An official website of the United States government
Abstract This work describes the apparatus for NIST-F4, an updated cesium atomic fountain at the National Institute of Standards and Technology (NIST), and presents an accuracy evaluation of the fountain as a primary frequency standard. The fountain uses optical molasses to laser cool a cloud of cesium atoms and launch it vertically in a fountain geometry. In high-density mode, the fractional frequency stability of NIST-F4 is , whereτis the measurement time in seconds. The short-term stability is limited by quantum projection noise and by phase noise from the local oscillator, an oven-controlled crystal oscillator operating at 5 MHz. Systematic frequency shifts and their uncertainties have been evaluated, resulting in a systematic (type B) fractional frequency uncertainty .
more »
« less
Advancements in the NRC-FCs2 primary frequency standard
Abstract We have made improvements in the stability, accuracy, and performance of the NRC-FCs2 fountain clock. The dominant systematic effects have been re-evaluated. Optically-stabilized microwaves are used to improve the short-term stability, now reaching $$\sigma_y = 3\;\times10^{-14}\;\tau^{-\frac{1}{2}}$$. We evaluate the distributed cavity phase shift using absorption imaging. This technique dramatically reduces the evaluation time and final uncertainty. We have re-evaluated biases due to microwave leakage and synchronous phase transients with higher accuracy. The total systematic uncertainty of NRC-FCs2 is now $$1.1 \times 10^{-16}$$ in fractional frequency, a factor of $$2$$ improvement over its most recent evaluation.
more »
« less
- Award ID(s):
- 2012117
- PAR ID:
- 10583218
- Publisher / Repository:
- IOP Publishing
- Date Published:
- Journal Name:
- Metrologia
- ISSN:
- 0026-1394
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract We present a way to dramatically accelerate Gaussian process models for interatomic force fields based on many-body kernels by mapping both forces and uncertainties onto functions of low-dimensional features. This allows for automated active learning of models combining near-quantum accuracy, built-in uncertainty, and constant cost of evaluation that is comparable to classical analytical models, capable of simulating millions of atoms. Using this approach, we perform large-scale molecular dynamics simulations of the stability of the stanene monolayer. We discover an unusual phase transformation mechanism of 2D stanene, where ripples lead to nucleation of bilayer defects, densification into a disordered multilayer structure, followed by formation of bulk liquid at high temperature or nucleation and growth of the 3D bcc crystal at low temperature. The presented method opens possibilities for rapid development of fast accurate uncertainty-aware models for simulating long-time large-scale dynamics of complex materials.more » « less
-
Abstract The performance of a caesium fountain frequency reference for use in precision measurements of trapped antihydrogen in the ALPHA experiment at CERN is evaluated. A description of the fountain is provided together with a characterisation of systematic effects. The impact of the magnetic environment in the Antimatter Factory, where the fountain is installed, on the performance of the fountain is considered and shown to be insignificant. The systematic fractional frequency uncertainty of the fountain is 3.0 × 10-16. The short-term frequency stability of the measured frequency from the ALPHA-HM1 maser is 1.5 × 10-13τ-1/2, whereas the fountain itself shows a stability limit of 4.7 × 10-14τ-1/2. We find a fractional frequency difference of (1.0 ± 2.2 (stat.) ± 6.5 (syst.)) × 10-16 in a comparison with Terrestrial Time via a GNSS Common View satellite link between January 2023 and June 2024. The fountain will enables a significant increase in frequency precision in antihydrogen spectroscopic measurements, and paves the way for improved limits on matter-antimatter comparisons.more » « less
-
Abstract Multicomponent oxides have received significant recent attention due to their potential for improved property tunability. In simple structures, compositionally complex oxides can be stabilized by increased configurational entropy and are sometimes called “high entropy” ceramics. In phases with multiple cation sublattices or complex stoichiometries, it is more difficult to achieve high configurational entropy. However, there is limited knowledge about the factors influencing stability and solubility limits in many systems. This study investigated the limits on the stability of rare earth (RE) aluminates containing mixtures of RE cations including Gd, La, Nd, Yb, and Y in cases where (i) a fixed RE:Al ratio attempts to constrain the material into a single‐phase aluminate or (ii) a two‐phase aluminate, and in equilibrium with RE zirconates that readily dissolve multiple RE3+. The results show that it is difficult to form single‐phase, equimolar mixed‐RE aluminates encompassing a range of RE3+sizes. Instead, the RE3+selectively partition into specific phases based on RE‐size trends in the constituent binary systems. The results are discussed in terms of the phase stability and cation partition trends and potential applications.more » « less
-
Abstract The ICESat‐2 and GEDI missions were launched in 2018, becoming the new generation of space‐borne laser altimeters. These missions provide unprecedented global geodetic elevations, opening great opportunities for water level monitoring. The potential of these altimeters has been demonstrated in open‐water environments such as lakes, rivers, and reservoirs. However, detailed evaluations in vegetated environments, such as wetlands, floodplains, and other areas not constrained by water canal networks, are essential for continued improvement and further hydrological application. We developed a systematic accuracy assessment of ICESat‐2 ATL08, and GEDI L2A products to monitor spatial‐temporal water level and depth dynamics over the South Florida Everglades wetlands. The evaluation was performed on data acquired between 2020 and 2021, using gauge‐based water level and depth estimates as references. The results showed an RMSE of 0.17 m (water level) and 0.15 m (water depth) for ICESat‐2 and 0.75 m (water level) and 0.37 m (water depth) for GEDI. The analysis suggested that nighttime acquisitions were more accurate for both missions than daytime ones. The low‐power beams achieved slightly higher accuracies than those of the high‐power beams over the evaluated wetlands. Water level retrieval was more problematic in densely vegetated areas; however, we derived a correction model based on the leaf area index that improved the accuracy by up to 75% for water depth retrievals from GEDI. Furthermore, the analysis provides new insights to understand the potential of the altimeters in monitoring the spatial‐temporal dynamics of water levels in the evaluated wetlands.more » « less
