The transport properties of strongly coupled quantum systems constitute an important interdisciplinary problem in high-energy theoretical physics, condensed matter physics, and quantum gravity. Among them, the ratio of the shear viscosity coefficient to the entropy density, η/s, is a core physical quantity for characterizing fluids. Early studies based on gauge/gravity duality found that, in a broad class of strongly coupled quantum field theories with Einstein gravity duals, this ratio takes the universal value η/s= 1/4π, giving rise to the well-known Kovtun–Son–Starinets (KSS) bound conjecture. In recent years, developments in Jackiw–Teitelboim (JT) gravity and AdS₂ holography have shown that important quantum fluctuation effects exist in the near-horizon region of near-extremal black branes, enabling systematic investigations of quantum corrections to black brane thermodynamics and related dynamical processes. Recently, a team led by Associate Professor Jun Nian from the International Centre for Theoretical Physics Asia-Pacific at the University of Chinese Academy of Sciences, together with collaborators, published an article in the Journal of High Energy Physics, a leading journal in high-energy theoretical physics. The study systematically investigates how quantum fluctuations in the infrared region of near-extremal black branes modify η/s in strongly coupled systems.

Within the framework of gauge/gravity duality, the thermodynamic and transport properties of strongly coupled quantum field theories can be studied through the gravitational dynamics of higher-dimensional black branes or black holes. For systems at finite chemical potential, the near-horizon limit of charged near-extremal black branes develops an AdS₂ geometry. In recent years, it has been recognized that this region is not entirely classical: quantum fluctuations described by JT gravity can significantly modify the low-temperature thermodynamics of near-extremal black branes. This naturally raises an important question: can these quantum effects originating from the near-horizon region of black branes also affect the transport coefficients of strongly coupled quantum systems, especially the low-temperature behavior of η/s?

This study is centered on this question. We first start from the standard holographic computation of η/s at finite chemical potential and analyze how the stress-tensor correlation functions in the AdS₄ boundary field theory are related to the correlation functions in the near-horizon AdS₂ region. The study then combines the infrared AdS₂ region of near-extremal black branes with quantum fluctuations in JT gravity, computes the quantum-corrected retarded Green’s function, and further extracts the quantum-corrected shear viscosity coefficient through the Kubo formula. In contrast to the classical case, where η/s is fixed at 1/4π, quantum corrections introduce a nontrivial temperature dependence, leading to richer behavior of η/s in the low-temperature regime.

The paper focuses on two distinct temperature regimes: the semiclassical regime and the lower-temperature quantum regime, where quantum effects dominate. In the semiclassical regime, the study finds that η/s deviates from the classical universal value as the temperature decreases and reaches a minimum below the KSS bound near a certain critical temperature. This minimum does not arise from nonmonotonic behavior of the shear viscosity coefficient itself, but is instead driven by the nonmonotonic behavior of the quantum-corrected entropy: the quantum-corrected entropy density develops a peak relative to the classical entropy density, thereby producing a minimum in η/s. This result shows that even small quantum corrections can have appreciable effects on the transport properties of strongly coupled systems.

In the lower-temperature quantum regime, the authors find that η/s ncreases rapidly and becomes significantly larger than 1/4π. However, the paper also points out that, at extremely low temperatures, the conventional hydrodynamic description and the expression for the quantum-corrected entropy may no longer be reliable, so the interpretation of results in this regime requires particular caution. In particular, when quantum corrections lead to a negative value in the entropy expression, this signals the breakdown of the semiclassical approximation, meaning that the existing formulae cannot be simply extrapolated to strictly zero temperature. Nevertheless, this study provides important clues for understanding how quantum fluctuations of near-extremal black branes affect low-temperature transport.

In addition, the authors compare the resulting shear viscosity coefficient with the quantum-corrected absorption cross section of near-extremal black branes. The results show that, in the two temperature regimes studied, the temperature dependence of η is consistent with the quantum-corrected absorption cross section. This mutual consistency supports the self-consistency of the holographic transport calculation and further demonstrates that near-horizon quantum effects of black branes not only modify thermodynamic quantities but also enter dynamical responses and transport processes.

This study shows that near-horizon quantum fluctuations can alter the low-temperature transport properties of strongly coupled systems and cause η/s to exhibit temperature-dependent behavior distinct from the classical universal result. These findings not only extend the applications of JT gravity and quantum corrections to near-extremal black branes in holographic transport studies, but also provide new theoretical insights for further understanding quantum effects in black branes, the dynamics of strongly coupled quantum matter, and the question of the η/s bound.

This research was supported by the National Natural Science Foundation of China under Grant Nos. 12375067, 12547104, and 12247103. The work was carried out by Xiao-Long Liu, a master’s graduate of the International Centre for Theoretical Physics Asia-Pacific and currently a Ph.D. student at Lehigh University, under the joint supervision of his master’s advisor, Jun Nian, and his Ph.D. advisor, Professor Sera Cremonini of Lehigh University. The collaborator was Professor Li Li from the Institute of Theoretical Physics, Chinese Academy of Sciences.

Article link：

https://link.springer.com/article/10.1007/JHEP06(2026)193