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Optics Express

Spatial mapping of cold atom clouds using velocity-selective Raman pulses in differential atom interferometers

Kanxing Weng, Cheng Chu, Junjie Tang, Mengxin Zheng, Peishuang Ding, Qi Wang, J. G. Bian, Wenfeng He, Zecheng Zhang, Bing Cheng, Bin Wu, Xiaolong Wang, Qiang Lin

Published Sep 1, 2025• 2 citations
Publisher

Abstract

A method for detecting the internal characteristic (including density, velocity and temperature) distribution of cold atom clouds is reported in this paper. During the detection process, the spatial resolution of the cold atom cloud is achieved at sub-mm by precisely adjusting the spatial position of the Raman laser beam. Meanwhile, to minimize the impact of frequency and intensity fluctuations in the Raman laser on measurement results, we propose to utilize a differential atomic interferometer to suppress common-mode noise, thereby enhancing measurement accuracy. This method enables precise detection of the internal structure of cold atom clouds, providing a reference for in-depth studies of atom cloud properties.

DOI
10.1364/oe.571559

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A method for detecting the internal characteristic (including density, velocity and temperature) distribution of cold atom clouds is reported in this paper. During the detection process, the spatial resolution of the cold atom cloud is achieved at sub-mm by precisely adjusting the spatial position of the Raman laser beam. Meanwhile, to minimize the impact of frequency and intensity fluctuations in the Raman laser on measurement results, we propose to utilize a differential atomic interferometer to suppress common-mode noise, thereby enhancing measurement accuracy. This method enables precise detection of the internal structure of cold atom clouds, providing a reference for in-depth studies of atom cloud properties.
We have developed a cold atom Sagnac interferometer with a single large Raman beam. In combination with a time-of-flight measurement performed by thin probe beams, the variation in spatial contrast of the atomic Mach-Zehnder interference was analyzed by considering the spatial intensity profile of the single large Raman beam and the spatial density profile of the atomic cloud. Based on our analysis, we determined that contrast enhancement is attainable using the spatial selection of the atomic cloud but it reduces angular velocity sensitivity.
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.
Velocity-selective Raman transitions are widely used in atom interferometers to prepare atomic ensembles with narrowly defined momentum distributions. However, differential light shifts between atomic energy levels generate velocity distributions that are correlated with the Raman beam intensity, and therefore with the position of the atoms within the laser beam. We show that these spatially inhomogeneous velocity distributions interact with detuning-dependent systematic effects in a Bragg diffraction-based simultaneous conjugate Ramsey-Bordé interferometer. These interactions can induce systematic phase shifts of order 10 milliradians in the interferometer phase. We further identify a "magic" detuning for velocity selection and show that operating at this detuning suppresses the systematic phase shift. Magic velocity selection eliminates systematic errors arising from correlations between atom velocity and position, facilitating high-resolution atom interferometry experiments targeting sub-part-per-billion accuracy.
We present an elegant application of matter-wave interferometry to the velocimetry of cold atoms whereby, in analogy to Fourier transform spectroscopy, the one-dimensional velocity distribution is manifest in the frequency domain of the interferometer output. By using stimulated Raman transitions between hyperfine ground states to perform a three-pulse interferometer sequence, we have measured the velocity distributions of clouds of freely expanding 85Rb^{85}\mathrm{Rb}85Rb atoms with temperatures of 34 and $18\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{K}$. Quadrature measurement of the interferometer output as a function of the temporal asymmetry yields velocity distributions with excellent fidelity. Our technique, which is particularly suited to ultracold samples, compares favorably with conventional Doppler and time-of-flight techniques, and it reveals artefacts in standard Raman Doppler methods. The technique is related to, and provides a conceptual foundation of, interferometric matter-wave accelerometry, gravimetry, and rotation sensing.
We demonstrate a minimally perturbative detection method—Raman driven spin noise spectroscopy (RDSNS)—to directly probe the local density of a cloud of cold 39K atoms in real time. The technique relies on detecting the polarization fluctuations of a far-off resonant focused probe light passing through the atoms, driven by two coherent Raman lasers connecting adjacent Zeeman spin states. This is an easily implementable scheme with a wide-ranging applicability in detecting cold atoms. Unlike traditional fluorescence and absorption imaging techniques, this approach does not require stochastic photon scattering from trapped atoms, thereby leaving the cold cloud unperturbed. The data can be collected in MHz refresh rate, allowing for near real-time detection. We validated this technique by monitoring the strength of the RDSNS signal by varying the key parameters of the three-dimensional magneto-optical trap (MOT) and the cold atom source (two-dimensional MOT). We performed experiments to probe local densities across the cold atom cloud using our technique and compared it with the inverse Abel-transformed density profiles extracted from the fluorescence of a spherically symmetric atomic cloud and found excellent agreement. This detection protocol is particularly advantageous for probing systems that lack inherent symmetry, where the Abel transformation is inapplicable. Moreover, RDSNS imaging is suitable for extracting the dynamical properties of systems in real time.
We present a precision atom interferometry gyroscope with a Sagnac area of 6.0 cm 2 . To drive the cold atom interferometry gyroscope, we utilize two sets of Raman beams with a pulse sequence of π /2- π - π - π /2. We present a procedure for aligning the two sets of Raman beams at the microradian level by making use of the atom interferometer itself in different configurations. Additionally, we measured the atomic cloud trajectory and implemented a spatial symmetry alignment for the atom interferometer. With these improvements, we construct a sensitive atom gyroscope with a stability of 1 nrad/s at an integration time of 10000 s. We analyzed the influence of Raman beams and the limitations of the system, and present the technical limits to the sensitivity and long-term stability. Finally, we point out the primary sources of noise in the atom interferometry gyroscope, clearing the way for the construction of further large-area atom gyroscopes.