Emergent structures and dynamics in a quantum gas with cavity-mediated long-range interactions

Abstract

Synthetic materials created by ultra-cold atoms open the way to explore properties beyond actual solid materials. Since the atoms are cooled to the ground state in motion, the interactions between atoms can significantly affect the macroscopic properties of the synthetic material. We use optical cavity modes and a coherent off-axis pump beam to couple atoms in different motional states, and achieve effective long-range interactions between atoms by exchanging photons. Therefore, we can artificially control the form of long-range interaction by introducing different optical modes, to spontaneously generate different types of static and dynamic crystal structures. We study structural phases in an atomic Bose-Einstein condensate (BEC) of a Rubidium gas by inducing cavity-mediated long-range interactions. The BEC is placed inside an optical cavity mode and driven transversally by a repulsive pump lattice beam combining a standing wave and a running wave component. Two crystalline phases arise from the couplings to different quadrature of the cavity mode, induced by mode softening in the P-band and S-band of the pump lattice, respectively. The two super-radiant crystals are connected by a first-order structural phase transition which is related to a change of crystal polarization. We measure in real-time the transient dynamics of the order parameters across the phase transition by recording the cavity photon field, where the relaxation frequencies reveal the excitation spectrum of the self-organized crystal. In our latest experiment, cavity dissipation, drives coherent dynamics between the two structural phases. The inherent photon loss of the cavity mirrors adds dynamically a phase advance on the photon field and drives the super-radiant phases of different quadratures into cycling dynamics. The non-stationary dynamics can be interpreted by the non-Hermiticity of the microscopic Hamiltonian with cavity dissipation. The continuous evolution of the crystal polarization, together with the dissipative light force, leads to a novel atomic transport phenomenon with chiral nature.