Angular Momentum Measurement And Phase Stabilization Of Vortex Light Field For Ultracold Atom Manipulation

Since the realization of Bose-Einstein condensates(BEC)in the laboratory in 1995,the study in the field of ultra-cold atoms has been rapidly developing.In ultra-cold atomic systems,some novel quantum properties caused by quantum coherence have emerged.It has not only become an important experimental platform for simulating novel quantum phases in condensed matter physics,but also developed into an important tool for various precision measurements.On the other hand,as an important light field for manipulating condensates,the properties and interference characteristics of vortex beams have also arisen widespread attention.We measured the orbital angular momentum(OAM)of the vortex beam by observing the doublebeam interference phenomenon,and improved the stability of the relative phase between two vortex beams by building a displaced Sagnac interferometer.The vortex beams with stable relative phase and definite OAM are applied to the existing 87Rb atom BEC experimental platform.The vortex BEC is prepared,and the stable vortex atom interference pattern is obtained.These works provide an experimental basis for the condensed matter interferometer to be applied to the precise measurement.The main achievements of this paper are summarized as follows:Firstly,the orbital angular momentum of vortex beam is measured by doublebeam interference.The temporal rotation and radial twist of the interference pattern between vortex beam and its conjugate copy are investigated by building a Mach-Zehnder(MZ)interferometer with adjustable frequency and optical path.By modifying the conjugate beam with a frequency difference or a curvature difference,we determine the sign from the rotation direction of the interference pattern or the twist direction of the interference fringe.We further measure two parameters,the angular rotation velocity Ω and twist parameter α,respectively,to quantitatively characterize the effects of the sign of the OAM on the interference pattern.The magnitude of the OAM is simply determined by the number of interference fringes.The theory of the optical interference between two vortex beams has quantitative agreements with the experimental results.Secondly,Using Raman beams and Radio Frequency(RF)magnetic field,the interference pattern of vortex matter waves in ultracold atoms was observed.We generalize the data processing method in the quantitative measurement of vortex beams,and apply it to extract the information of atom interference pattern.This method is suitable for interference patterns with angular periodicity,and the process include finding the maximum point and the center point by neighborhood averaging method,determining the angular orientation and evaluating the contrast.This complete data processing method is also help to select the best experimental parameters and evaluate the stability of the interferograms.Finally,the relative phase instability of Raman beams manipulating cold atoms will lead to the random rotation of orientation angle of vortex atom interference image in repeated experiments.We tested the stabilities of four types of optical interferometers(MZ interferometer,Sagnac interferometer,equal-armed Michelson interferometer,and displaced Sagnac interferometer),and selected the displaced Sagnac interferometer to prepare the Raman beams required for the experiment.Then,we build a displaced Sagnac interferometer which can be used in the atom interference experiment,and obtained stable atom interference patterns.By controlling the phases of RF process and Raman process,the rotation manipulation of the atom interference pattern is realized.The interference pattern and the phase correspondence of the rotation manipulation in experiments are found to be in good agreement with the theoretical analysis.