Strongly Correlated Quantum Fluids: Ultracold Quantum Gases, Quantum Chromodynamic Plasmas, and Holographic Duality

TL;DR

The article surveys strongly correlated quantum fluids across three domains—ultracold Fermi gases, quark–gluon plasmas, and holographic duality—highlighting universal hydrodynamic behavior and a low viscosity-to-entropy ratio near the holographic bound. It synthesizes experimental and theoretical advances in unitary gases, contrasts them with QGP results, and explains how holography provides a unifying framework to study strong coupling where quasiparticles fail. Core contributions include a detailed exposition of the holographic dictionary, quantitative connections between transport coefficients (such as $\eta/s$ and $D$) and coupling strength, and the emergence of universal hydrodynamics across vastly different scales. The work underscores the cross-pollination between cold-atom experiments, heavy-ion physics, and holographic models, illuminating how insights from one domain inform the others and guiding future explorations of nonperturbative quantum matter.

Abstract

Strongly correlated quantum fluids are phases of matter that are intrinsically quantum mechanical, and that do not have a simple description in terms of weakly interacting quasi-particles. Two systems that have recently attracted a great deal of interest are the quark-gluon plasma, a plasma of strongly interacting quarks and gluons produced in relativistic heavy ion collisions, and ultracold atomic Fermi gases, very dilute clouds of atomic gases confined in optical or magnetic traps. These systems differ by more than 20 orders of magnitude in temperature, but they were shown to exhibit very similar hydrodynamic flow. In particular, both fluids exhibit a robustly low shear viscosity to entropy density ratio which is characteristic of quantum fluids described by holographic duality, a mapping from strongly correlated quantum field theories to weakly curved higher dimensional classical gravity. This review explores the connection between these fields, and it also serves as an introduction to the Focus Issue of New Journal of Physics on Strongly Correlated Quantum Fluids: from Ultracold Quantum Gases to QCD Plasmas. The presentation is made accessible to the general physics reader and includes discussions of the latest research developments in all three areas.

Figures (25)

• Figure 1: Temperature and pressure scales of extreme quantum matter. Ultracold quantum gases are the coldest matter produced to date, while the quark-gluon plasma is the hottest, together spanning about 18 orders of magnitude in temperature and more than 40 orders of magnitude in pressure. Yet these systems exhibit very similar hydrodynamic behavior, as characterized by the shear viscosity to entropy density ratio shown in Fig. \ref{['fig:ratio']}). We also include two other well known quantum fluids, liquid helium and hot proto-neutron star matter, as well as a classical fluid, water, and a classical plasma, the Coulomb plasma in the sun.
• Figure 2: Transport properties of strongly correlated fluids. Ratio of shear viscosity $\eta$ to entropy density $s$ as a function of $(T-T_c)/T_c$, where $T_c$ is the superfluid transition temperature in the case of ultracold Fermi gases, the deconfinement temperature in the case of QCD, and the critical temperature at the endpoint of the liquid gas transition in the case of water and helium. The data for water and helium are from nist, the ultracold Fermi gas data are from NJPfocusissue4_thomas, the quark-gluon plasma point (square) is taken from the analysis of Song:2011hk, the lattice QCD data (open squares) from Meyer:2007ic, and the lattice data for the ultracold Fermi gas (open circles) are the $8^3$ data from Wlazlowski:2012jb. The dashed curves are theory curves from Arnold:2000drPrakash:1993btMassignan:2004Mannarelli:2012su. The theories are scaled by overall factors to match the data near $T_c$. The lines labeled "holographic bounds" correspond to the KSS bound $\hbar/(4\pi k_B)$Kovtun:2004de and the Gauss-Bonnet bound $(16/25)\hbar/(4\pi k_B)$Brigante:2008gz. Similar compilations can be found in Kovtun:2004deCsernai:2006zzLacey:2006bc.
• Figure 3: Ultracold Fermi gas phase diagram. Sketch of the Bardeen-Cooper-Schrieffer (BCS) to Bose-Einstein condensate (BEC) crossover for ultracold Fermi gases. When the scattering length $a_s$ passes through a pole, so that $1/(k_F a_s) \to 0$, one obtains a strongly correlated fluid, the unitary gas. The critical temperature $T_c$ for the phase transition only approaches the pairing temperature $T_{\mathrm{pair}}$ in the limit $1/(k_F a) \to -\infty$. The crossover region is the strongly interacting regime, loosely defined by $|1/(k_F a_s)| < 1$. Note that we denote the scattering length by $a$ in the text. Used with permission from Ref. sademelo2008.
• Figure 4: Experimental images. Elliptic flow of a strongly-interacting Fermi gas as a function of time after release from a cigar-shaped optical trap: from top to bottom, $100\,\mu$s to 2 ms after release. The pressure gradient is much larger in the initially narrow directions of the cloud than in the long direction, causing the gas to expand much more rapidly along the initially narrow directions, inverting the aspect ratio. Achieving nearly perfect elliptic flow requires extremely low shear viscosity, as is the case also for a quark-gluon plasma. The sequence of images is created by recreating similar initial conditions and destructively imaging the cloud at different times after release OHara:2002.
• Figure 5: Ultracold quantum gas experimental apparatus. Left: sketch of the experimental apparatus for ultracold fermions. Right: Apparatus for the Duke experiments (currently at North Carolina State University). Compare to sketch of QGP experiment at the LHC in Fig. \ref{['fig_atlas']}: the quantum gas experiment is about ten times smaller (2.5 meters vs. 26 meters), but the size of the trapped ultracold gas is 11 orders of magnitude larger (a few hundred micrometers vs. a few femtometers). The ultracold quantum gas is at nanokelvin temperatures, or pico-eV, compared to the deconfinement temperature of $\simeq 2\times 10^{12}$K in the QGP, or 200 MeV, created by colliding gold nuclei at energies of 100 GeV/nucleon.