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Optics Express

Characterizing atom clouds using a charge-coupled device for atom-interferometry-based G measurements

Hua-Qing Luo, Yao-Yao Xu, Xin-Ke Chen, Heng Zhang, Xiao-Bing Deng, De-Kai Mao, Min-Kang Zhou, Xiao-Chun Duan, Zhong-Kun Hu

Published Mar 10, 2022• 3 citations
Publisher

Abstract

Precise information of positions and sizes of atom clouds is required for atom-interferometry-based G measurements. In this work, characterizing atom clouds using a charge-coupled device (CCD) is presented. The parameters of atom clouds are extracted from fluorescence images captured by the CCD. For characterization, in-situ calibration of the magnification of the imaging system is implemented using the free-fall distance of atom clouds as the dimension reference. Moreover, influence of the probe beam on measuring the positions of atom clouds is investigated, and a differential measurement by reversing the direction of the probe beam is proposed to suppress the influence. Finally, precision at sub-mm level for characterizing atom clouds is achieved.

DOI
10.1364/oe.454660

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Precise information of positions and sizes of atom clouds is required for atom-interferometry-based G measurements. In this work, characterizing atom clouds using a charge-coupled device (CCD) is presented. The parameters of atom clouds are extracted from fluorescence images captured by the CCD. For characterization, in-situ calibration of the magnification of the imaging system is implemented using the free-fall distance of atom clouds as the dimension reference. Moreover, influence of the probe beam on measuring the positions of atom clouds is investigated, and a differential measurement by reversing the direction of the probe beam is proposed to suppress the influence. Finally, precision at sub-mm level for characterizing atom clouds is achieved.
A fluorescence detection scheme coupled to a highly sensitive nitrogen-cooled CCD camera is used to image the spatial distribution of a low-density falling rubidium atomic cloud released from an optical trap. The falling cloud passes through a thin probe laser beam tuned to resonance. The performance of the scheme is illustrated in the analysis of cold atomic clouds collimated by pinholes during their free fall under the influence of gravity. Clouds of approximately 10(4) atoms and with typically 10(6) at./cm(3) density are analyzed spatially with 24-mum resolution. This method is compared with different atomic cloud imaging techniques.
Atom interferometers (AIs) are promising tools for precision measurement with applications ranging from geophysical exploration and tests of the equivalence principle of general relativity to the detection of gravitational waves. Their optimal sensitivity is ultimately limited by their detection noise. We review resonant and near-resonant methods to detect the atom number of the interferometer outputs, and we theoretically analyze the relative influence of various scheme dependent noise sources and the technical challenges affecting the detection. We show that for the typical conditions under which an AI operates, simultaneous fluorescence detection with a charge-coupled device sensor is the optimal imaging scheme. We extract the laser beam parameters such as detuning, intensity, and duration required for reaching the atom shot noise limit.
Abstract Cold atom experiments often use images of the atom clouds as their exclusive source of experimental information. The most commonly used technique is absorption imaging, which provides accurate information about the shapes of the atom clouds, but requires care when seeking the absolute atom number for small atom samples. In this paper, we present an independent, absolute calibration of the atom numbers. We directly compare the atom number detected using dark-ground imaging to the one observed by fluorescence imaging of the same atoms in a magneto-optical trap. We normalise the signal using single-atom resolved fluorescence imaging. In order to be able to image the absorption of the very low atom numbers involved, we use diffractive dark-ground imaging as a novel, ultra-sensitive method of in situ imaging for untrapped atom clouds down to only 100 atoms. We demonstrate that the Doppler shift due to the acceleration of the atoms by the probe beam has to be taken into account when measuring the atom-number.
A charge-coupled device (CCD) camera capable of taking high-quality images of ultracold atomic samples can often represent a significant portion of the equipment costs in atom trapping experiment. We have modified the commercial control software of a CCD camera designed for astronomical imaging to take absorption images of ultracold rubidium clouds. This camera is sensitive at 780 nm and has been modified to take three successive 16-bit images at full resolution. The control software can be integrated into a Matlab graphical user interface with fitting routines written as Matlab functions. This camera is capable of recording high-quality images at a fraction of the cost of similar cameras typically used in atom trapping experiments.
Accurately measuring the three-dimensional position of an atomic cloud is essential for improving the precision of atom interferometers. In the work presented here, the three-dimensional position of an atomic cloud in an atom interferometer is measured with only one laser beam, generated by a phase modulator. The vertical position is determined from the change in the additional-sideband effect with the time interval TTT between adjacent Raman pulses in the atom interferometer. Based on this method, the vertical position of the atomic cloud is measured with an uncertainty of 0.2 mm; therefore, the uncertainty in a gravity measurement contributed by the gravity gradient is not limited by the uncertainty in the measurement of the vertical position at the level of $0.1\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{Gal}$. Furthermore, a method of measuring Rabi oscillations at different horizontal positions to determine the horizontal position of the atomic cloud is proposed and is demonstrated experimentally. With this method, the horizontal position can be determined with an uncertainty of less than 0.1 mm, and the horizontal velocity of the atomic cloud can also be deduced. The methods demonstrated in this paper are helpful for evaluating systematic errors in atom interferometers, and also useful in precision measurements based on an atomic fountain.