First accuracy evaluation of the NPL-CsF2 primary frequency standard

Abstract

An accuracy evaluation of the caesium fountain NPL-CsF2 as a primary frequency standard is reported. The device operates with a simple one-stage magneto-optical trap as the source of cold atoms. Both the uncertainty in and magnitude of the cold collision frequency shift are reduced by taking advantage of the dependence of the cross section on the effective collision energy in an expanding atomic cloud. The combined type B uncertainty (typically 4 × 10−16) is dominated by an estimate of the frequency shift due to the distributed cavity phase. When operated at single density, the short-term fractional frequency instability of NPL-CsF2 is 1.7 × 10−13 at 1 s and limited by the noise of the room temperature quartz-based local oscillator. During a typical frequency measurement campaign, the fountain is operated in an alternating mode at high and low density in order to measure and correct for a residual collision shift. This increases the effective fractional frequency instability to 5.4 × 10−13 at 1 s; consequently the averaging time required for the type A uncertainty level to match that of the type B is 20 days.

Evaluation of Doppler Shifts to Improve the Accuracy of Primary Atomic Fountain Clocks

“…shifts [1][2][3][4][5][6][7]. These Doppler shifts occur when the cold but moving atoms interact with a field inside the microwave cavity that has a spatial phase variation (Fig.…”

“…1a, and the purity of the standing wave. Uncertainty estimates are as low as ±3×10 −16 , excepting a few cases where it is incorrectly evaluated [1][2][3][4][5][6][7]. Here, we present precise measurements of a primary clock's frequency and large [8].…”

Building a better atomic clock

“…routinely attain accuracies near their quoted 1 day stabilities. Ultimate results will far exceed these numbers, but this is after years or decades of research [4,5,6,7,8]. Next, accuracy is the ability of one's measurement to be traceable back to an internationally agreed upon standard.…”

“…This will allow spectroscopic resolution of single pancakes of atoms in our optical lattice, a technique that could be especially interesting for looking at atomic noise correlations [34]. Construction of these recessed viewports was accomplished by the United Kingdom Atomic Energy Authority (UKAEA) 5 . In addition to their expertise in constructing these specialized UHV windows, they have extensive experience building these windows in a piecewise manner.…”

Measurement of the magnetic field profile in the atomic fountain clock FoCS-2 using Zeeman spectroscopy

“…This approach has made possible important advances in time and frequency metrology in such a way that the SI second is nowadays realized with an uncertainty below 10 −15 [1][2][3]. However, state-of-the-art fountain clocks are all based on a pulsed mode of operation: the atoms are sequentially laser-cooled, launched vertically upwards and interrogated during their ballistic flight before the cycle starts over again [4].…”