Observation of vortex-antivortex pairing in decaying 2D turbulence of a superfluid gas

TL;DR

This study demonstrates vortex sign detection in a 2D superfluid BEC and observes vortex-antivortex pairing during decaying turbulence, providing insights into 2D turbulence regimes in quantum fluids.

Contribution

Introduces a vortex sign detection method using Bragg scattering and observes vortex-antivortex pairing in decaying 2D superfluid turbulence.

Findings

Vortex-antivortex pairing develops in turbulent BECs.

Weak spatial pairing indicates a vortex-dipole gas regime.

Method enables quantitative study of 2D turbulence in superfluids.

Abstract

In a two-dimensional (2D) classical fluid, a large-scale flow structure emerges out of turbulence, which is known as the inverse energy cascade where energy flows from small to large length scales. An interesting question is whether this phenomenon can occur in a superfluid, which is inviscid and irrotational by nature. Atomic Bose-Einstein condensates (BECs) of highly oblate geometry provide an experimental venue for studying 2D superfluid turbulence, but their full investigation has been hindered due to a lack of the circulation sign information of individual quantum vortices in a turbulent sample. Here, we demonstrate a vortex sign detection method by using Bragg scattering, and we investigate decaying turbulence in a highly oblate BEC at low temperatures, with our lowest being $\sim 0.5 T_c$, where $T_c$ is the superfluid critical temperature. We observe that weak spatial pairing between vortices and antivortices develops in the turbulent BEC, which corresponds to the vortex-dipole gas regime predicted for high dissipation. Our results provide a direct quantitative marker for the survey of various 2D turbulence regimes in the BEC system.