Speaker
Description
Superfluidity encompasses a variety of remarkable physical phenomena, including zero viscosity in transport, nucleation of quantized vortices, and nonclassical rotational inertia. Recent advances in ultracold-atom experimental techniques have enabled unprecedented control over superfluids in quantum systems, such as bosonic superfluidity in interacting Bose-Einstein condensate (BEC) and fermionic superfluidity in Bardeen-Cooper-Schrieffer (BCS) superconductivity. However, inelastic atomic collisions and molecular chemical reactions inevitably induce particle loss, rendering these systems intrinsically open. One of the fundamental questions is the behavior and properties of dissipative superfluidity in open quantum systems.
In this talk, I will introduce our recent works on a comprehensive superfluid transport theory for open quantum many-body systems. In the first part, I will introduce our work on bosonic superfluid theory for a molecular BEC subject to uniform two-body loss. By employing the Schwinger-Keldysh formalism, we reveal that dissipation acts as an effective repulsive interaction that suppresses density fluctuations and generates superfluidity even in the absence of interaction. Furthermore, we also show that the two-body loss can enhance the stability of a molecular BEC against collapse. In the second part, I will introduce our work on fermionic superfluid transport theory. By using the Schwinger-Keldysh formalism, we show the Ward-Takahashi identity in open quantum systems for a Lindbladian dynamics possessing weak U(1) symmetry. We demonstrate that gauge invariance follows directly from the weak U(1) symmetry. This framework enables the calculation of superfluid density in the presence of dissipation. Finally, we derive the low-energy excitation spectrum for dissipative BCS superconducting systems and show that two-body loss induces a diffusive propagation in the low-energy mode.