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Chinese Physics B

Loss of cold atoms due to collisions with residual gases in free flight and in a magneto-optical trap

Jingfeng Xiang, He-Nan Cheng, Xiangkai Peng, Xinwen Wang, Wei Ren, Jingwei Ji, Kangkang Liu, Jian-Bo Zhao, Lin Li, Qiuzhi Qu, Tang Li, Bin Wang, Meifeng Ye, Xin Zhao, Yuanyuan Yao, Desheng Lü, Liang Liu

Published Jul 1, 2018• 10 citations
Publisher

Abstract

The loss rate of cold atoms in a trap due to residual gas collisions differs from that in a free state after the cold atoms are released from the trap. In this paper, the loss rate in a cold rubidium-87 atom cloud was measured in a magneto-optical trap (MOT) and during its free flight. The residual gas pressure was analyzed by a residual gas analyzer, and the pressure distribution in a vacuum chamber was numerically calculated by the angular coefficient method. The decay factor, which describes the decay behavior of cold atoms due to residual gas collisions during a free flight, was calculated. It was found that the decay factor agrees well with theoretical predictions under various vacuum conditions.

DOI
10.1088/1674-1056/27/7/073701

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Bulletin of the American Physical Society•May 1, 1993•0 citations
The loss rate of cold atoms in a trap due to residual gas collisions differs from that in a free state after the cold atoms are released from the trap. In this paper, the loss rate in a cold rubidium-87 atom cloud was measured in a magneto-optical trap (MOT) and during its free flight. The residual gas pressure was analyzed by a residual gas analyzer, and the pressure distribution in a vacuum chamber was numerically calculated by the angular coefficient method. The decay factor, which describes the decay behavior of cold atoms due to residual gas collisions during a free flight, was calculated. It was found that the decay factor agrees well with theoretical predictions under various vacuum conditions.
In this work, the UHV background pressure as low as 1.1×10−10 Torr has been measured using the loss rate characteristics of a vapor-loaded magneto-optical trap (MOT) formed on an atom chip in a UHV chamber. The loss rate due to non-rubidium (Rb) gases in the background in the chamber has been estimated by operating the MOT in low cooling beam intensities and low Rb pressure regimes simultaneously. Using this approach, we minimized the contributions of the intra-trap collisions as well as background MOT species collisions to the measured trap loss rate in our setup. These results can be useful for development of cold-atom based UHV pressure standards.
This thesis introduces an empirical method for determining and controlling the excited-state fraction of atoms in a magneto-optical trap (MOT), which is essential for the use of cold atoms as a sensor when they are held in a MOT since their interactions with other particles and fields are quantum state dependent. A four-level theoretical atomic model was used to describe the transitions of the atoms in a MOT, and the fluorescence emitted from a fixed number of atoms under different laser conditions were measured to determine the saturation parameters empirically. Two saturation parameters P_sat= 1.15 (0.06) mW and P_r,sat= 2.05 (0.59) mW were successfully extracted from the model, and the excited-state fraction in the four-level model was accurately calculated as a function of the MOT trap parameters, which ranges from 0.045 to 0.415 for the experimental settings currently available. We also observed minor deviations from the four-level model for the photon scattering rate, and a hypothesis of atom pinning under high powers was proposed to explain the problem. We plan to use this simple excited-state fraction determination method to distinguish the ground and excited state collision cross section of Rubidium atoms with species in residual gas of the vacuum. This is the first step to establishing atom loss rates from a MOT as an atomic primary pressure standard.
This thesis describes an investigation and application of the loss of laser cooled atoms from a trap induced by background collisions. The loss rate constant depends on the density of background gas and the velocity averaged collisional loss cross section due to collisions. The velocity averaged collisional loss cross section can be calculated and its dependence on trap depth was verified using a magneto-optical trap. This verification involved measurements of the loss rate constant for a quadrupole magnetic and a magneto-optical trap and measurement of the density of Ar background gas using a residual gas analyzer. The second part of the thesis focuses on an application of these measurements of the loss rate constant to measure the pressure of the background gas. The experimental progress to date on the atom pressure sensor is provided.
Collisions with background atoms are known to induce a significant shift in the frequency of state-of-the-art optical atomic clocks and contribute to state decoherence in cold atom experiments. The effects of these collisions can be quantified by measuring their cross sections. We experimentally measured the collision cross section between 88^{88}88Sr−-−N2_{2}2​ in a Magneto-Optical Trap (MOT). The measurement was carried out by monitoring the atom number loss rate as a function of background pressure of N2_{2}2​ and the cross section thus obtained was 8.1(4)×10−18\times 10^{-18}×10−18 m2^{2}2. The measured collision cross section has been utilized for the determination of C6_{6}6​ coefficient of the ground state (1S0{^1S}_01S0​) of 88^{88}88Sr atom, which can be useful to estimate the relative frequency shift in the clock transition. We also estimate the loss rate induced by the combined effect of the decay of atoms in the long-lived 3P0{^3P}_03P0​ state and temperature-induced atomic losses from the capture volume of the MOT. We find that the contribution due to the latter is dominant in comparison to the other atomic loss channels and must be included in the studies that rely on the total loss rate measurement.
Collisions between ultra-cold (T << 1K) atoms confined in a magneto- optical trap (MOT) may result in the ejection of atoms from the trap. These collisional loss processes, which involve ground state/excited state collisions, fundamentally limit the density attainable in MOT traps. Although accurate calculations of collision rates between ultra-cold atoms require precise knowledge of the molecular potentials over a long range, the loss rate for Li was recently predicted to be significantly less than for the other alkali metals. Motivated by this result, we are measuring the collisional loss rate of Li in a vapor-cell MOT using a red-detuned laser beam to catalyze the collisions, similar to recent experiments using Rb and Cs. We will report our progress to measure the trap loss rate for {sup 6}Li and {sup 7}Li, which will be compared to investigate an isotopic dependence, as was recently observed for Rb.