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Physical results of the year - 2020


The past year turned out to be difficult, but nonetheless rich in fundamental discoveries and technological breakthroughs. Today we will talk about the most memorable results.image Credit: scitechdaily.com
Superconductivity at room temperature
image Credit: Adam Fenster It is hoped that ordinary hydrogen may be the simplest high-temperature superconductor. True, for this it must be metallic, for which it will have to be compressed to pressures of over 500 gigapascals (this is about five million atmospheres). In general, such enormous pressures are created between diamond anvils - polished edges of high-quality diamonds tens of microns in size. The problem is that at 500 GPa, the anvils begin to burst: this is exactly what happened shortly after the first discovery of metallic hydrogen. It is much easier to stabilize hydrogen by using its compounds with other elements and to operate at more acceptable pressures of about 100-200 GPa. This led to success in 2015, when a group from Mainz showed that hydrogen sulfide, compressed to 155 GPa, becomes superconducting already at -70 degrees Celsius. The result improved several times, and finally, in the past year, a group from the University of Rochester in the USA (ironically led by the same researcher who lost the world's only sample of metallic hydrogen) showed superconductivity in sulfur hydride with the addition of carbon at a pressure of 270 GPa and room temperature of +15 C! To do this, the authors placed a mixture of carbon and sulfur between the anvils and passed hydrogen through it for several hours, illuminating the mixture with a green laser, which played the role of a photocatalyst. Due to the enormous pressure, practical applications are still very far away, but the result is undoubtedly impressive.
Fast radio bursts from magnetars
image Credit: Pitris / dreamstime.com From time to time, radio telescopes detect fast radio bursts - powerful pulses of an extraterrestrial nature with a duration of the order of milliseconds. Until last year, they all came from outside our Galaxy, and specific sources remained elusive, as did their nature. In narrow circles, there were jokes that there were more theories of the origin of radio bursts than were recorded. That all changed on April 27, when two orbiting telescopes detected several X-ray and gamma-ray bursts from the magnetar (neutron star with a huge magnetic field) SGR 1935 + 2154 in the Milky Way and alerted other observatories of the increased activity via The Astronomer's Telegram . Two observatories in Canada and the United States decided to join the observations of it, and after a few hours they saw an unusually powerful radio burst! After that, several more telescopes immediately joined the work, and half a day later, when the Earth turned in the right direction, the newest Chinese radio telescope FAST joined them. As a result, astronomers not only became convinced that magnetars can emit fast radio bursts, but also clearly measured how its radiation in all ranges - from radio waves to gamma rays - changes over time. According to the most harmonious theory describing these observations, the magnetar periodically emits shock waves, and a burst of radiation occurs when one of the waves catches up with the previous one and collides with it.
A hint of CP violation
image Credit: Kamioka Observatory / Institute for Cosmic Ray Research / The University of Tokyo Our world is woven of matter, but antimatter is almost never found in it. This is surprising, because at the dawn of the Universe, matter and antimatter were equally divided. To balance is disturbed and the world became what it is now, must be broken CP-symmetry (charge-parity symmetry): the laws of physics should be changed, if we flip a physical system and replace all particles by antiparticles. Generally speaking, CP-symmetry breaking was discovered back in the 60s during the decay of K mesons (in 1980 they received the Nobel Prize for this), and later it was observed in B and D mesons. However, it was too weak to explain the disappearance of antimatter from the early universe. But besides quarks (of which all mesons are composed), there is another type of elementary particles - leptons. Among them are three types of neutrinos that can transform from one to another (this is called neutrino oscillations ), and comparing the oscillation frequency of neutrinos and antineutrinos would be a good test of CP symmetry. The difficulty is that neutrinos are very difficult to detect: they practically do not interact with anything and can fly right through the Earth. But nothing is impossible. This year, the Japanese collaboration summed up the results of a long-term experiment in which a neutrino beam was generated at the accelerator in Tokai (for this they irradiated a graphite target with protons), and was detected in the famousSuper-Kamiokanda (an excellent overview of this work on the Elements ). Scientists have registered 90 oscillations of a certain type with neutrinos, and only 15 with antineutrinos. This indicates the violation of the lepton CP symmetry with a 95% confidence level, which is still insufficient for the discovery. Nevertheless, this is a serious claim for success, and the experiment will certainly continue.
Maximum speed of sound
image Credit: Gerd Altmann We know very well that sound is a longitudinal wave in which compressions of an elastic medium alternate with stretching. The speed of sound is highly dependent on the environment. On the one hand, sound travels faster in dense materials. On the other hand, the lighter the atoms of a substance, the less their inertia and the easier it is to move them. Therefore, the speed of sound in aluminum is higher than in steel, and the highest known speed of sound - 18 km / s - is observed in diamond. This year, a collaboration from Moscow, London and British Cambridge has proposed a surprisingly simple model for the speed of sound in elementary substances, which includes only one parameter (the atomic mass of an elementimage ) and four fundamental constants: the electron massimage , proton massimage , fine structure constantimage and the speed of lightimage :image The result is surprising in that the fundamental constants that usually describe the microcosm and quantum effects turned out to be decisive for the description of sound, a classical effect that manifests itself on incommensurably large scales. And it also follows from this model that the highest speed of sound should be observed in the already known metallic hydrogen. It is about 36 km / s, which is in good agreement with modeling solid hydrogen at pressures up to 1000 GPa. As we remember from the note about superconductivity, it is still unrealistic to achieve such pressures; nevertheless, this could be an interesting blueprint for future research into metallic hydrogen.
Discovery of Abelian anyions
image Credit: 5W Infographics / Quanta Magazine There are two types of particles around us: fermions and bosons. Bosons have a whole spin, fermions have a half-integer spin; identical fermions repel, bosons do not. There is another important difference, clear from a very thought experiment. Let's take two particles and make one of them a circle around the other as in the left picture. In a three-dimensional world, making a circle around the second particle is the same as making a small circle before reaching it, or doing nothing at all. The result will be the same: the particle will return to its place, the probability of meeting it there is equal to unity. In the quantum world, probability is the square of the amplitude, so the amplitude can only be +1 (these are bosons) or -1 (fermions). There is no third. Everything changes in the 2D world in the right picture. To make a circle around another particle is not the same as just standing still: we no longer have a third dimension to collapse into a point. Having made a turn, the particle can return, being not a fermion or a boson, but anything else. This is where the name any-on comes from. The two-dimensional world is full of surprises. For example, the fractional quantum Hall effect (the tricky behavior of the resistance of two-dimensional structures in enormous magnetic fields) is caused by composite perturbations that behave like particles with a fractional charge. Last year, a group from Paris was able to clearly show that it is precisely such disturbances that are prominent representatives of the Anion family. For this, the authors prepared an " anyon collider ": a two-dimensional sample with cuts was placed in a magnetic field so that anyions would propagate along the cut boundaries. Where the cuts came close to each other, charge tunneling was observed, the properties of which perfectly confirmed the nature of the anyions. Despite the complexity and unintuitive nature of such work, this is a very promising direction: anyons can be used in topological quantum computers for unsurpassed reliable quantum memory.
Straight-gap silicon
image Credits: nature.com Dreams of integrated optoelectronics - for example, fiber optic receivers built into a processor or video cameras on the same chip as a GPU - remain dreams for a very fundamental reason: all modern electronics are based on silicon, which is extremely poorly suited for working with light. The problem lies in the cubic crystal lattice of silicon and the law of conservation of momentum. When light is emitted, an electron in silicon passes from the valence band to the conduction band, while strongly changing its momentum. A photon cannot compensate for such a large momentum, and this has to be done by the crystal lattice itself, which by orders of magnitude reduces the probability of emission or absorption of light. In contrast to silicon and similar indirect-gap materials, optoelectronics uses direct-gap semiconductors, in which the electron momentum is small and is easily compensated by the photon momentum. Last year , a group from Eindhoven made a breakthrough: they were able to obtain a direct-gap alloy of silicon and germanium not with a cubic, but with a hexagonal crystal lattice (on the right in the picture). To do this, they grew gallium arsenide nanowires, which served as seeds for the growth of a silicon-germanium alloy with the desired crystal lattice. The resulting alloy emitted light with a wavelength of about 2 microns (this is a promising range for fiber-optic communication), while the wavelength could be adjusted by changing the germanium content of the alloy. So far, this technology is not very compatible with the silicon industry, but the ability to grow many emitters / detectors next to each other makes this discovery very promising for practical problems.
Photonic quantum computer
image Credit: Hansen Zhong One of the breakthroughs of 2019 was the demonstration of quantum superiority: a quantum chip with 53 superconducting qubits in a few minutes solved a problem on which a classical computer would have spent immeasurably more time. Designing such quantum chips to operate at ultra-low temperatures is a daunting task that becomes exponentially more difficult as new qubits are added. Therefore, although superconducting qubits remain the leaders of the quantum race, a lot of efforts are being made to find alternative systems. At the end of the year, news came from China: a group of Professor Peng (who created the quantum satellite Internet) demonstrated quantum superiority on a photonic quantum computer. The role of quantum memory in it is played by specially prepared light. All the components of such a device - quantum light sources, interferometers, photodetectors - are well known, but truly Chinese meticulousness was needed to assemble and adjust all the optics to emulate 50 qubits. Among the indisputable advantages of a photonic computer are operation at room temperature and the possibility of relatively simple addition of new qubits. And for light, you can use optical fibers or waveguides on a chip, which will greatly simplify the setup and increase the dimension of such a device. These are the achievements we will remember the past year. I hope this year we will also learn a lot of interesting things about the world around us.
xially 12 february 2021, 21:14
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