China Science and Technology Quantum Simulation Team Successfully Measured Second Sound Attenuation

The University of Science and Technology of China Pan Jianwei, Yao Xingcan, Chen Yuao, etc., in collaboration with Australian scientist Hu Hui, observed the critical divergence behavior of entropy wave decay in Fermi superfluids at the strong interaction (unitary) limit for the first time. It is revealed that there is an appreciable phase transition critical region in the system, and important transport coefficients such as thermal conductivity and viscosity coefficient are obtained. This work provides important experimental information for understanding quantum transport phenomena in strongly interacting Fermi systems, and is a paradigm for using quantum simulation to solve important physical problems. On February 4, the results were published in the international authoritative academic journal Science in the form of a research article.

More than 80 years ago, Landau established the two-fluid theory, successfully explained the superfluid phenomenon of helium-4 liquid (strongly interacting Bose system), and predicted that entropy or temperature would propagate in the superfluid in the form of waves. The properties of an entropy wave are similar to that of a conventional sound wave, and it gradually decays as it propagates, so Landau named it the second sound. The propagation and attenuation of the second sound is directly coupled with the superfluid order parameter, a unique quantum transport phenomenon that exists only in superfluids. Studying the decay behavior of the second sound in a Fermi superfluid can not only answer the long-standing question of whether the two-fluid theory can describe the low-energy physics of the strongly interacting Fermi superfluid, but also characterize the strongly interacting Fermi The critical transport phenomenon of the system at the superfluid phase transition.

The superfluid formed by the ultracold Fermi atoms in the strong interaction (unitary) limit has excellent purity and controllability, which brings a new opportunity to study the attenuation of the second sound, which is also the ultracold atom. An important goal in the field of quantum simulation. To observe the attenuation of the second sound, it is not only necessary to prepare high-quality Fermi superfluids with uniform density, but also to develop methods to detect weak temperature fluctuations. Although Fermi superfluidity has been realized for nearly 20 years, the above two key technologies have not been broken through, so the attenuation of the second sound cannot be studied.

In this work, after more than 4 years of hard work, the research team of USTC has built a new ultra-cold lithium-dysprosium atomic quantum simulation platform, which integrates the development of gray sticky groups, algorithmic cooling, box-type optical potential wells and other advanced technologies. At the same time, the research team also based on the low-noise traveling wave optical lattice and high-resolution in situ imaging technology, experimental realization and theoretical interpretation Bragg spectroscopy with low momentum transfer (about 5 percent of Fermi momentum) and high energy resolution (better than one thousandth of the Fermi energy), enabling high-resolution measurements of system density responses . Based on the above two key technological breakthroughs, the research team successfully observed the second sound signal in the density response of the unitary Fermi superfluid (as shown in Fig. 1(D)), and obtained a complete The density response spectrum of the unitary Fermi superfluid, the experimental results are in good agreement with the description based on the dissipative two-fluid theory.

Further, the research team obtained the attenuation rate (sound diffusion coefficient) of the second sound, and then accurately determined the thermal conductivity and viscosity coefficient of the system. The research results show that the transport coefficients of unitary Fermi superfluids all reach the universal quantum mechanical limit, for example, the second acoustic diffusion coefficient is about ℏ/m, and the thermal conductivity is about nℏkB/m. These limits are determined only by the reduced Planck (ℏ) and Boltzmann constant (kB), the particle mass m and the density n. In addition, they also observed the critical divergence behavior of the above-mentioned transport quantities near the superfluid phase transition, and found that the unitary Fermi superfluid has an appreciable critical region (about 100 times that of the liquid helium superfluid). This discovery lays the foundation for further quantum simulation studies using this system to understand the anomalous transport phenomenon in strongly correlated Fermi systems.

 

Figure 1. (A) Schematic of the device. (B) Schematic diagram of the detection scheme. (C) The first acoustic signal. (D) Second acoustic signal.

Reviewers for Science magazine spoke highly of the work, saying that the work "presents an astonishing, experimental masterpiece" (This paper presents spectacular, "tour de force," experiments,...)," This is an extremely impressive paper" (This is an extremely impressive paper...) "This work could be a milestone in quantum simulation..." ).

The research work was supported by the Ministry of Science and Technology, the National Natural Science Foundation of China, the Chinese Academy of Sciences, Anhui Province, and Shanghai.

Paper link:https://www.science.org/doi/10.1126/science.abi4480