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Acta Physica Sinica

Temperature measurement of cold atoms in a cesium magneto-optical trap by means of short-distance time-of-flight absorption spectrum

Tao Geng, 闫树斌 Yan Shubin, Yanhua Wang, Yang Hai-Jing, Tiancai Zhang, Junmin Wang, 量子光学与光量子器件国家重点实验室，山西大学光电研究所,太原 030006

Published Jan 1, 2005• 7 citations

Abstract

We report the basic idea and experimental demonstration of measuring the temperature of cold cesium atomic cloud confined in a magneto-optical trap (MOT) by analyzing absorption spectrum observed in the short-distance time-of-flight (TOF) method. Compared with the conventional TOF scheme, we measure the temperature of cold atomic cloud in MOT by short-distance TOF method for our small vapor cell. If the cold atomic cloud is released, it will fall and expand due to gravity and its initial velocity distribution. Utilizing a cylindrical resonant probe beam positioned several millimeters beneath the MOT (in this paper, we analyzed the cases for 3mm, 5mm, and 8mm of center-to-center distance between the MOT region and the probe beam, respectively), we can record the short-distance TOF absorption spectrum with a photodiode when cold atoms go through the probe region. Analysis of the short-distance TOF absorption spectrum yields the effective temperature of the cold cloud in MOT.

DOI: 10.7498/aps.54.5104

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The basic model and the experimental measurement method of short-distance time-of-flight absorption spectra for determining the temperature of cold atoms are described. Based on the realization of temperature measurement of cold cesium atoms in a magneto-optical trap，the influences of measurement errors of the three experimental parameters，namely the falling distance h，the initial Gaussian radius σ0 of the cold cloud, and the Gaussian radius σp of probe beam, on the determination of temperature have been analyzed and compared.

The four-beam 3D near-resonance optical lattice is introduced and experimentally realized based on the magneto-optical trap and optical molasses.The temperature of cold cesium atoms loaded into the lattice is measured via the short-distance time of flight(TOF) absorption spectrum.By changing the intensity and detuning of lattice beams,the lowest temperature in that case is obtained.The parameter dependence of the temperature of the cold atoms in lattice is experimentally investigated,and a comparison with the case of optical molasses is briefly discussed.

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The experimental setup for cooling and trapping~(87)Rb atoms in magneto-optical trap(MOT) and the process of them are briefly introduced in this paper.The number of cold atoms is measured by detecting the fluorescence emitted from~(87)Rb atoms.The laser probe beam is set at the positions which are 4mm,7mm,10mm below the atoms cloud,the short-distance time-of-flight(TOF) Absorption signals are obtained during the falling of atoms.The average temperature of cold atoms is obtained by numerical simulating.The results show that the number of cold atoms is about 10~(9),the density is about 10~(11)/cm~3 and the cooling temperature of atomic cloud is about 200 μK,such cold atoms can be used as good EIT medium.

The number of cold atoms trapped in magneto-optical trap(MOT) can be derived from probe absorption signal. After the MOT is switched off, the cold cloud freely expands in gravitational field with an initial velocity distribution. So the measurement process can be described with the interaction model between the individual stationary two-level atom and single mode resonate laser. Absorption method can largely improved the measurement precision. In this paper we introduce the physical principle of measurement and demonstrate the whole experimental procedure in detail. The number of atoms trapped in our MOT, (8±0.3)×106 is measured in experiment. And the number of steady-state atoms and the lifetime of atom trapped in MOT was used to deduce the capture rate of MOT.

We analyse the time-of-flight method of measuring the temperature of cold trapped atoms in the specific case of short distances of the probe beam from the trap centre and finite atomic cloud size. We theoretically examine the influence of the probe beam shape and its distance from the initial position of the cloud on the temperature evaluation. These results are then verified with a three-dimensional Monte Carlo simulation and applied to our experimental data to show that the proposed procedure allows accurate and reliable determination of the temperature.