Rubidium 2 is a newly constructed apparatus which is designed to study rubidium atoms with optical lattices, Raman coupling, and Feshbach resonances.

Ultracold atoms are an excellent system to realize important models in condensed matter physics, owing to its nearly disorder-free and tunable properties. One of the most intriguing topics is the quantum-Hall physics of 2D electronic systems in a strong magnetic field. Current experiments mostly use systems of rotating gases to create synthetic magnetic fields for neutral atoms in the rotating frame. However, rotating systems are limited by various technical matters, and consequently it is difficult to create sufficiently large fields required for the quantum-Hall regime. Recent theoretical proposals avoiding this limitation mostly involve a spatially dependent light-atom coupling between internal states. The atoms adiabatically remain in a spatially varying dressed state, where the Berry’s phase corresponds to an effective vector potential. With appropriate spatial dependence, the vector potential has a nonzero curl and yields a synthetic magnetic field.

We have experimentally realized such a synthetic magnetic field for the first time, and observed vortices in a neutral 87Rb Bose-Einstein condensate. Fig. 1ab show the setup of our BEC dressed by a Raman coupling within the F=1 manifold of the electronic ground state. The Raman coupling is characterized by a Rabi frequency ΩR and detuning δ. Fig 1c illustrates the modified dispersion curves (colored) of the Raman-dressed states, where our BEC is loaded into the lowest energy branch (red) with an effective vector potential A*(δ) along x, set by the Raman detuning δ (Fig. 1d). Consequently, a spatial gradient of detuning δ′ along y gives rise to a gradient in vector potential, whose curl is equal to a synthetic magnetic field B*=∇×A*=δ′ (dA*/dδ) along z. Fig. 1ef show the spin-resolved TOF images of the dressed BEC in a zero detuning gradient (e), and in a nonzero gradient with vortices (f). Fig. 2 shows that the vortex number increases with the detuning gradient δ′, which is proportional to the magnetic field B*. The vortices appear only when the field exceeds a critical value, indicating the energy cost for creating a single vortex given a finite system size.

Our experiment has demonstrated an optically induced magnetic field which is stable in the laboratory frame. This is in contrast to the systems of rotating BECs, where due to the complication of rotation, the condensate with vortices is either in a metastable excited state or is subject to dissipation from time-dependent perturbations. Another superior feature in our approach is we can easily add optical lattices. The lattice along the direction of synthetic field can divide the BEC into an ensemble of 2D systems. With suitable lattice configuration, we should be able to reach the quantum-Hall regime with ~ 200 vortices and < 200 atoms in each 2D system, and a practical ~20nK interaction energy.

References:

1. Synthetic magnetic fields for ultracold neutral atoms

Y.-J. Lin, R. L. Compton, K. Jiménez-García, J. V. Porto, & I. B. Spielman., Nature 462, 628-632 (2009)

2. Bose-Einstein Condensate in a Uniform Light-Induced Vector Potential

Y.-J. Lin, R. L. Compton, A. R. Perry, W. D. Phillips, J. V. Porto and I. B. Spielman., Phys. Rev. Lett. 102, 130401 (2009).

New! Synthetic magnetic fields for ultracold atoms (Dec. 2009)

New! Spin-orbit coupling in a BEC (Feb. 2011)

Spin–orbit (SO) coupling—the interaction between a quantum particle’s spin and its momentum—is ubiquitous in physical systems. In condensed matter systems, SO coupling is crucial for the spin-Hall effect and topological insulators; it contributes to the electronic properties of materials such as GaAs, and is import- ant for spintronic devices. Quantum many-body systems of ultra- cold atoms can be precisely controlled experimentally, and would therefore seem to provide an ideal platform on which to study SO coupling. Although an atom’s intrinsic SO coupling affects its electronic structure, it does not lead to coupling between the spin and the centre-of-mass motion of the atom. Here, we engineer SO coupling (with equal Rashba and Dresselhaus strengths) in a neutral atomic Bose–Einstein condensate by dressing two atomic spin states with a pair of lasers. Such coupling has not been realized previously for ultracold atomic gases, or indeed any bosonic system. Furthermore, in the presence of the laser coupling, the interactions between the two dressed atomic spin states are modified, driving a quantum phase transition from a spatially spin- mixed state (lasers off) to a phase-separated state (above a critical laser intensity). We develop a many-body theory that provides quantitative agreement with the observed location of the transition. The engineered SO coupling—equally applicable for bosons and fermions—sets the stage for the realization of topological insulators in fermionic neutral atom systems.

New! Synthetic partial waves in ultracold atomic collisions (Jan. 2012)

Interactions between particles can be strongly altered by their environment. We demonstrate a technique for modifying interactions between ultracold atoms by dressing the bare atomic states with light, creating an effective interaction of vastly increased range that scatters states of finite relative angular momentum at collision energies where only s-wave scattering would normally be expected. We collided two optically dressed neutral atomic Bose-Einstein condensates with equal, and opposite, momenta and observed that the usual s-wave distribution of scattered atoms was altered by the appearance of d- and g-wave contributions. This technique is expected to enable qIuantum simulation of exotic systems, including those predicted to support Majorana fermions.