Heat can propagate in the form of wave in some specific mediums, which is similar to the propagation pattern of sound. Thus, researchers name the phenomenon “second sound” to distinguish it from original sound (namely first sound). Second sound is a mysterious while significant project to investigate on, for the observation of second wave attenuation can prove the dynamic scaling theory. In a film shot in 1963, Alfred Leitner introduced how to measure second sound with helium 2. But this study was hindered by the narrow critical region and specific medium requirement. 

Measurement of second sound attenuation was a hard nut to crack until recently. A group led by Prof. PAN Jianwei from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences and his collaborators developed sophisticated setup and measured the second sound attenuation near quantum criticality. The experimental results proved the existing dynamic scaling theory while opening up the path to systematic exploration of quantum critical region. This work was published in Science. 

First sound transports in the air, while second sound can be observed in superfluid and some dielectric crystals. Being the frequently used medium in studying second sound, superfluid helium is hard to handle: this liquid has a viscosity of zero, which leads it to creep along the container automatically as long as it stays in the superfluid state. 

The team led by Prof. PAN chose a homogeneous unitary Fermi gas of lithium-6 rather than helium as the medium for the observation, which enhanced the controllability of the experiment. The development of Bragg spectroscopy technique with small wave number and high energy resolution also facilitated their study. 

Based on their ultra-cold lithium-dysprosium atomic quantum simulation platform, Prof. PAN and his group not only determined transport coefficients, conductivity and viscosity coefficients with elevated accuracy, but also verified the predictions of dynamic scaling theory. 

Their result confirmed the prospects of research on strongly interacting quantum many-body systems, while providing the prototype of experimental setup for homogeneous Fermi superfluid study. By accomplishing this work, they also testified the fundamental theories in quantum simulation, illuminating the way to the improvement of quantum communication.