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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)

Demonstration and Observation of a Frequency Standard Based on an Expanding Cold Atoms Cloud of Cesium

Stella Müller, Daniel Varela Magalhães, Aida Bebeachibuli, M. Silva Santos, Vanderlei Salvador Bagnato

Published Jan 18, 2006• 3 citations
Publisher

Abstract

This work reports the first results obtained in a laboratory system implemented to be a primary standard based on an expanding cloud of cold 133Cs. An atomic sample is prepared through the magneto-optical trapping technique, and after the loading procedure the atomic cloud is left to freely expand. A microwave interrogation signal is applied to the atoms, probing them in the ground state transition 6S1/2(F = 3)- 6S1/2(F = 4). The microwave interrogation was applied either in one or two pulses methods. The system could be also locked in a commercial standard to perform its stability evaluation. The first results showed a stability of 9times10-11tau-frac12, promising for such use of simple type of frequency standard

DOI
10.1109/freq.2005.1573945

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Laser Physics•Jan 1, 2004•5 citations
This work reports the first results obtained in a laboratory system implemented to be a primary standard based on an expanding cloud of cold 133Cs. An atomic sample is prepared through the magneto-optical trapping technique, and after the loading procedure the atomic cloud is left to freely expand. A microwave interrogation signal is applied to the atoms, probing them in the ground state transition 6S1/2(F = 3)- 6S1/2(F = 4). The microwave interrogation was applied either in one or two pulses methods. The system could be also locked in a commercial standard to perform its stability evaluation. The first results showed a stability of 9times10-11tau-frac12, promising for such use of simple type of frequency standard
A compact frequency standard based on an expanding cold Cs133 cloud is under development in our laboratory. In a first experiment, Cs cold atoms were prepared by a magneto-optical trap in a vapor cell, and a microwave antenna was used to transmit the radiation for the clock transition. The signal obtained from fluorescence of the expanding cold atoms cloud is used to lock a microwave chain. In this way the overall system stability is evaluated. A theoretical model based on a two-level system interacting with the two microwave pulses enables interpretation for the observed features, especially the poor Ramsey fringes contrast.
The accelerated development of laser methods for manipulating the internal state and motion of atoms, and the recent primary evaluation in France of a cesium frequency standard using cold atoms in an atomic fountain configuration, allow the frontiers of time metrology, accuracy, and stability to be pushed near parts in the 1016 level, a factor of 100 over the present primary laboratory clocks. These positive results revived the development of more accurate cesium atomic clocks, and the Institute of National Measurement Standards (INMS), of the National Research Council of Canada (NRC) is undertaking a cesium-fountain project aimed at reaching these new levels of accuracy and stability. This paper reviews its present status. To appreciate the considerable impact of cold atoms in time and frequency metrology, we begin by recalling the background principles of traditional cesium frequency standards, their typical performance and main limitations. After introducing the important theoretical results essential to understanding the techniques of laser cooling and magneto-optical trapping, we focus on describing the characterization and control of our magneto-optical trap (MOT) and report the parameters that allow us to fill the trap with a few million atoms in less than a second and cool them to a temperature lower than 5 muK using optical molasses. We will describe the specifics of our cesium-fountain prototype, which operates with a 2-dimensional moving molasses. This special configuration is applicable for multipulse operation of the fountain, which can greatly relax the local oscillator requirements for time keeping. Finally, progress toward our objective of launching cooled atoms, at a rate of one ensemble per second, at speeds of 7 m/s, for a fountain with an interrogation time of about 1 s, will be analysed.PACS Nos.: 06.20, 06.30, and 32.80P
Our laboratory has been developing an atomic reference system in which an ensemble of cold atoms of cesium is produced already in the center of a microwave cavity. The timeline of the project is divided in three parts: proof of principle with an antenna, design of a microwave cavity with viewports to produce the magneto-optical trap (MOT) and a movable system with all the parts embedded in a single package. In this work we present the first two parts of the global project with some relevant aspects for the construction of this compact frequency standard.
We have demonstrated the possibility for a compact frequency standard based on a sample of cold cesium atoms. In a cylindrical microwave cavity, the atoms are cooled and interrogated during a free expansion and then detected. The operation of this experiment is different from conventional atomic fountains, since all the steps are sequentially performed in the same position of space. In this paper we report the analysis of a Ramsey pattern observed to present a 47 Hz linewidth and a stability of 5x10^-13 for an integration time longer than 100s. Some of the main limitations of the standard are analyzed. This present report demonstrates considerable improvement of our previous work (S. T. Muller et al, J. Opt. Soc. Am. B, v.25, p.909, 2008) where the atoms were in a free space and not inside a microwave cavity.
The well-known MOT technique is used to trap and cool 133 Cs atoms to reproduce the SI second in an atomic fountain. About 10 8 cesium atoms are caught in a magnetooptical trap from a vapor cell at room temperature. The fountain has a (1,1,1)-geometry for the laser cooling and launching operation. The three upper beams have the same polarization, opposite to the three lower ones. The interrogation and preparation cavities have also been constructed, as well as characterized, concerning the temperature tunability in vacuum. The microwave synthesizer has been constructed in collaboration with BNM-SYRTE, time-and-frequency group. The synthesizer has been compared to a very stable oscillator at BNM-SYRTE resulting in a short-term stability of 9.7 × 10 -14 τ -1/2 .