Orbital angular momentum (OAM) of light represents a fundamental optical freedom that can be exploited to manipulate quantum state of atoms. In particular, it can be used to realize spinorbital-angular-momentum (SOAM) coupling in cold atoms by inducing an atomic Raman transition using two laser beams with differing OAM. Rich quantum phases are predicted to exist in manybody systems with SOAM coupling. Their observations in laboratory, however, are often hampered by the limited control of the system parameters. In this work we report, for the first time, the experimental observation of the ground-state quantum phase diagram of the SOAM coupled Bose-Einstein condensate (BEC). The discontinuous variation of the spin polarization as well as the vorticity of the atomic wave function across the phase boundaries provides clear evidence of firstorder phase transitions. Our results open up a new way to the study of phase transitions and exotic quantum phases in quantum gases. arXiv:1806.06263v2 [cond-mat.quant-gas]

We experimentally and theoretically observe the expansion behaviors of a spherical Bose-Einstein condensate. A rubidium condensate is produced in an isotropic optical dipole trap with an asphericity of 0.037. We measure the variation of the condensate size during the expansion process. The free expansion of the condensate is isotropic, which is different from that of the condensate usually produced in the anisotropic trap. The expansion in the short time is speeding and then after a long time the expansion velocity asymptotically approaches a constant value. We derive an analytic solution of the expansion behavior based on the spherical symmetry, allowing a quantitative comparison with the experimental measurement. The interaction energy of the condensate is gradually converted into the kinetic energy at the beginning of the expansion and the kinetic energy dominates after a long-time expansion. We obtain the interaction energy of the condensate in the trap by probing the expansion velocity, which is consistent with the theoretical prediction.

We have experimentally produced rubidium Bose-Einstein condensate in an optically-plugged magnetic quadrupole (OPQ) trap. A far blue-detuned focused laser beam with a wavelength of 532 nm is plugged in the center of the magnetic quadrupole trap to increase the number of trapped atoms and suppress the heating. A radio frequency (RF) evaporative cooling in the magneto-optical hybrid trap is applied to decrease the atom temperature into degeneracy. The atom number of the condensate is 1.2(0.4)×105 and the temperature is below 100 nK. We have also studied characteristic behaviors of the condensate, such as phase space density (PSD), condensate fraction and anisotropic expansion.PACS numbers: 67.10.Ba; 64.70.fm; 37.10.De Since the experimental observation of Bose-Einstein condensate (BEC) in a dilute gas [1][2][3], the ultracold quantum gas has become a reachable tabletop to carry on a wide range of research, such as accurate measurement on physical constants [4,5] [19,20], and magneto-optical combination trap [21]. Compared to many other kinds of traps, the optically-plugged magnetic quadrupole (OPQ) trap has many advantages, which has been demonstrated in some groups [3,[22][23][24][25]. First, the tight confinement allows for fast radio frequency (RF) evaporative cooling and the large trapping volume offered by the magnetic quadrupole trap facilitates loading of a large number of atoms from the magneto-optical trap (MOT). Secondly, the atom cloud is positioned exactly in the center of the glass cell on the symmetry axis of the quadrupole coil pair, which ensures very good optical access to atoms. Third, the quadrupole coil pair allows to create large homogeneous magnetic fields by switching to copropagating currents, which we can use to address Feshbach resonances.In this paper, we produce rubidium BEC in an OPQ trap. Here we show that the laser beam with a wavelength of 532 nm can be used to efficiently obtain 87 Rb BEC despite the large detuning of the optical plug laser * Electronic address: kjjiang@wipm.ac.cn from the rubidium transition line at 780 nm. The atom number of the condensate is 1.2(0.4) × 10 5 and the temperature is less than 100 nK, which matches the basic requirements for further advanced studies. We have also studied characteristic behaviors of the condensate, such as phase space density (PSD), condensate fraction and anisotropic expansion. In the near future we will use the obtained rubidium BEC to study the collective oscillations of the quantum gas, and generate an ultracold Bose-Femi mixture using the sympathetic cooling in the same experimental setup where we have cooled fermionic atoms like 6 Li and 40 K [26,27].We use the two-MOTs configuration to realize 87 Rb BEC. The optical arrangement is shown in Fig.1. Two diode lasers (DLs) are individually frequency locked on the two crossover transitions |F = 1 → |F ′ = 1, 2 and |F = 2 → |F ′ = 2, 3 , respectively, using the standard saturation absorption spectroscopy (SAS) method. DL2 affords cooling, probing and pushing beams, after passing...