654 research outputs found
High-accuracy optical clock based on the octupole transition in 171Yb+
We experimentally investigate an optical frequency standard based on the 467
nm (642 THz) electric-octupole reference transition 2S1/2(F=0) -> F7/2(F=3) in
a single trapped 171Yb+ ion. The extraordinary features of this transition
result from the long natural lifetime and from the 4f136s2 configuration of the
upper state. The electric quadrupole moment of the 2F7/2 state is measured as
-0.041(5) e(a0)^2, where e is the elementary charge and a0 the Bohr radius. We
also obtain information on the differential scalar and tensorial components of
the static polarizability and of the probe light induced ac Stark shift of the
octupole transition. With a real-time extrapolation scheme that eliminates this
shift, the unperturbed transition frequency is realized with a fractional
uncertainty of 7.1x10^(-17). The frequency is measured as 642 121 496 772
645.15(52) Hz.Comment: 5 pages, 4 figure
Interrogation of caesium atoms in a fountain clock by a femtosecond laser microwave oscillator
A caesium fountain clock is operated utilizing a microwave oscillator that
derives its frequency stability from a stable laser by means of a fiber-laser
femtosecond frequency comb. This oscillator is based on the technology
developed for optical clocks and replaces the quartz based microwave oscillator
commonly used in fountain clocks. As a result, a significant decrease of the
frequency instability of the fountain clock is obtained, reaching 0.74E-14 at
100 s averaging time. We could demonstrate that for a significant range of
detected atom numbers the instability is limited by quantum projection noise
only, and that for the current status of this fountain clock the new microwave
source poses no limit on the achievable frequency instability.Comment: 4 pages, 4 figure
Chemical and ecological health of white sucker (Catostomus commersoni) in Rock Creek Park, Washington, D.C., 2003-04 / by Cherie V. Miller ... [et al.] ; prepared in copoeration with the National Park Service
Several classes of chemicals that are known or suspected contaminants were found in bed sediment in Rock Creek, including polyaromatic hydrocarbons (PAHs), phthalate esters, organochlorine pesticides, dioxins and furans, trace metals and metalloids (mercury, arsenic, cadmium, chromium, cobalt, copper, lead, nickel, silver, and zinc), and polychlorinated biphenyls (total PCBs and selected aroclors). Concentrations of many of these chemicals consistently exceeded thresholdor chronic-effects guidelines for the protection of aquatic life and often exceeded probable effects levels (PELs). Exceedance of PELs was dependent on the amount of total organic carbon in the sediments. Concurrent with the collection of sediment-quality data, white sucker (Catostomus commersoni) were evaluated for gross-external and internal-organ anomalies, whole-body burdens of chemical contaminants, and gut contents to determine prey. The histopathology of internal tissues of white sucker was compared to contaminant levels in fish tissue and bed sediment. Gut contents were examined to determine preferential prey and thus potential pathways for the bioaccumulation of chemicals from bed sediments. Male and female fish were tested separately. Lesions and other necroses were observed in all fish collected during both years of sample collection, indicating that fish in Rock Creek have experienced some form of environmental stress. No direct cause and effect was determined for chemical exposure and compromised fish health, but a substantial weight of evidence indicates that white sucker, which are bottom-eeding fish and low-order consumers in Rock Creek, are experiencing some reduction in vitality, possibly due to immunosuppression. Abnormalities observed in gonads of both sexes of white sucker and observations of abnormal behavior during spawning indicated some interruption in reproductive success
Cancellation of the collisional frequency shift in caesium fountain clocks
We have observed that the collisional frequency shift in primary caesium
fountain clocks varies with the clock state population composition and, in
particular, is zero for a given fraction of the |F = 4, mF = 0> atoms,
depending on the initial cloud parameters. We present a theoretical model
explaining our observations. The possibility of the collisional shift
cancellation implies an improvement in the performance of caesium fountain
standards and a simplification in their operation. Our results also have
implications for test operation of fountains at multiple pi/2 pulse areas
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