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Physical Review A

Direct measurement of loading and loss rates in a magneto-optical trap with atom-number feedback

Youngwoon Choi, Seokchan Yoon, Sungsam Kang, Wookrae Kim, Jai-Hyung Lee, Kyungwon An

Published Jul 10, 2007• 6 citations
Publisher

Abstract

We have measured the loading and loss rates in a magneto-optical trap only with a few atoms by directly counting atom-number changing events. Unambiguous formulas are presented for the calculation of those rates from a step-wise time sequence of the fluorescence of the trapped atoms. With a recently developed atom-number feedback technique we could efficiently measure the loading rate as a function of the magnetic field gradient for the initial number of trapped atoms of zero. We could also measure the one- and two-atom loss rates as functions of the trap laser intensity for a precisely prepared initial number of trapped atoms. Each of these rates has been measured independently by directly counting the corresponding atom-number-changing events, without any influence from or inference to the other rates.

DOI
10.1103/physreva.76.013402

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We have measured the loading and loss rates in a magneto-optical trap only with a few atoms by directly counting atom-number changing events. Unambiguous formulas are presented for the calculation of those rates from a step-wise time sequence of the fluorescence of the trapped atoms. With a recently developed atom-number feedback technique we could efficiently measure the loading rate as a function of the magnetic field gradient for the initial number of trapped atoms of zero. We could also measure the one- and two-atom loss rates as functions of the trap laser intensity for a precisely prepared initial number of trapped atoms. Each of these rates has been measured independently by directly counting the corresponding atom-number-changing events, without any influence from or inference to the other rates.
A quantitative study on characteristics of a magneto-optical trap with a single or a few atoms is presented. A very small number of 85 Rb atoms were trapped in a micron-size magneto-optical trap with a high magnetic-field gradient. In order to find the optimum condition for a single-atom trap, we have investigated how the number of atoms and the size of atomic cloud change as various experimental parameters, such as a magnetic-field gradient and the trapping laser intensity and detuning. The averaged number of atoms was measured very accurately with a calibration procedure based on the single-atom saturation curve of resonance fluorescence. In addition, the number of atoms in a trap could be controlled by suppressing stochastic loading events by means of a real-time active feedback on the magnetic-field gradient.
In our experiment, firstly, we carry out the loading of single atoms in the magneto-optical trap (MOT) by increasing the quadrupole magnetic field gradient, improving the background vacuum, and reducing the diameters of the cooling and trapping laser beams. Secondly, we get the single atomic fluorescence signal of a high signal-to-background ratio in the MOT by means of reducing the detuning of cooling light and increasing its intensity appropriately, and using the polarization spectroscopy locking technique to suppress the fluctuations of cooling laser. In addition, with the real-time feedback on quadrupole magnetic field gradient, we demonstrate a probability of loading single atoms in the MOT as high as 98%. We also measure the statistical properties of the single atomic fluorescence which is excited by continuous light in the MOT; the measured second-order correlation is g(2)( = 0) = 0.09.
We present a detailed analysis of the cold collision measurements performed in a high-gradient magneto-optical trap with a few trapped Cs atoms first presented in Ueberholz et al ( J. Phys. B: At. Mol. Opt. Phys. 33 (2000) L135 ). The ability to observe individual loss events allows us to identify two-body collisions that lead to the escape of only one of the colliding atoms (up to 10% of all collisional losses). Possible origins of these events are discussed here. We also observed strong modifications of the total loss rate with variations in the repumping laser intensity. This is explained by a simple semiclassical model based on optical suppression of hyperfine-changing collisions between ground-state atoms.