How Non-Hermitian Superfluids are Special? Theory and Experiments

Abstract

Ultracold atoms emerge as a promising advanced platform for researching the principles of quantum mechanics. Its development of scientific understanding and technology enriches the toolbox for quantum simulations and quantum computations. In this dissertation work, we describe the methods we applied to build our new high-resolution 87Rb Bose-Einstein condensate (BEC) machine integrated with versatile quantum control and measurement tools. Then we describe the applications of these tools to the research of novel superfluidity and non-Hermitian physics.

Superfluids and normal fluids were often studied in the context of Landau’s two-fluid model, where the normal fluid stemmed from thermally excited atoms in a superfluid background. But can there be normal fluids in the ground state of a pure BEC, at near zero temperature? Our work addressed the understanding of this scenario, and then measured the anisotropic superfluid density in a density-modulated BEC, where the result matched the prediction of the Leggett formula proposed for supersolids. We further considered and measured this BEC in rotation and found a non-classical moment of inertia that sometimes turns negative. We distinguished the roles of superfluid and normal fluid flows, and linked some features to the dipolar and spin-orbit coupled supersolids.

As a second direction, we describe our capability to create non-Hermiticity with Raman lasers, digital-micromirror device (DMD), and microwave, and present our work in engineering the real space non-Hermitian skin effect with a spin-orbit coupled BEC. By use of a spin-dependent dissipative channel, we realized an imaginary gauge potential which led to nonreciprocal transport in the flat box trap. We studied the system dynamics by quenching the dissipation, and further prepared stationary edge states. We link our discoveries to a non-Hermitian topological class characterized by a quantized winding number. Finally, we discuss the exciting promises of using these tools to study many-body physics open quantum systems.