One-Dimensional Beam Scanner Based on Refractive Galvanometer for Cold Atom Experiment
Objective In recent years,single-atom-array-based quantum information processing has caught intense attention.In single-atom-array-based quantum simulators,programmable quantum processors,and fault-tolerant quantum computing,the transportation and addressing of single atoms play crucial roles.Additionally,the transportation of cold atoms over a long distance from a magneto-optical trap(MOT)loading chamber to a science chamber can maintain lower vacuum pressure or better optical access for experiments.Traditionally,atom addressing and transportation often employ acousto-optic deflectors(AODs)to control the deflection of laser beams on demand.However,there are some restrictions for AODs.The deflected laser frequency varies as the RF driving of AOD changes during atom addressing,and an auxiliary acousto-optical modulator(AOM)is needed to compensate for the frequency,which often leads to a more complex experimental setup.Meanwhile,the clear aperture of the acousto-optic crystal limits the beam size of the transmitted laser and the limited diffraction efficiency of AOD results in substantial optical insertion loss.We experimentally demonstrate a one-dimensional beam scanner based on a refractive galvanometer,which is compatible for laser beams with large cross-sections and has low insertion loss.By adjusting the rotation of the wedged prism of the scanner,we can adiabatically transport the position of an optical dipole trap(ODT)in one dimension over 7 mm within 22.5 ms.The 3 dB bandwidth of this beam scanner is 56 Hz,with an ODT waist of(21.9±0.4)µm.During transportation,the waist size is constant,the variation in the optical power of the ODT is±3.45％,and the ODT position perpendicular to the transportation is±1.7％with respect to the confocal parameter of the ODT.Methods The refractive one-dimensional beam scanner consists of a rotatable wedge prism(optical wedge)and an electromotor[Fig.1(b)].The prism controlled by the electromotor to rotate around the z-axis deflects a collimated incident laser beam along the transverse direction(x-axis).In the movable ODT setup[Fig.2(a)],an achromatic doublet focuses the collimated beam along the y-axis.As the prism is placed at the focal point of the lens,the prism rotation produces a displacement of the focused beam along the x direction.The beam waist is kept in the focal plane.The deflection angle of the laser is much smaller than the rotation angle of the prism(motor)itself to make the accuracy of the beam deflection greater than that of the motor rotation.Additionally,when the scanner is adopted to transport cold atoms,as the refracted laser is insensitive to the vibration of the electromotor in the galvanometer,this setup configuration can greatly improve the spatial stability of the ODT.Furthermore,by designing the moving function of the electromotor driving voltage carefully,we can move the dipole trap(atoms)adiabatically over a long distance.Results and Discussions We construct a one-dimensional beam scanner system based on a refractive galvanometer,which is further manipulated as a movable ODT for transporting cold atoms over a long distance.By measuring the Bode diagram of the scanner,a 3 dB bandwidth of 56 Hz is obtained.During ODT transportation,the ODT waist is maintained at(21.9±0.4)μm,and the variation in the ODT position in the y direction is±48 μm[Fig.2(b)],which is±1.7％of the confocal parameter.Meanwhile,we measure the variation in ODT power with respect to the prism rotation,and obtain a variation of±3.5％in ODT power[Fig.2(c)].To transport atoms over a long distance,we should consider the heating of the atoms in the optical trap during transportation.Therefore,we employ an adiabatic process to avoid heating the atoms.Then,a sine-type acceleration with respect to time in the transportation of the dipole trap is designed to meet adiabatic conditions.The ODT displacement and the deflection angle of the galvanometer are nonlinearly related,while the deflection angle of the prism is linearly related to the galvanometer motor voltage.We deduce the relations between the displacement and driving voltage,and by designing the time function of the electromotor driving voltage carefully,the adiabatic movement of the dipole trap is realized.The ODT can be moved more than 7 mm within 22.5 ms.The experimental results of the ODT trajectory are reasonably consistent with the expected trajectory(Fig.3).Atoms can be transported over 5 mm within 30 ms,with atomic transfer efficiency exceeding 90％and a temperature change of less than 5 μK.Furthermore,when the transportation duration is extended to 45 ms,the transfer efficiency reaches(99.6±4.6)％.By enhancing the motor's response,atoms can be adiabatically transported over 1 mm in 10 ms to yield atomic transfer efficiency of(94.3±3.6)％(Fig.4).Conclusions We experimentally demonstrate a one-dimensional beam scanner based on a refractive galvanometer,which is compatible with laser beams with large spot size and low insertion loss.This scanner can be employed to transport cold atoms over a long distance or realize atom addressing.By adjusting the rotation of the wedged prism in the scanner,we can adiabatically move the ODT in one dimension over 7 mm within 22.5 ms.The 3 dB bandwidth of our beam scanner is 56 Hz.During the movement,the ODT waist is maintained at(21.9±0.4)μm,the variation in the optical power of the ODT is±3.45％,and the ODT position perpendicular to the moving direction is±1.7％relative to the confocal parameter of the ODT.By utilizing this system,atoms can be transported over 5 mm within 30 ms,with atomic transfer efficiency exceeding 90％and a temperature change of less than 5 μK.Furthermore,when the transportation duration is extended to 45 ms,the transfer efficiency reaches(99.6±4.6)％.By enhancing the motor's response,atoms can be adiabatically transported over 1 mm in 10 ms to achieve atomic transfer efficiency of(94.3±3.6)％.Compared to the dipole trap moving system using AOD,this method avoids the requirement for AOD to have a specific polarization for the incident light,providing a larger aperture for light,smaller optical losses,lower noise,and a constant dipole trap light frequency during the movement.In comparison with the reflective galvanometer system,this method exhibits smaller fluctuations in beam directionality,which is beneficial for the application in cold atom addressing experiments.
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