Research

Ultracold atoms

Ultracold atoms are at the temperature range from micro-kelvin down to nano-kelvin; these systems are simple and can be precisely controlled, where quantum physics in our textbooks can be manifested. We can use them as a platform to study fundamental quantum physics, simulate complicated models in condensed matter physics, and create new type of interaction with designed properties, leading to new systems which have no counterparts in material systems.

Laser coolings and trappings

We make Bose-Einstein condensates (BEC) of Rb atoms from trapping and cooling atoms with lasers. The atoms are first heated to vapor in an oven, pass through a Zeeman slower and then captured in a Magneto-Optical-Trap. We then perform polarization gradient cooling and load the atoms in a quadrupole magnetic trap. To reach BEC, we further cool the atoms by evaporative cooling. Eventually, we achieved BEC with 3x10⁵ atoms in a cross dipole trap with each experimental cycle time ~ 15 s.

Ref: Phys. Rev. A. 79, 063631 (2009)

Synthetic gauge fields for ultracold atoms

The charge neutrality of the atoms has limited physicists from simulating interesting phenomena where charged particles are under electromagnetic fields, like those in quantum Hall physics and topological phases.

A key experimental technique to lift the constraint is to “synthesize a magnetic field for neutral atoms" with a synthetic vector gauge potential. This is equivalent to generating a Lorentz force for moving atoms, simulating charged particles in real magnetic fields.

Ref: Nature Physics 7, 531 (2011), Nature 462, 628 (2009), Phys. Rev. Lett. 102 130401 (2009).

Spin Orbit Couplings for ultracold atoms

Raman coupling between internal spin states while transferring photon momentum leads to the spin-linear-momentum coupling (SLMC). This is a type of general "spin-orbit-coupling” (SOC), referring to coupling between the atomic spin and the center-of-mass motion of the atoms.

Going beyond SLMC, we demonstrated another class of SOC where the atomic spin is coupled to the orbital-angular-momentum of the atoms which we refered to spin-orbital-angular-momentum coupling (SOAMC). We acheived this by Raman-coupling bare spin states, where one of the Raman beams is a Laguerre Gaussian beam . We characterize its spin textures and the gauge potentials.

Ref: Nature 471, 83 (2011), Phys. Rev. Lett. 121 113204 (2018)

Azimuthal gauge potentials in SOAMC BEC

In SOAMC systems, the atoms dressed by LG Raman beams experience azimuthal gauge potentials, effectively rotating our the atoms. This presents the first demonstration to rotate atoms in the thermodynamic ground state under a stationary light induced Hamiltonian in contrast to earlier works on mechanical rotations that suffers from mechanical issues and dynamical instabilities. We further exploited our effective rotation and explored the quantum phases by demonstrating the Hess-Fairbank effect, which is the analogue of the Meissner effect in superconductors.

Ref: Phys. Rev. Lett. 121 250401 (2018)

Creating topologically nontrivial spin textures

In our SOAMC systems, we demonstrated the capability to creating topological excitations by tailoring atom-light interaction for the very first time. By adiabatically changing the atom-light coupling such that the spin state follows, one can directly create topological spin textures. Versatile design of the atom-light coupling term allows creation of topological excitations in spinor BECs, where the rich variety of order parameters accommodate various types of topological defects

Ref: Phys. Rev. Lett. 121 113204 (2018), Phys. Rev. Lett. 121 250401 (2018)

Toward persistent superfluid flows

In combination with our light-induced azimuthal gauge potential, an effective rotation for ultracold atoms, and a ring trap we have been building, we hope to create persistent superfluid flows (persistent currents, persistent superflows). Interesting applications have been proposed in atomtronics and other applications.

Google Sites

Report abuse