For decades, improved theory and experiment of the n=2 3 P fine structure of helium have allowed for increasingly-precise tests of quantum electrodynamics, determinations of the fine-structure constant α, and limitations on possible beyond-the-Standard-Model physics. Here we use the new frequency-offset separated-oscillatory-fields (FOSOF) technique to measure the 2 3 P2→ 2 3 P1 interval. Our result of 2 291 176 590(25) Hz represents a major step forward in precision for helium fine-structure measurements.

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A high-aspect-ratio absorption grating with a pitch of several µm is a key component of X-ray grating interferometery, which is an X-ray phase imaging technique that allows for highly sensitive X-ray imaging with a compact laboratory X-ray source. Here, we report that X-ray phase imaging was successfully performed at 15 keV by using a 23 ± 1-µm-height, 9-µm-pitch absorption grating (10 × 10 mm2) based on Gd (Gd60Cu25Al15) fabricated by a metallic glass imprinting technique. The imprinting technique is cost-efficient and has a high-production rate, and will be widely used for fabricating gratings not only for X-rays but also neutrons in the near future.

We produce cold and dense clouds of atomic ions (Ca + , Dy + ) by laser ablation of metal targets and cryogenic buffer gas cooling of the resulting plasma. We measure the temperature and density of the ion clouds using laser absorption spectroscopy. We find that large ion densities ( 10 9 cm −3 ) can be obtained at temperatures as low as 6 K. Our method opens up new ways to study cold neutral plasmas, and to perform survey spectroscopy of ions that cannot be laser-cooled easily.