Nearly Perfect Fluidity: From Cold Atomic Gases to Hot Quark Gluon Plasmas

TL;DR

This paper reviews the properties of quantum fluids with extremely low shear viscosity to entropy density ratios, including liquid helium, ultracold Fermi gases, and quark-gluon plasma, highlighting theoretical bounds and experimental observations.

Contribution

It provides a comprehensive summary of theoretical models and experimental findings on nearly perfect fluids across different physical systems.

Findings

Liquid helium exhibits low eta/s ratios.

Ultracold Fermi gases show hydrodynamic behavior.

Quark-gluon plasma approaches the theoretical bound.

Abstract

Shear viscosity is a measure of the amount of dissipation in a simple fluid. In kinetic theory shear viscosity is related to the rate of momentum transport by quasi-particles, and the uncertainty relation suggests that the ratio of shear viscosity eta to entropy density s in units of hbar/k_B is bounded by a constant. Here, hbar is Planck's constant and k_B is Boltzmann's constant. A specific bound has been proposed on the basis of string theory where, for a large class of theories, one can show that eta/s is greater or equal to hbar/(4 pi k_B). We will refer to a fluid that saturates the string theory bound as a perfect fluid. In this review we summarize theoretical and experimental information on the properties of the three main classes of quantum fluids that are known to have values of eta/s that are smaller than hbar/k_B. These fluids are strongly coupled Bose fluids, in particular liquid helium, strongly correlated ultracold Fermi gases, and the quark gluon plasma. We discuss the main theoretical approaches to transport properties of these fluids: kinetic theory, numerical simulations based on linear response theory, and holographic dualities. We also summarize the experimental situation, in particular with regard to the observation of hydrodynamic behavior in ultracold Fermi gases and the quark gluon plasma.