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International Symposium on Advanced Optical Manufacturing and Testing Technologies (AOMATT)

Optimized design of high-flux 2D magneto-optical trapping

Wei Song, SEN JIA, XIANHUA WANG, WU Cui-gang

Published Jan 18, 2019• 0 citations

Abstract

High-precision cold atomic interferometry requires a large flux of cold atomic groups, the length of 2D-MOT largely influences the number of cold atomic clusters in the 3D-MOT flux.The longer the 2D-MOT length, the longer the interaction of the atoms with the light field, so that the trajectory of the atom impinges on the inlet of the differential pump tube so that its divergence is small enough to reach the tube, but the length of 2D-MOT is not the longer,the better.There are two reasons for this: first, higher atomic flux and faster atoms are trapped as the length of 2D-MOT increases, resulting in an increase in the number of collisions between lateral radicals.Second, the average speed in the atomic beam increases.Above an optimum MOT length every increase in length will only add faster atoms to the beam, thus increasing the value for the mean velocity.In summary, choosing the best 2D-MOT length plays a key role in getting the number of cold radicals entering 3D-MOT.In this paper, through mathematical modeling and finite element analysis,2D-MOT race-track anti-Helmholtz coils are numerically calculated.Analyzing the distribution of zero magnetic field for coils of different lengths and zero drift and magnetic field gradient changes caused by the error of asymmetrical coil position , the number of uniform turns and parallelism in the process of processing and assembly.The result provides reliable theoretical guidance for the design and manufacture of the magnetic field system of 2D-MOT high-precision cold-atom interferometers.

DOI: 10.1117/12.2506119

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A continuous beam of cold atoms is considered as an alternative to improve the performance of caesium frequency standards. A two-dimensional magneto-optical trap (2D MOT) is outlined. The assembling of three counter-propagating circularly polarised laser beams and of two magnetic field gradients along the horizontal axes leads to a transverse cooling and trapping, whereas in the vertical z-direction the atoms are only cooled and not trapped because of the zero magnetic field in this direction. In order to predict the characteristics of the beam of cold atoms, a simulation of the atomic trajectories was carried out by using a two-dimensional model of Doppler cooling and magneto-optical trapping. The velocity distribution of the atoms, which, due to the trapping and cooling process, enter the zone, where they are launched, is calculated.

We have constructed in our laboratory two rectangular shaped current carrying coils in accordance with the parameters used in our design so that they can be put in use in the Two Dimensional Magneto Optical Trap (2D-MOT)to obtain very high value of Optical Depth(OD). The dimensions of the coils are 8cm x 10cm. When put in anti-Helmholtz configuration as required in a 2D-MOT, these coils generate magnetic field such that it becomes zero along a line bisecting the space between the two coils. The magnetic field gradient near the zero magnetic field line is measured to be 3.33 Gauss/cm when the current passing through the coils is 3.5A. We have also made measurements at other values of current and found, as expected, this gradient increases with current. These results match nicely with the estimated values and hence a significant step forward towards the experimental realization of the design proposed by us.

In a cold atom interferometer, a bias magnetic field, usually generated by a solenoid or a set of coils, is used to select specific Raman transitions of atoms in the interference region, and its inhomogeneity leads to a Zeeman-effect-induced error for the measurement. To improve the measurement’s accuracy further, we propose a simple method to compensate for the non-uniformity of the bias field by reversing the direction and tuning the value of the current running through the pair of quadrupole coils used for magneto-optical trap (MOT). To smoothen the field with MOT coils, the current applied should be controlled timely and precisely, which was done with a fast-responding electric circuit we designed and built. We also used Raman spectroscopy to characterize the magnetic field profile without and with our method applied, which shows that the length of the homogeneous field (where the fluctuation of magnetic strength is less than 10%) is expanded from 81.87 mm to 101.64 mm. From our calculations, with the free evolution time T of 30 ms, the quadratic Zeeman-effect-induced phase shift Δϕzeeman can be reduced by 76%, which reduces the quadratic Zeeman-effect-induced error of measured gravity Δgzeeman to a quarter of that without our method applied (from 1.745 mGal to 0.418 mGal). Simulation and experiment prove that this method is effective. This method can also be used in other atom interferometers, especially compact systems.

This thesis reports on the generation of MOT for laser cooling and trapping of Rubidium 87 atoms. Probe-pump technique have been used in Rb gas cell and doppler free absorption peaks has been observed. Peak at the crossover frequency will be locked for laser cooling and repump beams. This locked freuqency used in MOT to trap Rb atoms. The technique of optical molasses is used to cool the atoms and quadrupole magnetic field applied to confine or trap atoms at the center of MOT. Two chambers are constructed for 2D and 3D MOT with different pressure. Rb atoms loaded efficiently from the high flux cold atom beam generated in a compact 2D MOT. This is the first step of Bose Einstein Condensate (BEC). A performance of 2D MOT was tested by another main chamber of 3D MOT contains 3 pairs of counter-propagating beams. Over 5x10⁶ atoms trapped in a 3D MOT with loading rate of 1:8x10⁶ atoms/sec. This experiment provided an excellent starting point for future experiment on Rydberg atoms in dense ultracold quantum gases.

A cold atom source is important for quantum metrology and precision measurement. To reduce the quantum projection noise limit in optical lattice clock, one can increase the number of cold atoms and reduce the dead time by enhancing the loading rate. In this work, we realize an enhanced cold mercury atom source based on a two-dimensional (2D) magneto-optical trap (MOT). The vacuum system is composed of two titanium chambers connected with a differential pumping tube. Two stable cooling laser systems are adopted for the 2D-MOT and the three-dimensional (3D)-MOT, respectively. Using an optimized 2D-MOT and push beam, about 1.3×106 atoms, which are almost an order of magnitude higher than using a pure 3D-MOT, are loaded into the 3D-MOT for 202Hg atoms. This enhanced cold mercury atom source is helpful in increasing the frequency stability of a neutral mercury lattice clock.