On June 5, 2017 (May 11, 2017 in the lunar calendar), humans created "liquid light" for the first time, which is called the fifth state of matter. If light becomes a liquid like water, what will happen to it? This is not a brain hole. Recently, scientists have created liquid light at room temperature for the first time, allowing light to flow, bend and circle around an object like water. Picture | Artist's Imagination Animation of Liquid Light The breakthrough was jointly completed by researchers at the CNR Institute of Nanotechnology in Italy and the Montreal Institute of Technology in Canada. The related paper was published in Nature Physics on June 5, 2017. The successful implementation of this research paves the way for further development of quantum fluid mechanics, and may also provide inspiration for the realization of room temperature superconductivity and new electronic components. The above picture shows the reaction of ordinary liquids encountering obstacles; the following picture shows the reaction of liquid light encountering obstacles. In fact, under certain circumstances, light can indeed become a liquid and become a superfluid. However, to achieve this effect requires very harsh conditions, because liquid light belongs to the Bose-Einstein condensate - this condensed state is also known as the "fifth state of matter" (there are six states of matter, the other five are gas, liquid, solid, plasma, and fermion condensate). Under normal circumstances, similar states can only appear at low temperatures close to absolute zero (minus 273 degrees Celsius). Daniele Sanvitto, the team's lead scientist from the CNR Institute of Nanotechnology in Italy, said: "The most unusual thing about this work is that we have shown that superfluids can also be achieved under ambient conditions at room temperature". The two leaders of the project, Daniele Sanvitto, and Stéphane Kéna-Cohen, another leader of the research team of Stéphane Kéna-Cohen, describe a more dramatic effect of liquid light: unlike ordinary liquids, liquid light will only pass smoothly when it encounters obstacles, without generating any ripples and swirls, showing zero friction and zero viscosity. As the energy increases, the fluid gradually takes on the properties of superfluids as it passes through matter. The four sets of comparison diagrams describe the distribution, intensity, momentum, and density of electromagnetic polarons respectively. It is not difficult to see that the preparation method of liquid light is similar to the realization method of metal superconductivity: both can only be observed at extremely low temperatures, and the duration is very short. Figure Shu Optical setting of the experiment, there is an extremely thin layer of organic molecules between the two lenses. So, how did scientists create liquid light at room temperature this time? According to Stéphane Kéna-Cohen, to achieve this goal, they placed a 130-nanometer-thick slice of organic molecules between two extremely reflective lenses, forming a sandwich-like structure. Then, the researchers bombarded the system with laser pulses with a period of 35 femtoseconds, causing photons to eject back and forth between the lenses. In the process, the photons rapidly intersect with the organic molecules in the middle, forming a kind of liquid light with the dual property of light-matter. In short, the photons and electrons in the organic molecules are coupled to form liquid light. Figure Shu Polaritons This coupler in this experiment is called a polariton, which is a quasiparticle. It is created by strong coupling between electromagnetic waves and excitation with electric or magnetic dipoles. In simple terms, the formation of a polariton can also be regarded as an excited photon. The concept of polaritons-superfluids was first proposed in 2007, when researchers hypothesized that one of the greatest features of this type of superfluid is the possibility of being realized at room temperature. Figure Shu Comparison of other measurements at low and high energy reflects the formation of superfluid states. This breakthrough will have a huge impact on future academic research and practical applications. In academic research, in addition to allowing scientists to study fundamental phenomena related to Bose-Einstein condensates at room temperature, liquid light can also provide a better research object for quantum fluid mechanics. As for its practical utility, Stéphane Kéna-Cohen says: "This result not only demonstrates the fundamental properties of Bose-Einstein condensates, but also inspires the design of future photonic superfluid devices, which are likely to achieve zero energy loss." Previously, in superconductor studies similar to this experimental principle, the fabrication of materials with near-zero electrical resistance often required rigorous extreme freezing treatment. Using this method of liquid light fabrication, engineers can produce more efficient superconducting material devices such as lasers, Light Emitting Diodes, solar panels and photovoltaic cells at room temperature, and these devices can largely avoid the energy loss caused by photon contact with obstacles. Extended reading: Bose-Einstein condensates Extended reading: Bose-Einstein condensates In recent years, scientists have created more and more new types of substances, and the preparation methods are constantly becoming more and more routine. Whether it is the previous metallic hydrogen, time crystals, negative mass superfluids, or this time liquid light, these imaginative discoveries have exploited the strange state of matter in extreme situations. Among them, the Bose-Einstein condensate mentioned above is one of them. This condensate is also known as the "fifth state of matter" (there are six states of matter, the other five are gas, liquid, solid, plasma, and fermion condensates). It follows quantum mechanics rather than classical physics, and is also the most important theoretical pillar in this study. In 1953, Satyendra Nath Bose stared at a photo of Einstein. Eighty years earlier, Einstein and the Indian physicist Bose predicted the existence of this state of matter based on quantum mechanics. Einstein even doubted his theory because of its peculiar properties. The state appeared experimentally in 1938, when scientists discovered the isotope of helium at a temperature of 2.17K, and helium-4 suddenly changed from a normal fluid to a superfluid with zero viscosity. However, at that time, people had not yet connected the superfluid phenomenon to Einstein's theory. It was not until 1995 that Eric Cornell and Carl Wieman of the University of Colorado created a true Bose-Einstein condensed matter - a polymer of about 2,000 rubidium atoms. In order to cool this pile of atoms, the two scientists were also able to fight hard. First, they used laser technology to forcibly cool down, and then used a magnetic field to cut off the hotter atoms bit by bit, and finally dropped to a temperature of 100nK (one millionth of a degree). However, the research results eventually won them the Nobel Prize in 2001. Figure Shu 1995 experiment: using a laser (red arrow) and a magnetic field (blue arrow) to cool down the rubidium atoms (green area), and finally the Bose-Einstein condensate appeared in the green area. It can be seen that this time the room temperature liquid light is almost 6 to fly! In the near future, we expect this state of matter to exhibit more magical physical properties and continue to bring unexpected new discoveries to mankind. The amazing superfluid phenomenon Zero viscosity, zero friction, no wrinkles when encountering obstacles... Why does liquid light have these incredible properties? This has to start with a special phenomenon called superfluids. Superfluids are liquids or gases that exhibit zero viscosity under extreme conditions. Because there is no friction around the flow, its mechanical energy loss is also zero. If we place the superfluid in a ring-shaped container, since there is no friction, it can flow endlessly. And the substance that exhibits this property is called a superfluid, and the liquid light produced this time is the light in the superfluid state. As you can see, superfluids do not produce any ripples when they flow through obstacles. Superfluids generally only exist in extreme environments close to absolute zero, because most superfluids are the embodiment of Bose-Einstein condensates. When a polymer of particles cools to a certain extent, they will condense in the lowest energy state. At this time, they are in a semi-quantum state formed based on wave-particle duality, so fermions can condense in a small space like bosons. For the convenience of everyone's understanding, it is a cloud of particles that are held together when it is very cold. After they clump together, they overlap with each other and do not distinguish between you and me, as if many small water droplets are agglomerated into a large pool of water. Therefore, because of its highly coherent quantum nature, it is not surprising that the friction and viscosity disappear when flowing. Superfluids are a manifestation of Bose-Einstein condensates, but not all superfluids are in Bose-Einstein condensates. Corresponding to this, there are also fermion condensates, which are suitable for the theoretical description of superconductors, but "History Today" LSJT will not continue here.