How does heat travel in matter? This is usually done by diffusion, i.e. a gradual decrease in temperature from near to far. However, in some cases it may also propagate in the form of waves, much like sound waves. Therefore, this phenomenon is called the second tone, and the opposite ordinary sound waves are called the first tone. The second sound does not appear in ordinary substances, but only in certain special substances, such as superfluid helium.
If you don't know what superflow is, let me explain it a little: superflow is when viscosity becomes 0. For example, a superfluid in an open cup can crawl out spontaneously. For example, if a vortex is generated in an ordinary liquid, it will gradually disappear, while the vortex in a superfluid will not decay and will exist forever. This sounds a lot like superconductivity, because superconductivity is when the resistance becomes 0, and the current in the superconductor does not decay and will remain forever. That's right, superconductivity and superfluidity belong to the same broad category of phenomena. They are all macroscopic quantum phenomena, specifically from the Bose-Einstein condensation.
By measuring the second sound and its associated thermal transport phenomena in liquid helium, a universal theory called Dynamical Scaling Theory was established. This theory has important implications for phase transitions in many quantum systems, such as high-temperature superconductivity, because it points out that the phase transitions of many different systems follow the same certain universal functions. However, it is difficult to accurately measure these universal functions in liquid helium because of its narrow critical region and limited maneuverability. In short, it was through liquid helium that people discovered the phenomenon of a second sound, but it was difficult to go deeper.
Recently, my colleagues at HKUST, such as Pan Jianwei, Yao Xingcan, Chen Yuao, and others, have made breakthroughs. They used another system to measure the universal function of the second sound correlation very accurately. These systems are called ultracold fermonic atoms, which are actually cold and thin Li-6 atomic gases. They published a long article in Science titled "Second Sound Attenuation near Quantum Criticality." Come to think of it, why isn't there a system in this title? Because as mentioned earlier, these functions are universal!
Let me read a press release (
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 advanced ultra-cold atom control technologies such as gray clay mass and algorithm cooling and box-shaped optical potential traps, and finally successfully realizes the world's leading preparation of homogeneous Fermi gas. At the same time, based on the low-noise traveling-wave optical lattice and high-resolution in-situ imaging technology, the research team experimentally realized and theoretically interpreted the Bragg spectroscopy method with low momentum transfer (about 5% Fermi momentum) and high energy resolution (better than 1/1000 Fermi energy), and used it to achieve high-resolution measurement of the density response of the system. On the basis of the above two key technological breakthroughs, the research team successfully observed the signal of the second sound in the density response of the unitary Fermi superfluid (as shown in Fig. 1(d)), and obtained the complete density response spectrum of the unitary Fermi superfluid, and the experimental results are in good agreement with the description based on the dissipative two-fluid theory.
Let me note that this paragraph explains why they were able to do these experiments: because they had made technological breakthroughs and were able to do things that no one else could.
Read another press release:
Further, the research team obtained the attenuation rate (acoustic diffusion coefficient) of the second sound, and accurately determined the thermal conductivity and viscosity coefficient of the system. The results show that the transport coefficients of the unitary Fermi superfluids reach the universal quantum mechanical limits, for example, the second acoustic diffusion coefficient is about ? m, the thermal conductivity is about n?kb/m。These limit values are only determined by the reduced Planck constant (?) and the Boltzmann constant (kb), particle mass m, and density n. In addition, they observed the critical divergence behavior of these transport volumes in the vicinity of the superfluid phase transition, and found that the unitary Fermi superfluid has a considerable critical region (about 100 times greater than the critical region of the liquid helium superfluid). This discovery lays a foundation for further quantum simulation studies using the system to understand the anomalous transport phenomenon in the strongly correlated Fermi system.
Let me note that this paragraph explains the significance of their results. The measurements are indeed theoretical**, and this system of ultra-cold Fermi atoms is much better than liquid helium, and the critical zone is 100 times larger.
Read another press release:
Reviewers for the journal Science spoke highly of the work, saying it "showcased this **presents spectacular, "tour de force," experiments,..This is an extremely impressive article, which promises to be a milestone in the field of quantum simulationthis **could be a milestone in quantum simulation...
This paragraph seems to have nothing to comment on! However, I still need to note that one of the key words here is "quantum simulation", that is, simulating one quantum system with another. I recently published a popular science book, A Brief Introduction to Quantum Information, in which quantum computing is introduced. In fact, the basic idea of quantum computing and quantum simulation is very similar, both of which use one physical system to simulate another system, and transform a difficult problem into another relatively easy system to solve, so people often call the two together, called "quantum computing and quantum simulation".
Finally, there is a social phenomenon that I would like to ask you to think about. This social phenomenon is called "civil science". In the past, it was often said that quantum communication is **, and these people who engage in quantum communication are**! Later, these colleagues of mine made a lot of achievements in the field of quantum computing, and then Minke said angrily that quantum computing is also fake, and these people want to divert their attention from quantum communication through quantum computing! Now, scientists have made achievements in quantum simulation and many other fields, such as the recent experimental proof that complex numbers are indispensable in quantum mechanics (), what should Minke say? Is it to say that all of this is fake again?
There are some people in the world who are constantly doing practical things, and some people can't even understand what other people are doing, they only know how to twitter and spray others. What can we say about such people? It's sad and pathetic.