How many steps does it take to observe a T-Rex walking in the lab? The answer is two steps.
First, you need to buy a live chicken and a toilet bowl.
Second, put the toilet bowl on the chicken butt. Dangdang, you see who's coming?
Image credits are at the end of the article
Obviously, observing the Tyrannosaurus rex walking in the laboratory does not necessarily require the Tyrannosaurus rex to actually be invited into the laboratory. You can completely find an animal (chicken) and modify it. Just let it walk in a posture very similar to that of a T-Rex.
Proceeding from this line of thinking, physicists have found that some physical phenomena that cannot be observed closely, such as how the shell of a neutron star affects the rotation of neutron stars, and some physical phenomena that cannot be observed for a long time, such as the quark gluon plasma (which is believed to be the state of matter that existed in the first 20 or 30 microseconds after the Big Bang) can be studied carefully and for a long time in the laboratory to some extent.
All you have to do is find a material that creates the right conditions for its physical properties to resemble the shell of a neutron star and the quark gluon plasma of the early Big Bang.
Recently, the University of Science and Technology of China Pan Jianwei, Yao Xingcan, Chen Yuao, and others collaborated with Australian scientist Hu Hui to successfully prepare a peculiar substance in the laboratory, making it have similar physical properties to the shell of a neutron star and the quark gluon plasma.
The content of this experiment is called:
In the presence of strong interactions (monogram)
Limits under the Fermi superfluid
Observe the attenuation characteristics of the second type of sound waves.
Although the experimental content looks very tall, the experimental material they use is very ordinary, that is, a chemical element that everyone uses in their mobile phones every day: lithium!
What the hell is going on with this dazzling operation?
(1) What is Fermi superfluid
First of all, we must know that whether it is the shell of a neutron star or the quark gluon plasma of the early Big Bang, although they have different temperatures, different densities, different compositions of matter, and different internal interactions, they belong to the same state of matter in physics, called Fermi superfluids.
What is superfluidics? You can think of a superfluid as a fluid that "ignores" friction.
For example, if the temperature of liquid helium (helium-4) is reduced to near 2 Kelvin, it suddenly becomes a wonderful state - a superfluid state. At this time, when we put it into a thin tube, once it begins to flow, it will flow non-stop, unlike the water we usually see, the flow will slowly stop under the action of friction.
Physicists speculate that the outer shells of neutron stars, and the quark gluon plasma of the early Big Bang, are in such a superfluid state that ignores friction. If you're going to ask why, I can only answer: because of quantum mechanics. (Superfluids are fluid states unique to quantum mechanics.) ）
Fermi superfluids are a special type of superfluid. Just like ordinary chicken is called chicken, but the "chicken" made of tofu should be called vegetarian chicken. Physicists call the superfluids they first learned about, composed of bosons such as helium-4 atoms, superfluidics, and the superfluids they later learned about, composed of fermions such as neutrons and quarks, called Fermi superfluids.
From this strange name we can see that if we want to "put a material under extreme conditions" in the laboratory, so that it forms a Fermi superfluid, and thus have the same physical properties as the shell of a neutron star and the quark gluon plasma, the best way is certainly not to study the helium-4 superfluid that physicists are most familiar with.
Because helium-4 atoms belong to bosons, the superfluids it forms do not belong to Fermi superfluids.
So, the research team took a step forward on the periodic table and identified the experimental material as lithium-6 atoms. Because lithium-6 atoms belong to fermions, if it can form superfluids, it should be a Fermi superfluid similar to the shell of a neutron star and the quark gluon plasma.
So, is it not possible to get the lithium-6 atom, reduce its temperature to near absolute zero, let it form a Fermi superfluid, and the physicists will be finished?
(2) What is strong interaction
Fermi superfluids at the limit
It's not that simple. This is because the shell and quark gluon plasma of a neutron star are not ordinary Fermi superfluids, but rather Fermi superfluids with strong internal interactions.
To simulate such a Fermi superfluid, physicists would have to follow the prescription so that lithium-6 atoms maintain "high-intensity interactions" as they enter the overcurrent state.
So, how high is the interaction strength of lithium-6 atoms? The research team did not stop and adjusted the intensity of the interaction of lithium-6 atoms to the limit. In the dark parlance of physics, it is called "scattering length infinity." Not strictly speaking, you can understand, "In this Fermi superfluid, no matter how far apart the two lithium-6 atoms are, their states will be related to each other." ”
In quantum mechanics, the intensity limit reached by the interaction is also called the unitary limit, so the state that the research team let the lithium-6 atoms enter is called "Fermi superfluid under the limit of strong interaction (unitary)".
One of the advantages of achieving the strong interaction (monogram) limit is that the physical characteristics of the Fermi superfluid at this time are universal, and it is not compatible with what materials you used to make it, and what form of interaction you used.
For example, when medical scientists experiment with new drugs, they always let mice eat them first, and then let people eat them. However, after all, the mouse is not exactly the same as the person, the mouse eats the effect, the person eats it may not work. Such mice lack universality.
However, if there is a "super mouse" in the world, in addition to looking like a white rat, its physiological characteristics are exactly the same as humans. Then, as long as a medicine works when it is eaten, it must work when people eat it. Such "super mice" are medically universal.
The Fermi superfluid under the strong interaction (monogram) limit achieved by the experimental group using lithium-6 atoms is such a "super guinea pig". As long as the physical characteristics studied from it must be the universal law of the strong interaction Fermi superfluid, it must be applied to the shell of the neutron star and the quark gluon plasma that are completely different from its composition, interaction, temperature, pressure, and density.
So, what physical features did the research team study?
(c) two sets of transport characteristics of superfluidics
What the research team wanted to figure out was the transport characteristics of the two sets of superfluids: viscosity and thermal conductivity.
The so-called viscosity is to use a coefficient to describe whether it is as viscous as syrup or as non-viscous as water.
(Image source: Wikipedia)
The so-called thermal conductivity is to use a coefficient to describe whether it is hot like an iron pot, or whether it is burned casually like a wooden handle.
According to the mainstream physical model of superfluids, the "difroid model", the research team believes that as long as they can measure these two sets of coefficients clearly, they will completely study the strongly interacting Fermi superfluids. These two sets of coefficients are related to the ability of the superfluid to transport energy (heat) and momentum, so they are both transport characteristics of the superfluid.
So, how can the research team measure these two sets of coefficients clearly in the experiment?
(4) Attenuation characteristics of the second type of sound waves
To figure out how the research team measured the two sets of coefficients of the superfluid in an experiment, we had to introduce a physical phenomenon that doesn't exist in the classical world at all: the second type of sound waves.
What is the second type of sound wave? Middle School Physics will tell you that sound waves are mechanical vibrations. When an object vibrates, it repeatedly squeezes the air, causing the pressure and density of the nearby air to change repeatedly. When this vibration of pressure and density reaches your ears, you hear the sound.
If you transmit this pressure and density vibration into the superfluid, the superfluid will also vibrate in pressure and density. Therefore, superfluids also transmit sound.
Note: According to the "difluid model" of the superfluid, n and s here represent the two seed components in the superfluid. Together, these two seed components form a superfluid.
However, physicists have found that in addition to vibrations of pressure and density, superfluids can also transmit a vibration of entropy and temperature. Since the former vibration is a sound wave, physicists have given another vibration a name, called the second type of sound wave.
In layman's terms, you can think of the second type of sound wave as a scene in a superfluid like a large group exercise performance. The particle groups scattered everywhere in the superfluid, like the actors in the group exercise, have no movement themselves, but they hold up red cards (becoming sub-components n in superfluids) and blue cards (sub-components in superfluids) for a while. From a close range, they all stay in their respective positions and are completely in motion. From a distance, their cards change back and forth, forming a uniform wave as a whole. In the eyes of physicists, such fluctuations are fluctuations in entropy (and temperature).
In summary, to measure the two sets of coefficients of the superfluid, the research group must first excite the sound waves and the second type of sound waves in the superfluid, and then measure their attenuation characteristics separately. Only by first measuring the attenuation characteristics of the sound waves can they calculate the two sets of coefficients of the superfluid. The question arises again, what does the attenuation characteristics of sound waves mean?
There are two ways to understand the attenuation characteristics of sound waves. One is to excite a sound wave (or type II sound wave) in the superfluid, and then see how fast it gets weaker and weaker. This approach is easy to understand, but it is not the approach adopted by the Study Group.
Another method is to stir up sound waves of different frequencies (or type II sound waves) at the same intensity in the superfluid, and then see which frequency of the sound wave is strong, which frequency of the sound wave is weak, and what is the width of the resulting distribution of strength and weakness. This is the method used by the research team to measure the attenuation characteristics of the sound waves. Although this method is a bit difficult to understand, it is completely equivalent to the first method (the two differ only once in the Fourier transform).
Speaking of which, the background of the experiment is basically explained. The research group is to use lithium-6 atoms to prepare Fermi superfluids under the limit of strong interaction (monogram), and by measuring the attenuation characteristics of its sound waves (or second type of sound waves), according to the mainstream model "two fluid model" in superfluidic research, the two sets of transport characteristic coefficients of this superfluid are obtained: viscosity and thermal conductivity.
So, how exactly did the research team do it?
(5) The results and significance of the experiment
First, the team put about 10 million lithium-6 atoms into an empty box in the shape of a cube.
In the dark words of physics, they built a potential well in a rectangular region through the tight and delicate combination of lasers and magnetic fields, and then successfully suspended the lithium atoms in it and reduced the temperature to about a few hundred millionths of Kelvin. At this time, the lithium atoms enter a superfluid state.
Then, by interfering with the two laser beams, they made the box rise and fall like a wave.
This is equivalent to loading a moving optical lattice onto a superfluid composed of lithium atoms.
At this time, a mysterious wave was spawned in the superfluid. The research team found that this wave contains both ordinary sound waves and the second type of sound waves unique to superfluids.
So, using the second method we just described, they measured the attenuation characteristics of sound waves (and second-class sound waves).
At the same time, they measured the critical divergence behavior exhibited by transport characteristics near the phase transition temperature of the superfluid, which physicists value.
(Note: As shown in the figure, the value of the transport characteristics suddenly increases near the critical temperature of the phase transition)
On February 4, 2022, their paper was published in the journal Science.
The results of the study group had at least four levels of significance.
First, it is very difficult for about 10 million lithium atoms to obediently form a Fermi superfluid with uniform density, precise temperature, stable for a long time, and precisely controlled by the research group at the limit of strong interaction (unitary positive). At the same time, it is very difficult to excite a second sound wave on such a superfluid and accurately measure its attenuation characteristics. The research team successfully completed experimental preparation and measurement, developed a multi-body quantum system that can be precisely regulated, and laid the foundation for quantum simulation research, which is very meaningful in itself.
Second, the dominant model of superfluids, the "difrofluid model", was originally used to describe conventional superfluids (such as helium-4 superfluidics). The research team's experiments have proved that it is also suitable for monoframic superfluidics.
Third, the measurement results of the research group show that the transport coefficients of the monoframic Fermi superfluids have reached the universal quantum mechanical limit, such as the diffusion coefficient of the second type of sound waves
, the thermal conductivity is approximate
。 This shows that it is indeed a kind of "super guinea pig" as physicists expect. The physical knowledge we gain from it can be safely extended to other similar Fermi superfluids.
Fourth, the research team found a critical region about 100 times larger than the critical region of helium-4 superfluids in this monofirmee superfluid. With a larger critical zone, it will be much easier for physicists to study the critical divergence behavior that they value so much. This finding lays the foundation for further quantum simulation studies using the system to understand anomalous transport phenomena in the strongly correlated Fermi system.
When it comes to the strong correlation Fermi system, you may feel strange. In fact, the high-temperature superconducting materials that physicists think about day and night belong to the strong correlation Fermi system.
We hope that in the future, physicists will be able to further simulate Fermi superfluids with ultracold atoms in the laboratory and conduct more in-depth research on it. These studies will not only help us understand the physical properties of the shells of neutron stars and the quark gluon plasma of the early Big Bang, but also further reveal the physical properties of the strongly correlated Fermi system, helping us to gain a comprehensive understanding of this uncharted territory.
With a comprehensive understanding of the strongly correlated Fermi system, we will be able to understand and design high-temperature superconducting materials with economic value!
Note: There is a key to this experiment. The research team divided the lithium atoms into two groups. They put one group of lithium atoms into the lowest-energy Stateman energy level, and the other group of Lithium Atoms into the Second Lowest Energy Level state. Why divide lithium atoms into two different states? Because if the lithium atoms are not divided into two different states, all lithium atoms are exactly the same fermions, that is, all the same fermions. According to the Pauli incompatibility principle, the all-identical fermions cannot overlap spatially, the interaction is very weak, and it is impossible to form a monogram system. Only after dividing the lithium atoms into two different states can the two groups of lithium atoms interact strongly, thus realizing the monogram system.
bibliography:
1. Li X, Luo X, Wang S, et al. Second sound attenuation near quantum criticality[J]. Science, 2022, 375(6580): 528-533.
2. Donnelly R J. The two-fluid theory and second sound in liquid helium[J]. Phys. Today, 2009, 62(10): 34-39.
3. Schaefer T. Quantum-limited sound attenuation[J]. Science, 2020, 370(6521): 1162-1163.
4. Patel P B, Yan Z, Mukherjee B, et al. Universal sound diffusion in a strongly interacting Fermi gas[J]. Science, 2020, 370(6521): 1222-1226.
图片来源：Grossi B, Iriarte-Díaz J, Larach O, et al. Walking like dinosaurs: chickens with artificial tails provide clues about non-avian theropod locomotion[J]. PloS one, 2014, 9(2): e88458.
End
Author: Sheldon
Draw: Appreciation
BEAUTY FINGER: Cow cat
Typography: Zhang Ying
Acknowledgements: Yao Xingcan, Hu Hui, Luo Xiang
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