Second sound with ultracold atoms: A brief historical account

TL;DR

This paper reviews the historical development and recent experimental advances in observing second sound in ultracold atomic gases, highlighting its significance as a hallmark of superfluidity and quantum transport phenomena.

Contribution

It provides a concise historical overview of key theoretical and experimental progress on second sound in ultracold gases, including recent uniform trap measurements and implications for quantum criticality.

Findings

First observation of second sound in a highly elongated harmonic trap

Recent measurement of second sound attenuation in a uniform setup

Observation of second sound in weakly interacting Bose condensates in 2D and 3D

Abstract

We briefly review the research on second sound in ultracold atomic physics, with emphasis on strongly interacting unitary Fermi gases with infinitely large $s$-wave scattering length. Second sound is a smoking-gun feature of superfluidity in any quantum superfluids. The observation and characterization of second sound in ultracold quantum gases has been a long-standing challenge, and in recent years there are rapid developments due to the experimental realization of a uniform box-trap potential. The purpose of this review is to present a brief historical account of the key research activities on second sound over the past two decades. We summarize the initial theoretical works that reveal the characteristics of second sound in a unitary Fermi gas, and introduce its first observation in a highly elongated harmonic trap. We then discuss the most recent measurement on second sound attenuation in a uniform setup, which may open a new era to understand quantum transport near quantum criticality in the strongly interacting regime. The observation of second sound in homogeneous weakly interacting Bose condensates in both two and three dimensions are also briefly introduced.