A research group, led by Prof. LIU Liang from the Shanghai Institute of Optics and Fine Mechanics (SIOM) of the Chinese Academy of Sciences (CAS), recently demonstrated one-dimensional DLC of atoms, and the related results are published in Physical Review on March 28, 2022.

In their research, a vacuum glass tube with a length of 105 cm and diameter of 2 cm, as shown in the upper part of Fig.1, is used to generate cold atoms. The lower part of Fig.1 shows a uniformly distributed cold ⁸⁷Rb-atom ensemble cooled by diffuse lasers along the axis with an atomic optical depth of more than 4 and a temperature of about 25μK.

Laser cooling of neutral atoms has been rapidly developed since it was proposed in 1975, and laser-cooled atoms are now applied extensively in fundamental physics, modern metrology, quantum sensing, precision measurements, space technology, etc. in many labs worldwide.

Typical configurations of laser cooling of atoms include magneto-optical trap (MOT) and optical molasses. Both methods require strict alignments of propagating direction, polarization and power of 3 pairs of counter-propagating laser beams, while MOT needs an anti-Helmholtz magnetic coil for the trapping of cooled atoms. Due to their principle, however, the number and size of cold atom ensemble in both MOT and molasses are limited, typically around 10⁹ atoms in 1 cm³ volume.

Proposed by Prof. Yu-Zhu Wang in 1979, a third method called diffuse laser cooling (DLC) has attracted many attentions recently due to its successful application in atomic clocks, which use diffuse laser cooled atoms in a spherical or cylindrical microwave cavity as the clock medium. The DLC does not need any alignment of laser beams and thus makes the laser cooling simple and robust. The success of DLC in a microwave cavity suggests that this method can be extended to many other shapes.

Such a result is remarkable. It is the first time to demonstrate experimentally cold atom ensemble at meter-long size, an incredible increase comparing with MOT or molasses. It is even more remarkable that the length can be extended to km or even longer, and it is also possible to have 2D cold atoms. In fact, the DLC can work in any shape at any size in principle, and such a feature can be realized only in DLC.

The DLC opens the door for the applications of cold atoms more broadly. For example, with the help of DLC, a large area quantum detector (LAQD) based on cold atoms can be constructed. The LAQD can sense signals simultaneously in large area and such an arrangement will greatly increase the time and space resolution of detection. There is no doubt that DLC has a great potential in future quantum sensing, and this work is an important milestone of laser cooling of neutral atoms.

Fig. 1. Experimental vacuum system and normalized cold atom number along the axis of the tube. (Image by SIOM)