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Journal of the Optical Society of America B

Free expanding cloud of cold atoms as an atomic standard: Ramsey fringes contrast

Stella Müller, Daniel Varela Magalhães, Aida Bebeachibuli, Tiago A. Ortega, Mushtaq Ahmed, Vanderlei Salvador Bagnato

Published May 14, 2008• 9 citations
Publisher

Abstract

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.

DOI
10.1364/josab.25.000909

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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.
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
We demonstrate a Ramsey-type microwave clock interrogating the 6.835 GHz ground-state transition in cold [Formula: see text]Rb atoms loaded from a grating magneto-optical trap (GMOT) enclosed in an additively manufactured loop-gap resonator microwave cavity. A short-term stability of [Formula: see text] is demonstrated, in reasonable agreement with predictions from the signal-to-noise ratio of the measured Ramsey fringes. The cavity-grating package has a volume of [Formula: see text]67 cm[Formula: see text], ensuring an inherently compact system while the use of a GMOT drastically simplifies the optical requirements for laser cooled atoms. This work is another step towards the realisation of highly compact portable cold-atom frequency standards.
A portable cold87Rb atomic clock is under development at NIM. The clock uses87Rb atoms which are trapped and cooled in a small three-dimensional magneto-optical trap (3D-MOT) chamber. Within the 3D-MOT chamber, the free-falling atoms are prepared in∣F=2,mF=0⟩\vert F=2, m_{F}=0\rangle∣F=2,mF​=0⟩clock state with optical pumping (OP) directly after post cooling by2−2′2-2^{\prime}2−2′OP light and repumping. Following optical state selection, the free-falling atoms enter a microwave cavity where they undergo microwave interrogation. Eventually, the number of atoms in two clock states are detected individually by the fluorescence detection method in the detection chamber. The preliminary results show that, the selected atom number has been improved about 3 times with the optical state selection compared to the microwave state selection method. And a Ramsey fringe has been obtained experimentally, and the contrast of the central Ramsey fringe exceeds 88% with a full width half maximum (FWHM) of less than 9 Hz.
We demonstrate a high-performance coherent-population-trapping (CPT) Cs vapor cell atomic clock using the push-pull optical pumping technique in the pulsed regime, allowing the detection of high-contrast and narrow Ramsey-CPT fringes. The impact of several experimental parameters onto the clock resonance and short-term fractional frequency stability, including the laser power, the cell temperature, and the Ramsey sequence parameters, has been investigated. We observe and explain the existence of a slight dependence on laser power of the central Ramsey-CPT fringe line-width in the pulsed regime. We report also that the central fringe line-width is commonly narrower than the expected Ramsey line-width given by 1/(2TR), with TR the free-evolution time, for short values of TR. The clock demonstrates a short-term fractional frequency stability at the level of 2.3×10−13 τ−1/2 up to 100 s averaging time, mainly limited by the laser amplitude modulation noise. Comparable performances are obtained in the conventional continuous wave regime, with the use of an additional laser power stabilization setup. The pulsed interaction allows to reduce significantly the clock frequency sensitivity to laser power variations, especially for high values of TR. This pulsed CPT clock, ranking among the best microwave vapor cell atomic frequency standards, could find applications in telecommunication, instrumentation, defense or satellite-based navigation systems.
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.
Abstract A compact platform for cold atoms opens a range of exciting possibilities for portable, robust and accessible quantum sensors. In this work, we report on the development of a cold-atom microwave clock in a small package. Our work utilises the grating magneto-optical trap and high-contrast coherent population trapping in the lin ⊥ $\perp $ lin polarisation scheme. We optically probe the atomic ground-state splitting of cold 87Rb atoms using a Ramsey-like sequence whilst the atoms are in free-fall. We have measured a short-term fractional frequency stability of 5 × 10 − 11 / τ 5×10−11/τ5{\times}{10}^{-11}/\sqrt{\tau }5×10−11/τ​ with a projected quantum projection noise limit at the 10 − 13 / τ 10−13/τ{10}^{-13}/\sqrt{\tau }10−13/τ​ level.