Improved uncertainty budget for optical frequency measurements with microkelvin neutral atoms: Results for a high-stability 40Ca optical frequency standard
G Wilpers, Christopher W. Oates, Leo W. Hollberg
Using a Ca optical frequency standard at 657 nm, we demonstrate a method for reducing uncertainties in absolute frequency measurements of optical transitions using freely expanding neutral atoms. Working with atoms that have been laser cooled to 10 υ, we have developed and employed a new technique that combines launching of cold atom clouds with atom interferometry to measure and optimize spectroscopy beam parameters. When applied to laser beams of high spatial quality, this approach has the potential to reduce residual Doppler effect uncertainties to well below one part in 1016. With Doppler uncertainties greatly suppressed, we investigate of other potential shifts at the hertz level with a multiplexed measurement system that takes advantage of the low instability of the calcium frequency standard (4x10-15 at 1 s). The resultant fractional frequency uncertainty for the standard is 6.6x10-15, the lowest reported to date for a neutral atom optical standard.