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arXiv:1401.7949 · Review of Scientific Instruments

Measurement of vacuum pressure with a magneto-optical trap: A pressure-rise method

R. Moore, Lucie A. Lee, Elizabeth A. Findlay, Lara Torralbo-Campo, Graham D. Bruce, Donatella Cassettari

Published Sep 1, 2015• 39 citations
arXivHTMLPDF

Abstract

The lifetime of an atom trap is often limited by the presence of residual background gases in the vacuum chamber. This leads to the lifetime being inversely proportional to the pressure. Here, we use this dependence to estimate the pressure and to obtain pressure rate-of-rise curves, which are commonly used in vacuum science to evaluate the performance of a system. We observe different rates of pressure increase in response to different levels of outgassing in our system. Therefore, we suggest that this is a sensitive method which will find useful applications in cold atom systems, in particular, where the inclusion of a standard vacuum gauge is impractical.

DOI
10.1063/1.4928154

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The lifetime of an atom trap is often limited by the presence of residual background gases in the vacuum chamber. This leads to the lifetime being inversely proportional to the pressure. Here, we use this dependence to estimate the pressure and to obtain pressure rate-of-rise curves, which are commonly used in vacuum science to evaluate the performance of a system. We observe different rates of pressure increase in response to different levels of outgassing in our system. Therefore, we suggest that this is a sensitive method which will find useful applications in cold atom systems, in particular, where the inclusion of a standard vacuum gauge is impractical.
We present a simple, reliable, and nondestructive method for the measurement of vacuum pressure in a magneto-optical trap. The vacuum pressure is verified to be proportional to the collision rate constant between cold atoms and the background gas with a coefficient k, which can be calculated by means of the simple ideal gas law. The rate constant for loss due to collisions with all background gases can be derived from the total collision loss rate by a series of loading curves of cold atoms under different trapping laser intensities. The presented method is also applicable for other cold atomic systems and meets the miniaturization requirement of commercial applications.
Ultra-high vacuum measurement and extremely high vacuum (UHV/XHV) measurement play an important role in high-tech fields such as deep space exploration, particle accelerators, and nanoscience; with the continuous extension of the lower limit of measurement, especially when it reaches the order of 10–10 Pa, higher requirements are placed on the accuracy of the measurement. At present, in the field of UHV/XHV measurement, ionization gauges based on the principle of neutral gas ionization are commonly applied to the vacuum measurement. However, traditional ionization vacuum gauges during use can create electronic excitation desorption effects, soft X-rays, and the effect of hot cathode outgassing, thereby affecting the accuracy of measurement and limiting the lower limit of measurement. Compared with the traditional measurement technology, this method uses the relationship between the loss rate and pressure caused by the collision of cold atoms trapped in the trap depth with the background gas to calculate the gas density and inversely calculate the vacuum pressure. Based on the intrinsic quantum mechanical properties of cold atom collisions, this method is expected to be developed into a new vacuum traceability standard. In this paper, based on the small-angle approximation and impulse approximation under the quantum scattering theory, the loss rate coefficient of the collision of 6Li cold atoms with background gas molecules is calculated. According to the ideal gas equation, the pressure inversion formula is obtained. The collision loss rate is extracted by accurately fitting the loss curve of the cold atom. In order to improve the accuracy of vacuum inversion and reduce the influence of quantum diffractive collision on loss rate measurement, the trap depth under the conditions of a certain cooling laser intensity, detuning, and magnetic field gradient is determined by the photoassociation method. Finally, in a range of 1 × 10–8–5 × 10–6 Pa, the inverted pressure value is compared with the measured value of the ionization meter, proving that this method has good accuracy and reliability in the inversion of vacuum pressure. At present, the main factor restricting the improvement of accuracy is the influence of the collision between the excited atoms in the magneto-optical trap and the background gas on the loss rate measurement. In the future, with the proportion of excited atoms and the excited state C6 coefficient to be precisely determined, the uncertainty of vacuum pressure measurement can be further reduced.
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.
Cold atoms play a very important role in metrology. The ultra-high or extreme-high vacuum pressure can be accurately measured by investigating the collision of cold atoms and residual atoms in the environment. We report a vacuum pressure metrometer using cold 7Li atoms as sensor atoms in a magneto-optical trap and a magnetic trap. The accuracy, uncertainty, and feasibility of the measurements in such two different traps are studied and compared in the pressure range from 10−5 to 10−8 Pa. It is found that the vacuum pressure measurement in a magneto-optical trap is more advantageous with a higher accuracy, higher signal-to-noise ratio, and shorter measurement time under current conditions. The results may be used to realize an applicable cold atom quantum vacuum standard.
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.