# Ultracold Neutral Atoms Set into Rotation by Synthetic Gauge Potentials / Dr. Yu-Ju Lin, Associate Research Fellow

Dr. Lin’s group studies ultracold bosonic neutral atoms, which are dilute quantum gases in the degenerate regime. These ultracold gases are atoms present at microkelvin to nanokelvin temperature ranges; these systems are simple and can be well controlled because the effective interaction is short range and tunable, and the optical or magnetic trapping potentials are precise. Thus, quantum physics in our textbooks can be manifested in such systems. Dr. Lin uses them as a platform to study fundamental quantum physics, simulate complicated models in condensed matter physics, and create new types of coupling with designed properties that can lead to new systems with no counterparts in materials.

Dr. Lin’s research focuses on synthesizing a magnetic field B*=∇×A* for neutral atoms with a corresponding synthetic gauge potential A*; this is equivalent to generating a Lorentz force for moving atoms, thus simulating charged particles in a real magnetic field B (see Fig. 1). The motivation is that the charge neutrality of the atoms has limited physicists from simulating interesting phenomena where charged particles are under electromagnetic fields, such as those in quantum Hall physics and topological phases.  Therefore, through the generation of synthetic A* and B*, Dr. Lin expects to explore novel quantum phenomena resulting from gauge fields.

Dr. Lin’s group created an ultracold atomic gas, which is an 87Rb Bose–Einstein condensate (BEC) of N=2×105 atoms with temperatures of a few ×10 nK. The BEC is produced after laser cooling and evaporative cooling, and the atoms are trapped by optical tweezer beams (see our setup in Fig. 2). A synthetic gauge potential A* is then created along the azimuthal direction ϕ ̂. This is achieved by using two Raman beams, one of which is a Laguerre–Gaussian beam carrying orbital angular momentum (OAM) of light. The two-photon Raman coupling from the two beams transfers OAM of h to the center-of-mass of the atoms as the atomic spin state changes, thus creating a spin-OAM coupling (SOAMC). That is, the cold atoms initially spin polarized with zero angular momentum can be set into rotation by the OAM-carrying Raman beams, which couple the atoms into other spin states with nonzero angular momentum. To be precise yet technical, Fig. 3 displays the atoms in each spin state, mF= −1, 0, 1 after the Raman coupling is turned on. The atoms are initially prepared in spin mF = l−1 state with zero OAM l−1 =0 before the Raman coupling is turned on. The Raman beams couple spin mF with OAM lmF to spin mF+1 with OAM lmF+1 = lmF+h, and thus we have mF = −1, l-1=0 state, mF = 0, l0=ℏ, and mF = 1, l1=2ℏ states for each Raman detuning δ. Such a correlation between spin mF and OAM lmF is the SOAMC. We further employed the azimuthal gauge potential arising from the SOAMC as an effective rotation, and this can be used to study the rotational properties of superfluids under thermal equilibrium in a well-controlled manner.

References:

[1] I. Bloch, J. Dalibard, and W. Zwerger, Rev. Mod. Phys., 80, 885 (2008).
[2] N Goldman, G Juzeliūnas, P Öhberg and I B Spielman, Rep. Prog. Phys. 77, 126401 (2014).
[3] H.-R. Chen et al., Phys. Rev. Lett. 121, 113204 (2018).
[4] P.-K. Chen et al., Phys. Rev. Lett. 121, 250401 (2018).