| Ultracold molecules possess many unique features,including extremely low temperatures,additional internal rotational and vibrational degrees of freedom,and permanent electrical dipole moments.These characteristics provide a new platform for research in the fields of quantum chemistry,strongly correlated quantum systems,quantum information processing,and precision tests of fundamental physics.Creating dense samples of ultracold molecules has been a long-standing goal in the field.Ultracold 23Na40K ground state molecules,which are chemically stable in two-body collisions and possess a large dipole moment,are an ideal candidate for studying strongly interacting and dipolar Fermi gases.The main research focus of this thesis is the construction of an ultracold 23Na40K ground-state molecule setup,and the use of this setup to create ultracold 23Na40K polar molecules in the absolute rovibrational ground state.The main research contents of this paper include:1.Building an experimental setup for ultracold 23Na40K ground state molecules,which includes a vacuum system,a magnetic field system,a high-power laser system,a microwave radio frequency generation and control system,and a STIRAP laser system.This thesis describes the experimental subsystems for creating ultracold 23Na40K ground-state molecules,which include the vacuum system,magnetic field system,highpower laser system,microwave or radiofrequency generation and control system,and STIRAP laser system.2.Achieving quantum degeneracy in a mixture of ultracold 23Na and 40K gases.In the experiment,sodium and potassium atoms were first captured in a 2DMOT and then pushed into a dual-species 3DMOT.The 3DMOT is a dark trap that significantly increases the density of the captured atom samples.To reduce light-assisted losses between atoms,the potassium atoms were captured before the sodium atoms.After loading the atoms into the 3DMOT,a sequence of compressed magneto-optical trapping,gray molasses,and optical pumping was used to further cool the atoms and prepare them in a low-field seeking state that can be trapped in the magnetic trap.After loading the dual-species sample into an optically plugged magnetic quadrupole trap,the mixture was subjected to forced microwave evaporation cooling.During the evaporation process,the magnetic trap depth was reduced to minimize the three-body loss of the mixed gas.To further reduce the temperature of the atomic sample,the dual-species sample was loaded into a far detuning large volume crossed optical dipole trap.In order to minimize the three-body loss between atomic samples,sodium atoms were transferred from |2,2>state to |1,1>state.After the evaporation cooling in the optical trap,a degenerate Bose-Fermi mixture was experimentally obtained.3.Synthesizing 23Na40K Feshbach molecules based on a mixture of ultracold 23Na and 40K quantum degenerate gases.Near the Feshbach resonance,atoms can be bound together to form weakly bound Feshbach molecules.Two different methods,radiofrequency association and magneto-association,were employed to synthesize weakly bound Feshbach 23Na40K molecules.The magnetic association method was used to produce 3×104 weakly bound Feshbach molecules with a temperature of 251 nK and a conversion efficiency of 20%from atoms to molecules.The collision characteristics between Feshbach molecules and atomic mixtures were also experimentally studied.4.Using the stimulated Raman adiabatic passage,weakly bound Feshbach molecules were transferred to the rovibrational ground state.First,the position of the complex intermediate state B’∏|v=4>～C3∑+|v=25>is determined using the excitation spectrum of Feshbach molecules.Subsequently,the position of the absolute rovibrational ground state X1∑+|v=0,J=0>is determined using the EIT spectrum.Once the positions of both states are determined,2.2×104 23Na40K fermionic molecules in the absolute rovibrational ground state are synthesized using the stimulated Raman adiabatic transfer process.The temperature of the molecules is 247 nK,and the one-way STIRAP transfer efficiency is up to 75%.Innovative work includes:1.Designed and constructed a compact vacuum system.The vacuum system is divided into two regions:a 2D region with a vacuum pressure of 10-7 Pa and a 3D region with a vacuum pressure of 10-9 Pa.This configuration minimizes the impact of background gas on the atomic gas in the 3D region.The 2D region is used to capture atomic gases,while the 3D region is used to achieve quantum degeneracy and prepare molecular gases.Using this system,we successfully prepared a mixture of ultracold 23Na and 40K quantum degenerate gases in experiments.2.A high-power laser system is used to construct an optical dipole trap.Here,a simple and effective method is proposed to compensate for the beam deflection caused by the thermal lensing effect of the TeO2 crystal inside the acousto-optic modulator,thereby eliminating its impact on fiber efficiency.Experimentally,by constructing a thin lens imaging structure in the optical path,combined with some simple tuning techniques,the laser pointing deflection caused by the thermal lens effect can be greatly reduced.3.An economic microwave signal scanning scheme has been invented and verified.By using a microwave signal as the main frequency and mixing it with a radio frequency(RF)signal,a microwave signal with adjustable amplitude,frequency,and controllable scanning speed can be generated by scanning the RF signal.4.The STIRAP laser system is a necessary hardware for creating 23Na40K groundstate molecules.In the experiment,dual-sideband modulation PDH frequency stabilization technology is used to lock two external cavity diode lasers to the same ultrastable cavity.To maintain the stability of the ultrastable cavity,a vacuum system and corresponding isolation measures are configured for it.The measurement results show that the finesse of the ultra-stable cavity at the two wavelengths is above 30000,which can ensure that the frequency fluctuation of the two lasers is maintained at the level of kHz.5.Successfully demonstrated that Feshbach molecules in a weakly bound state can be transferred to the rovibronic ground state via a composite intermediate state B1∏|v=4>～c3∑+|v=25>.We experimentally synthesized 2.2×104 fermionic 23Na40K molecules in the absolute rovibronic ground state B1∏|v=4>～c3∑+|v=25>with a temperature of 247 nK,achieving a single STIRAP transfer efficiency as high as 75%. |
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