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Acta Sinica Quantum Optica

Measurement of the Number and Temperature of the Cold atoms EIT Medium

Hai Wang

Published Jan 1, 2007• 0 citations
OpenAlex

Abstract

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.

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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.
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.
The Gaussian radius and temperature of cold atomic cloud are important parameters in describing the state of cold atoms. The precise measuring of these two parameters is of great significance for studying the cold atoms. In this paper, we propose a new method named knife-edge to measure the Gaussian radius and temperature of the cold atomic cloud. A near-resonant and supersaturated laser beam, whose size is controlled by a knife-edge aperture, is used to push away the cold atoms in the free falling process of cold atomic cloud. By detecting the intensity of fluorescence signal, the numbers of residual atoms under different-sized near-resonant beams can be obtained. According to the characteristic of cold atoms′ distribution, we construct a theoretical model to derive the Gaussian radius of cold atomic cloud from the recorded residual atom number and near-resonant beam size. Since the Gaussian radius and temperature of cold atomic cloud are associated with each other, we can finally obtain the temperature of cold atomic cloud through the recorded residual atom number and beam size. By using this method, we successfully measure the Gaussian radii of cold atomic cloud at the heights of 10 mm and 160 mm below the center of 3D-MOT (three dimensional magneto-optical trap) to be (1.54 ± 0.05) mm and (3.29 ± 0.08) mm, respectively. The corresponding temperature of cold atomic cloud is calculated to be (7.50 ± 0.49) μK, which is well consistent with the experimental result obtained by using the time-of-flight method under the same condition. This experiment is conducted on the platform of Cesium atomic fountain clock of National Time Service Center, China.
Based on the real sense of the time-of-flight , we demonstrate an alternative method of measuring the temperature of cold atoms in magneto-optical traps (MOTs) using a high-finesse optical microcavity, which acts as a pointlike single-atom counter. A cloud of atoms trapped in magneto-optical traps is positioned about 5 mm above the cavity, and the atoms fall freely down through the cavity. The temperature of the cold atoms in the MOT is determined by counting the exact arrival times of the single atoms. A theoretical model based on a ballistic expansion of a cloud of trapped atoms falling in the earth’s gravitational field is used to fit the probability distribution of atom arrivals, and the fittings agree very well with the experimental results. This method could be used for systems with little room, where an extra probe beam is hard to involve, or with fewer atoms initially.
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 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.