Recently, the team of academician Pan Jianwei of the University of Science and Technology of China observed for the first time the critical divergence of entropy waves in the Fermi superflow, and Science magazine commented that "this work is expected to become a milestone in the field of quantum simulation".
Pan Jianwei, Yao Xingcan, Chen Yuao, and others from the University of Science and Technology of China, in collaboration with Australian scientist Hu Hui, observed the critical divergence behavior of entropy wave attenuation in a Fermi superfluid at the limit of strong interaction for the first time. The results were published in the journal Science on Feb. 4.
This major experimental breakthrough helped to carry out further quantum simulation studies using the strongly interacting Fermi system and laid the foundation for understanding the anomalous transport phenomena in the system.
Image from the Science magazine website
"Second sound"
Heat is usually propagated through diffusion, but in some cases it can also propagate in the form of waves like sound.
More than 80 years ago, Landau established the two-fluid theory, successfully explained the phenomenon of hyperflow of helium-4 liquids (strongly interacting Bose systems), and predicted that entropy or temperature would propagate in the form of waves in the supercurrent. The nature of entropy waves is similar to that of traditional sound waves, and it gradually decays during propagation. Landau named it "second sound."
The "second sound" is a unique quantum transport phenomenon that exists only in superfluids. Superflow is a macroscopic quantum phenomenon that refers to the flow of superfluidic fluids without resistance (viscous force).
Scientists have observed the "second sound" transmission of entropy waves in liquid helium and ultracool atomic systems, but their kinetic processes have never been measured.
Studying the attenuation behavior of the "second sound" in the Fermi superflow can not only answer the long-standing problem of "whether the two-fluid theory can describe the low-energy physics of the strongly interacting Fermi superflow", but also characterize the critical transport phenomenon of the strongly interacting Fermi system at the supercurrent phase transition.
Looking for the "Second Sound"
How to measure the attenuation of the "second sound" has always been a thorny problem internationally.
The superfluid formed by ultracold Fermi atoms under the limit of strong interaction has excellent purity and controllability, which brings new opportunities for studying the attenuation of "second sound", which is also an important goal in the field of ultracold atomic quantum simulation.
However, to observe the attenuation of the "second sound", it is necessary not only to prepare high-quality density uniform Fermi superflow, but also to develop methods to detect weak temperature fluctuations. Although Fermi superflow has been realized for nearly 20 years, the above two key technologies have not been breakthroughs, so it is impossible to study the attenuation of the "second sound" internationally.
After more than 4 years of hard work, the pan jianwei team of the University of Science and Technology of China has built a new ultra-cold lithium-dysprosium atom quantum simulation platform, integrated the development of gray sticky groups and advanced ultra-cold atom regulation technologies such as algorithmic cooling, box-type photopostatic well, etc., and cooled about 10 million strongly interacting Fermi lithium atoms to extremely low temperatures (near absolute zero), and finally successfully realized the world's leading uniform Fermi gas preparation.
At the same time, based on low-noise traveling-wave optical lattice and high-resolution in situ imaging techniques, the team experimented and theoretically interpreted the Bragg spectroscopic methods of low-momentum transfer (about five percent of Fermi momentum) and high energy resolution (better than one-thousandth of Fermi energy), and used it to achieve high-resolution measurements of the system density response.
Based on the above two key technological breakthroughs, the team successfully observed the "second sound" signal in the strongly correlated Fermi superfluid and obtained a complete density response spectrum. The experimental results confirm Landau's theoretical prediction that temperature will propagate in the form of fluctuations in the superfluid.
(A) apparatus schematic diagram, (B) detection scheme schematic diagram, (C) first sound signal, (D) second sound signal, picture from the University of Science and Technology of China
Observing the "Second Sound"
At the same time, the team of the University of Science and Technology of China quantitatively observed the kinetic process of entropy wave propagation, accurately determined the thermal conductivity and viscosity coefficient of the system, and obtained the attenuation rate of entropy waves for the first time in the world.
According to the results of the study, the transport coefficients of the strongly correlated Fermi superfluids have reached the universal quantum mechanical limit, for example, the second sound diffusion coefficient is about /m, and the thermal conductivity is about n kB/m. These limit values are determined only by the reduced Planck ( ) and Boltzmann constants ( kB ) , particle mass m , and density n.
The team also observed the critical divergence behavior of the above transport near the superluid phase transition and found that the strong interaction Fermi superfluid has a considerable critical region, which is about 100 times larger than the critical region of liquid helium superfluids. This considerable critical region will contribute to future systematic experimental research and quantitative quantum simulation of physical phenomena in the quantum critical region, such as the change of transport coefficients with temperature.
The core goal of quantum simulation is to use accurate and controllable artificial quantum systems to effectively simulate the basic laws of complex quantum systems that are difficult to control under some real conditions, so as to solve important physical problems that classical computers cannot solve, and provide ideas and verification for the discovery of universal physical laws.
The research results of Pan Jianwei's team at the University of Science and Technology of China not only reveal that there are considerable critical areas in the strong interaction Fermi system, but also obtain important dynamic transport coefficients, which provide important experimental information for understanding the quantum transport phenomenon of the system, and are an example of using quantum simulation to solve important physical problems. This law of universality will hopefully be extended to other strongly interacting fermion systems such as neutron stars and quark gluon plasmas.
Reviewers in the journal Science spoke highly of it, saying the study "showcases an amazing, experimental masterpiece" and "the work promises to be a milestone in the field of quantum simulation."