● Non-equilibrium superfluid dynamics

We observe experimentally the spontaneous formation of star-shaped surface patterns in driven Bose-Einstein condensates. Two-dimensional star-shaped patterns with l-fold symmetry, ranging from quadrupole (l=2) to heptagon modes (l=7), are parametrically excited by modulating the scattering length near the Feshbach resonance. An effective Mathieu equation and Floquet analysis are utilized, relating the instability conditions to the dispersion of the surface modes in a trapped superfluid. Identifying the resonant frequencies of the patterns, we precisely measure the dispersion relation of the collective excitations. The oscillation amplitude of the surface excitations increases exponentially during the modulation. We find that only the l=6 mode is unstable due to its emergent coupling with the dipole motion of the cloud. 

Reference: Physical Review Letters 127, 113001 (2021)

● Quantum atom optics

We report the observation of matter-wave jet emission in a strongly ferromagnetic spinor Bose-Einstein condensate of 7Li atoms. Directional atomic beams with |F =1, mF=1> and |F =1, mF=-1> spin states are generated from |F =1, mF=0>state condensates or vice versa. This results from collective spin-mixing scattering events, where spontaneously produced pairs of atoms with opposite momentum facilitates additional spin-mixing collisions as they pass through the condensates. The matter-wave jets of different spin states (|F =1, mF=±1>) can be a macroscopic Einstein-Podolsky-Rosen state with spacelike separation. 

Reference: Physical Review Letters 127, 043401 (2021)

● Ferromagnetic superfluid

We report the observation of strongly ferromagnetic F = 1 spinor Bose-Einstein condensates of 7Li atoms. The condensates are generated in an optical dipole trap without using magnetic Feshbach resonances so that the condensates have internal spin degrees of freedom. Studying the nonequilibrium spin dynamics, we have measured the ferromagnetic spin interaction energy and determined the s-wave scattering length difference among total spin f channels to be af=2 − af=0 = −18(3) Bohr radius. This strong collision-channel dependence leads to a large variation in the condensate size with different spin compositions. We were able to excite a radial monopole mode after a spin-flip transition between the |mF=0⟩ and |mF=1⟩spin states. The results indicate the spin-dependent interaction energy of our system is as large as 46% of the condensate chemical potential. 

Reference: Physical Review Research 2, 033471 (2020)

● Bose-Einstein condensation of lithium7 atoms

We demonstrate the production of large 7Li Bose-Einstein condensates in an optical dipole trap using D1 gray molasses. The sub-Doppler cooling technique reduces the temperature of 4 × 10^9 atoms to 25 μK in 3 ms. After microwave evaporation cooling in a magnetic quadrupole trap, we transfer the atoms to a crossed optical dipole trap, where we employ a magnetic Feshbach resonance on the |F = 1, m F = 1⟩ state. Fast evaporation cooling is achieved by tilting the optical potential using a magnetic field gradient on the top of the Feshbach field. Our setup produces pure condensates with 2.7 × 10^6 atoms in the optical potential for every 11 s. The trap tilt evaporation allows rapid thermal quench and spontaneous vortices are observed in the condensates as a result of the Kibble-Zurek mechanism.

Reference: Physical Review A 99, 053604 (2019)