Spin-orbit coupling and supersolidity in ultracold quantum gases

Abstract

Ultracold quantum gases provide a clean, isolated, and controllable platform for simulating and characterizing complex physical phenomena. In this thesis, I present several experiments on realizing one-dimensional spin-orbit coupling in ultracold 23Na gases and the creation of a new form of matter with supersolid properties using interacting spin-orbit coupled Bose-Einstein condensates. The first part describes the realization of spin-orbit coupling in optical superlattices which consist of stack of pancakes of imbalanced double-wells. The orbital levels, individual pancakes, in an superlattice potential are used as pseudospin states. Spinorbit coupling was induced by two-photon Raman transition between the pseudospin states, and was experimentally characterized by the spin-dependent momentum structure from this dressing. The realization suppresses heating due to spontaneous emission.

The system is highly miscible, allowing the study of novel phases in interacting spin-orbit coupled systems. Next, spin-orbit coupling was demonstrated by synchronizing a fast periodically modulating magnetic force with the Radio-Frequency (RF) pulses. The modulation effectively dressed the RF photons with tunable momentum. The consequent Doppler shifts for RF transitions were observed as velocity-selective spin flips. The scheme is equivalent to Floquet engineered one-dimensional spin-orbit coupling. Finally, I report experiments on creating a new form of matter, a supersolid, in ultracold quantum gases. An interacting spin-orbit coupled Bose-Einstein condensate in the stripe phase spontaneously breaks two continuous symmetries : the U(1) symmetry, observed as sharp interference peaks in momentum space, and the continuous translational symmetry, observed as a spontaneously formed density modulation. The density modulation is measured and characterized with Bragg scattering.

A system spontaneously breaking these two symmetries is a crystal and a superfluid simultaneously, and is considered as a supersolid.