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Optics Express

Experimental two-dimensional Raman cooling with orthogonal geometry

Xiangyu Dong, Yan Zhang, Zhennan Liu, Shuai Wang

Published Sep 23, 2025• 0 citations
Publisher

Abstract

Despite the remarkable achievements in precision measurement, the transverse diffusion of atom ensembles remains the main factor that hinders further improvements in the accuracy of matter-wave interferometry. This paper demonstrates a two-dimensional deep cooling method based on the orthogonal Raman geometry. After 35 milliseconds of cooling, the atom ensemble reaches a temperature lower than 100 nanokelvins in the horizontal plane, and a cooling efficiency of 30% approximately. In the meantime, the cooled atoms exhibit high uniformity in all directions. These characteristics render this cooling protocol more suitable for quantum sensing compared with ultracold atoms or optical molasses.

DOI
10.1364/oe.576353

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Despite the remarkable achievements in precision measurement, the transverse diffusion of atom ensembles remains the main factor that hinders further improvements in the accuracy of matter-wave interferometry. This paper demonstrates a two-dimensional deep cooling method based on the orthogonal Raman geometry. After 35 milliseconds of cooling, the atom ensemble reaches a temperature lower than 100 nanokelvins in the horizontal plane, and a cooling efficiency of 30% approximately. In the meantime, the cooled atoms exhibit high uniformity in all directions. These characteristics render this cooling protocol more suitable for quantum sensing compared with ultracold atoms or optical molasses.
We report the implementation of a two-dimensional Raman cooling scheme using sequential excitations along the orthogonal axes. Using square pulses, we have cooled a cloud of ultracold cesium atoms down to an rms velocity spread of 0.39(5) recoil velocities, corresponding to an effective transverse temperature of 30nK30\phantom{\rule{0.3em}{0ex}}\mathrm{nK}30nK (0.15Trec)(0.15{T}_{\mathrm{rec}})(0.15Trec​). This technique can be useful to improve cold-atom atomic clocks and is particularly relevant for clocks in microgravity.
Summary form only given. Laser cooled atoms play a critical role in modern designs of frequency standards such as atomic fountains. Recently, an elegant and efficient method of two-dimensional cooling to the vibrational ground state of a far-off resonance optical lattice has been demonstrated. This method is a variant of Raman sideband cooling based on the Raman transitions between the vibrational manifolds of adjacent Zeeman substates. A static magnetic field is used to tune the Zeeman levels so that Raman resonance occurs on the red sideband and results in cooling. Two circularly polarized fields are used to recycle the atoms for repetitive Raman cooling. Unfortunately, a direct application of this scheme to frequency standards meets with several difficulties. Stimulated by these concepts and results, we propose a new variant of transverse sideband cooling, where these problems are avoided, while still maintaining most of the attractive features. The field configuration used for the optical lattice consists of three linearly polarized beams having equal amplitudes and propagating in the xy plane with angles of 2/spl pi//3 between each other. The polarization vectors of these beams are tilted through a small angle with respect to the z-axis. All the beams have the same frequency far-detuned to the red of the D/sub 2/ resonance line. Raman transitions between vibrational levels of adjacent magnetic substates are induced by the small in-plane component of the field. Two other important requirements for Raman sideband cooling are a uniform energy shift of the magnetic sublevels and optical pumping.
Abstract Two approaches for solving the long-standing problem of deep laser cooling of neutral magnesium atoms are proposed. The first one uses optical molasses with orthogonal linear polarizations of light waves. The second approach involves a ‘nonstandard’ magneto-optical trap (NMOT) composed of light waves with elliptical polarizations (in general). Both the widely used semiclassical approach based on the Fokker–Planck equation and quantum treatment fully taking into account the recoil effect are employed for theoretical analysis. The results show the possibility of obtaining temperatures lower than 100 µ K simultaneously with a large number of cold atoms ~10 6 ÷ 10 7 . A new velocity-selective cooling technique allowing one to reach the microkelvin temperature range is also proposed. This technique may have some advantages over, for instance, the shallow-dipole-trap technique utilized by other authors. In the case of magnesium atoms this new technique may be used for obtaining a large number of ultracold atoms ( T ~ 1 µ K, N > 10 5 ). Such a large number of ultracold atoms is crucial issue for metrological and many other applications of cold atoms.
We demonstrate experimentally the generation of one-dimensional cold gases of 87Rb^{87}\mathrm{Rb}87Rb atoms by diffuse laser cooling. A horizontal slender vacuum glass tube with a length of 105 cm and diameter of 2 cm is used in our experiment. The diffuse laser light inside the tube, which is generated by multireflection of injected lasers, cools the background vapor atoms. With 250 mW of cooling light and 50 mW of repumping light, an evenly distributed 1-m-long profile of cold atom cloud is obtained. We observe a factor 4 improvement in the atomic optical density for a typical cooling duration of 170 ms and a sub-Doppler atomic temperature of 25 $\ensuremath{\mu}\mathrm{K}$. The central number of detected cold atoms remains constant for a free-fall duration of 30 ms. Such samples are ideal for many quantum optical experiments involving electromagnetically induced transparency, electronically highly excited (Rydberg) atoms, and quantum precision measurements.
Using a matter wave lens and a long time of flight, we cool an ensemble of ^{87}Rb atoms in two dimensions to an effective temperature of less than 50_{-30}^{+50} pK. A short pulse of red-detuned light generates an optical dipole force that collimates the ensemble. We also report a three-dimensional magnetic lens that substantially reduces the chemical potential of evaporatively cooled ensembles with a high atom number. By observing such low temperatures, we set limits on proposed modifications to quantum mechanics in the macroscopic regime. These cooling techniques yield bright, collimated sources for precision atom interferometry.
Abstract Isotropic laser cooling (ILC) is widely recognized for its distinct advantages and demonstrates significant potential in quantum precision measurements and quantum sensing technologies. The morphology and density distribution of the cold atomic cloud generated by ILC are strongly influenced by the distribution of cooling light and the structural geometry of the cavity, making precise characterization and optimization of cold atom distribution essential for practical applications. In this paper, we present an innovative flat diffuse cavity design with the dimensions approximating a quasi-two-dimensional configuration, which generates a sheet-like isotropic laser field inside the cavity through diffuse reflections. We thoroughly characterized the system’s performance under different optical parameter settings. A two-dimensional (2D) movable detection system was employed to detect the quasi-two-dimensional distribution of cold atoms. These results demonstrate the ability of ILC to produce diverse morphological and density distributions of cold atoms, which we anticipate will be suitable for quantum sensing.