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Journal of Time and Frequency

Optical molasses loaded from a slow atomic beam

Shougang Zhang

Published Jan 1, 2012• 0 citations
OpenAlex

Abstract

Preparation of the optical molasses is one of the key steps for developing a fountain clock.The optical molasses is loaded from a slow atomic beam,and the fluorescence collection method is applied to determine the number of atoms in the optical molasses,meanwhile the time-of-flight(TOF) method is used to measure the temperature of the optical molasses.The number of cold atoms in the optical molasses for loading time of 200 ms is determined to be(8.14±0.2)×107,and the temperature of cold atoms is determined to be(10.72±0.69) μK.

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•May 21, 1990•0 citations
Preparation of the optical molasses is one of the key steps for developing a fountain clock.The optical molasses is loaded from a slow atomic beam,and the fluorescence collection method is applied to determine the number of atoms in the optical molasses,meanwhile the time-of-flight(TOF) method is used to measure the temperature of the optical molasses.The number of cold atoms in the optical molasses for loading time of 200 ms is determined to be(8.14±0.2)×107,and the temperature of cold atoms is determined to be(10.72±0.69) μK.
The time-of-flight (TOF) method is one of the most common ways to measure the temperature of cold atoms. In the cold atomic fountain setup, the geometry of the probe beam will introduce the measurement errors to the spatial distribution of cold atomic cloud, which will lead to the measurement errors on atomic temperature. Using deconvolution, we recover the atomic cloud profile from the TOF signal. Then, we use the recovered signals other than the TOF signals to obtain a more accurate atomic temperature. This will be important in estimating the effects of cold atom collision shift and the shift due to transverse cavity phase distribution on an atomic fountain clock.
We demonstrate the efficient capture of cesium atoms from a low-velocity intense source (LVIS) of atoms into an optical molasses. The high load rates that are achievable with the technique can potentially improve the stability of and reduce collisional shifts in atomic fountain clocks. An LVIS is an atomic beam created from a small hole in one of the retroreflectors in an otherwise ordinary magnetooptical trap (MOT). Our typical LVIS flux was 10/sup 10/ atoms/s. The asymptotic value for the number of atoms captured in the optical molasses was 1.1(1) /spl times/ 10/sup 9/ atoms, and the fill time constant was /spl tau/ = 290(30) ms. The initial molasses fill rate was R/sub t=0/ = 3.8(5) /spl times/ 10/sup 6/ atoms/ms. At this rate, it would require 24 ms to capture and launch 10/sup 7/ state-selected atoms in an atomic fountain. The fill rate at short times indicates that approximately 40% of the LVIS atoms were being captured in the optical molasses.
Early experiments on ultracold optical molasses relied on time-of-flight and other ballistic techniques to measure the temperature of atoms released from the molasses.1,2 At the extremely low temperatures observed, the size of the molasses, the effects of gravity, the probe that detected the falling atoms, and the way in which the laser beams forming the molasses were turned off produced a large systematic uncertainty in the temperature determination. Here, we report a direct measurement of the Doppler width of the fluorescence spectrum of Na atoms still viscously confined in the molasses. We determined this spectrum using an optical heterodyne technique and we believe this to be the first time an atomic fluorescence spectrum has been heterodyned. The temperature deduced from the Doppler width is in good agreement with that determined using improved time-of-flight techniques. The heterodyne technique has the added advantage of providing spatially resolved temperature measurements. In addition to the nominally 600-kHz wide Doppler peak, we see a much narrower (60-kHz wide) peak which we tentatively ascribe to Dicke narrowing due to confinement by optical forces.
We report a novel method for temperature measurement in a cesium fountain clock and experimental investigation about its dependence respect to diverse operational parameters. The temperature of the Cs atomic cloud is inferred from the width of the velocity distribution, which is obtained from two measurements in different regions of the spatial distribution of the atomic cloud using the time of flight (TOF) method. We use a simple process to determine the velocity distribution by calculating the square speed point by point from these two TOF signals. This new method is demonstrated to characterize the temperature of the Cs atoms on the CENAM Cs fountain clock. The minimum temperature reached in the CENAM Cs fountain clock is 612(193) nK.
Since the first optical molasses1 came out, the temperature of the atomic vapor in the optical molasses, which was determined by measuring the time of flight (TOF), was found to be well below the Doppler cooling limit,2 which is based on the assumption that a single frequency laser interacts with the two-energy-level atoms. The new theory3 proposes that this lower temperature is due to the nonadiabatic motion of the atoms with Zeeman sublevels in the optical field with polarization gradients. A bimodal speed distribution in optical molasses was also observed.4