Topic: Caesium fountain clocks: reaching the ultimate accuracy

Abstract:
Primary frequency standards provide independent realisations of the SI unit of time, the second, by measurement of the microwave frequency of a hyperfine transition in the ground state caesium atom. The most precise standards operate in a fountain configuration, where samples of laser-cooled caesium atoms are tossed up and fall freely under gravity. Achieved are long times of interaction with the interrogating microwave field, which has led to significant improvements of performance compared to earlier thermal beam standards. The accuracy of the fountain standards is limited by a number of systematic effects, generally related to the interaction of the atomics with stray electromagnetic fields, collisions between the atoms, and technical details of the method of interrogation. Another source of inaccuracy is the statistical uncertainty with which the clock frequency can be determined, after a given time of averaging.
In the talk, improvements introduced to the NPL-CsF2 primary clock will be presented, including: operation in the vicinity of the zero-collisional shift point; a technique for accumulation of the atomic population in the mF=0 clock sublevel; and re-evaluation of the frequency shift due to distributed cavity phase. Prospects for further accuracy enhancements, together with the use of a fountain clock for direct steering of a local timescale, will also be discussed.
The development and operation of the caesium fountain standard at NPL is a part of a wider time and frequency research activity, which includes work on optical clocks, femtosecond laser frequency combs, transfer and dissemination of time signals via satellite and fibre links, and optimisation of timescale algorithms.