level of 15%. We show that the
ionization modifies the velocity distribution of the metastable atoms, the line
shape of the \sts transition, and has an influence on the derivation of its
absolute frequency.Comment: 10 pages, 5 figure
Measurement noise floor for a long-distance optical carrier transmission via fiber
We investigated the measurement floor and link stability for the transfer of
an ultra-stable optical frequency via an optical fiber link. We achieved a
near-delay-limited instability of 3x10^(-15)/(tau x Hz) for 147 km deployed
fiber, and 10^(-20) (integrations time tau = 4000 s) for the noise floor.Comment: 5 pages, 3 figures, 7th Symposium on Frequency Standards and
Metrology (Pacific Grove,USA, Oct 2008
New Limits to the Drift of Fundamental Constants from Laboratory Measurements
We have remeasured the absolute - transition frequency in atomic hydrogen. A comparison with the result of the previous
measurement performed in 1999 sets a limit of Hz for the drift of
with respect to the ground state hyperfine splitting in Cs. Combining this result with the recently published
optical transition frequency in Hg against and a
microwave Rb and Cs clock comparison, we deduce separate limits
on yr and the
fractional time variation of the ratio of Rb and Cs nuclear magnetic moments
equal to
yr. The latter provides information on the temporal behavior of the
constant of strong interaction.Comment: 4 pages, 3 figures, LaTe
Optical Clocks in Space
The performance of optical clocks has strongly progressed in recent years,
and accuracies and instabilities of 1 part in 10^18 are expected in the near
future. The operation of optical clocks in space provides new scientific and
technological opportunities. In particular, an earth-orbiting satellite
containing an ensemble of optical clocks would allow a precision measurement of
the gravitational redshift, navigation with improved precision, mapping of the
earth's gravitational potential by relativistic geodesy, and comparisons
between ground clocks.Comment: Proc. III International Conference on Particle and Fundamental
Physics in Space (SpacePart06), Beijing 19 - 21 April 2006, to appear in
Nucl. Phys.
Antibaryon density in the central rapidity region of a heavy ion collision
We consider (anti-)baryons production in heavy ion collisions as production
of topological defects during the chiral phase transition. Non-zero quark
masses which explicitly break chiral symmetry supress the (anti-)baryon
density. Hardly any (anti-)baryons will be produced in the central rapidity
region of a heavy ion collision.Comment: 3 pages in RevTex, 3 .ps file
Ultra-cold atoms in an optical cavity: two-mode laser locking to the cavity avoiding radiation pressure
The combination of ultra-cold atomic clouds with the light fields of optical
cavities provides a powerful model system for the development of new types of
laser cooling and for studying cooperative phenomena. These experiments
critically depend on the precise tuning of an incident pump laser with respect
to a cavity resonance. Here, we present a simple and reliable experimental
tuning scheme based on a two-mode laser spectrometer. The scheme uses a first
laser for probing higher-order transversal modes of the cavity having an
intensity minimum near the cavity's optical axis, where the atoms are confined
by a magnetic trap. In this way the cavity resonance is observed without
exposing the atoms to unwanted radiation pressure. A second laser, which is
phase-locked to the first one and tuned close to a fundamental cavity mode
drives the coherent atom-field dynamics.Comment: 7 pages, 7 figure
RACE-OC Project: Rotation and variability in the open cluster M11 (NGC6705)
Rotation and magnetic activity are intimately linked in main-sequence stars
of G or later spectral types. The presence and level of magnetic activity
depend on stellar rotation, and rotation itself is strongly influenced by
strength and topology of the magnetic fields. Open clusters represent
especially useful targets to investigate the rotation/activity/age connection.
The open cluster M11 has been studied as a part of the RACE-OC project
(Rotation and ACtivity Evolution in Open Clusters), which is aimed at exploring
the evolution of rotation and magnetic activity in the late-type members of
open clusters with different ages. Photometric observations of the open cluster
M11 were carried out in June 2004 using LOAO 1m telescope. The rotation periods
of the cluster members are determined by Fourier analysis of photometric data
time series. We further investigated the relations between the surface
activity, characterized by the light curve amplitude, and rotation. We have
discovered a total of 75 periodic variables in the M11 FoV, of which 38 are
candidate cluster members. Specifically, among cluster members we discovered 6
early-type, 2 eclipsing binaries and 30 bona-fide single periodic late-type
variables. Considering the rotation periods of 16 G-type members of the almost
coeval 200-Myr M34 cluster, we could determine the rotation period distribution
from a more numerous sample of 46 single G stars at an age of about 200-230 Myr
and determine a median rotation period P=4.8d. A comparison with the younger
M35 cluster (~150 Myr) and with the older M37 cluster (~550 Myr) shows that G
stars rotate slower than younger M35 stars and faster than older M37 stars. The
measured variation of the median rotation period is consistent with the
scenario of rotational braking of main-sequence spotted stars as they age.Comment: Accepted by Astronomy and Astrophysics on Dec 15, 200
The Space Optical Clocks Project: Development of high-performance transportable and breadboard optical clocks and advanced subsystems
The use of ultra-precise optical clocks in space ("master clocks") will allow
for a range of new applications in the fields of fundamental physics (tests of
Einstein's theory of General Relativity, time and frequency metrology by means
of the comparison of distant terrestrial clocks), geophysics (mapping of the
gravitational potential of Earth), and astronomy (providing local oscillators
for radio ranging and interferometry in space). Within the ELIPS-3 program of
ESA, the "Space Optical Clocks" (SOC) project aims to install and to operate an
optical lattice clock on the ISS towards the end of this decade, as a natural
follow-on to the ACES mission, improving its performance by at least one order
of magnitude. The payload is planned to include an optical lattice clock, as
well as a frequency comb, a microwave link, and an optical link for comparisons
of the ISS clock with ground clocks located in several countries and
continents. Undertaking a necessary step towards optical clocks in space, the
EU-FP7-SPACE-2010-1 project no. 263500 (SOC2) (2011-2015) aims at two
"engineering confidence", accurate transportable lattice optical clock
demonstrators having relative frequency instability below 1\times10^-15 at 1 s
integration time and relative inaccuracy below 5\times10^-17. This goal
performance is about 2 and 1 orders better in instability and inaccuracy,
respectively, than today's best transportable clocks. The devices will be based
on trapped neutral ytterbium and strontium atoms. One device will be a
breadboard. The two systems will be validated in laboratory environments and
their performance will be established by comparison with laboratory optical
clocks and primary frequency standards. In this paper we present the project
and the results achieved during the first year.Comment: Contribution to European Frequency and Time Forum 2012, Gothenburg,
Swede
On the protection of extrasolar Earth-like planets around K/M stars against galactic cosmic rays
Previous studies have shown that extrasolar Earth-like planets in close-in
habitable zones around M-stars are weakly protected against galactic cosmic
rays (GCRs), leading to a strongly increased particle flux to the top of the
planetary atmosphere. Two main effects were held responsible for the weak
shielding of such an exoplanet: (a) For a close-in planet, the planetary
magnetic moment is strongly reduced by tidal locking. Therefore, such a
close-in extrasolar planet is not protected by an extended magnetosphere. (b)
The small orbital distance of the planet exposes it to a much denser stellar
wind than that prevailing at larger orbital distances. This dense stellar wind
leads to additional compression of the magnetosphere, which can further reduce
the shielding efficiency against GCRs. In this work, we analyse and compare the
effect of (a) and (b), showing that the stellar wind variation with orbital
distance has little influence on the cosmic ray shielding. Instead, the weak
shielding of M star planets can be attributed to their small magnetic moment.
We further analyse how the planetary mass and composition influence the
planetary magnetic moment, and thus modify the cosmic ray shielding efficiency.
We show that more massive planets are not necessarily better protected against
galactic cosmic rays, but that the planetary bulk composition can play an
important role.Comment: 7 figure
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