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Material posted : Administrator Publication date: 06-01-2018

The time has come to sum up the scientific end of 2017, along with the American physical community. This time the editors of the APS tried their best and produced a very entertaining compilation of the latest achievements of fundamental science. Today we will talk about them in detail.

Gravitational-wave astronomy and everything

Along with the Nobel prize in gravitational-wave astronomy brought new surprises. To the two gravitational wave detectors LIGO joined the European Advanced Virgo. Now the observation detectors can be independently confirmed by a device of different construction, being on another continent. Moreover, the presence of the three detectors allow to determine the direction to the source of gravitational waves. The long wait is 14 August, all three detectors are recorded , another signal from the merger of two black holes whose location (green marker in the picture) was able to identify much more precisely than with two detectors.

And after three days, the detectors saw a new event – this time the merge of black holes and neutron stars. By happy coincidence, simultaneously with this event the huge number of telescopes saw the flash from the merger of stars in the whole spectrum — from radio to gamma radiation. The ability to simultaneously record both light and gravitational waves are an incredible breakthrough for astronomy, so, bored in the near future astrophysicists definitely not necessary.

Prepare the crystal of time

In physics there is such a fundamental phenomenon as spontaneous symmetry breaking: it occurs when the energy state of the system loses the symmetry inherent in the describing equations. The most obvious example is the crystal: he turns an ordinary space, all points of which are identical with each other, in the structure with a strictly specified period. To put it a little more scientifically, the crystal breaks the continuous translational symmetry of space, making it discrete. Since space and time are entities of the same kind, then the question arises: is it possible to create a similar crystal for the time – that is, to make the ground state of the system was not stationary and periodically changed? Intuition says no: changing the system usually has nonzero kinetic energy, and therefore is not in main energy state. However, in 2012, it was shownthat if the momentum of the system is nonlinearly dependent on speed, it becomes possible. Soon this conclusion was generalized to the case of quantum systems.

Later, it became clear that in thermal equilibrium the crystals of time still can not exist. However, if the system turns the external periodic influence, becomes real to create discrete crystal time – it also periodically changes its state, it does it multiple times slower than the external disturbance. In other words, if the response time of the crystal is expanded in a Fourier series, we see a signal to one of the subharmonics of the external impact. In the past year, the experimental observation of this has published as many as two teams. Collaboration from Maryland and Berkeley used to do this, a chain of ions of ytterbium, periodically acting on the nuclear spins with laser pulses with a period T. In the intervals between the pulses of ions interacting with each other in such a way that the evolution of the whole system occurred with a period of 2T. This was the main evidence of crystal formation time. In one month, a group from Harvard reported on a similar experiment with an ensemble of NV-centers in diamond, the spins which were excited by microwave pulses. Here the authors were able to observe oscillations with double and triple period. In addition to fundamental significance, this work opens new possibilities for studying the dynamics of quantum systems and can also be interesting for storage of quantum States.

Causation in the quantum world

If two things correlate with each other, either one can cause the other. Or may not be. For example, there is a definite correlation between the number of tsunamis in Japan and Chile; none of them affects the other because they both have a completely different cause – the earthquake in the Pacific ocean. In matters of causality correlated phenomena sometimes helps to understand the Reichenbach principle: if you know that the root cause two phenomena occurred, the correlation between them is lost.

The quantum world is much more complicated. Peritrichida many phenomena (e.g., correlations of entangled particles) for a long time searched for in hidden parameters, inaccessible to the observer. However, the experiments of bell's inequality has shown that hidden parameters do not exist (at least in any of the known types). Therefore, in the quantum world, the question itself is arranged in another way: not what is the causeand what is the quantum causality. Progress in this matter, achieved the collaboration of Britain and Canada. The authors suggested to override the principle of Reichenbach, moving from deterministic classical evolution to unitary evolution, which obey a quantum system. The result was the first consistent model, capable enough to strictly describe quantum causality. Despite matematicheski, this work sheds light on the nature of quantum correlations and may provide an opportunity to visualize quantum phenomena in causal language.

Wi-Fi: radar that is always with you

The idea is to use the radiation of Wi-Fi module for radar nearby objects is not new (for example, work 2005). In practice it is complicated by the fundamental features of Wi-Fi transmitters. First of all, they, unlike radars that emit in all directions. This gives rise to multiple reflections from the objects and okrujayuschih great complicates the analysis of the signal. In principle, the task could be simplified by sending short pulses – but it is difficult because of osmolarnosti Wi-Fi.

The original solution to the problem suggested a group from the technical University of Munich. They record the wave front behind the object under study, after which rekonstruiruet its shape using well-known algorithms for optical holography. In the experiment, the resolution was about 3 cm for a Wi-Fi router at 5 GHz. A nice bonus is the fact that the source can transmit any signal reconstruction will work in any case. Difficulties of recording the wave front has to produce a pixel-by-pixel, physically moving the receiver. The use of an array of receivers would help to simplify this process, raising the frame rate to 10 fps.

Cuprate superconductors

The high-temperature superconductors remain cuprate connection, including a copper oxide, such as YBaCuO. The Champions go into the superconducting state at 134 K (-139 ° C), while the nature of this superconductivity is still under question. In any case, it was considered that it is not described by the theory of BCS, which has worked well for many other superconductors (also called superconductors of type II). In particular, the BCS theory predicts the existence of Abrikosov vortices, the contour of which flows a continuous current, while inside the vortex, superconductivity disappears. Such vortices appear in a magnetic field, which cannot exist in a superconductor, but penetrates easily inside the non-superconducting vortex. The experimentally well observed Abrikosov vortices in superconductors of type II (confirming the theory BCS), and have never been seen in the cuprates.

In fact, not been seen before that year. Collaboration from Switzerland and Germany for the first time demonstrated the emergence of vortices in superconducting Y123 cuprate. For this purpose, the authors used a scanning tunneling microscope, by which they measured the conductivity of the sample on the area of 90x90 nm2 and found an ordered lattice of vortices (figure). Despite a number of experimental difficulties and ambiguities (mainly due to the contribution of signal from the non-superconducting electrons), the observed properties of these vortices are well described by BCS theory that can shed light on the nature of high-temperature superconductivity. Moreover, the approach that recognises the contribution of non-superconducting electrons to the total signal, it will be extremely important for future research.

The contribution of gluons to the proton spin

 

 

The COMPASS spectrometer at CERN, which measures the contribution of quarks to the proton spin. The picture is from here.

The nuclei of atoms costant of protons and neutrons, each of which, in turn, is composed of three quarks. Protons have a spin (intrinsic magnetic moment) equal to½; exactly the same spin and quarks. The more surprising results of the experiments showed that the total spin of the proton is only 30% determined by the spin of quarks. The reasons for this remain unclear, as does the nature of the remaining spin; however, candidates enough is and virtual quark-antiquark pairs, and the orbital momentum of the particles, and of course the gluons carry the strong interaction holding the quarks together.

This year a collaboration of four us universities for the first time calculated the spin contribution from gluons. This is done using a complex numerical simulation of quantum chromodynamics on a space-time lattice. It turned out that the total spin of the gluons is 0.25 ± 0.05 – in other words, the gluons account for almost half of the proton spin! A much smaller contribution from quarks is caused, apparently, by the transfer of angular momentum quarks to a cloud of virtual quark-antiquark pairs and pions; the role of gluons in this process were insignificant. Overall, these calculations allowed a better understanding of the internal structure of the proton, and their experimental confirmation is planned for the future us electron ion Collider.

In search of dark matter

As you know, the negative electrode is the same electrode, and the negative result – too result. Over the last 16 months the three largest detectors of dark matter (XENON1T Italian, Chinese PandaX-II American and LUX) and not managed to detect any traces of winow particles, presumably components of dark matter. This clearly shows that the existing theoretical understanding of vinah still far from reality. Given the failure of the searches for supersymmetry at the LHC is anyone who does puts the existence of these hypothetical particles.

The essence of the experiments to search winow is quite simple: as their detectors are huge tanks of liquid xenon, located deep underground to protect from cosmic radiation. The interaction of heavy wimp with the xenon atom leads to a flash of light and generate electrons, which are registered by photomultipliers at the top and bottom of the tank. Knowing the theoretical limits on energy vimov, we can estimate the expected number of events per unit of time. The fact that such events were registered too few means that the properties of winow very different from predicted. Seemingly, if wimpy exist, they have a different mass or a different scattering cross section for atoms (and maybe the others) and, therefore, their search will require a new generation of detectors.

Machine learning to recognize the topological state of the

Topological effects in physics – a very topical subject that is incredibly difficult to explain on the fingers. That is why it is virtually not covered in the popular scientific literature (and this despite the great success – to recall about graphene, quantum Hall effect or the Nobel prize 2016). In a nutshell, different topological States can not be translated into each other in a smooth continuous change how the video source system, which makes them very resistant against external disturbances. The simplest example – a two-dimensional lattice of atoms whose spins are either form or not form a vortex:

The picture here

Mathematically, these States differ in the topological charge – in this case, the number of vortices in the system with the plus sign if the vortex swirling clockwise and negative counterclockwise. The picture on the left the charge is 0, and on the right is -1. If topological charges are different, then the state can not smoothly move each other. The difficulty is that to calculate the topological charge can be very difficult. For example, if the size of the vortex is huge, and it is twisted somewhere on the borders, to calculate the charge will have to explore all the atoms in the system. But there are topological charges that are computed are much more complicated, making calculations new topological materials is almost overwhelming.

The solution to this problem suggested theorists from Cornell and the University of California. Its essence is that on the basis of the studied crystal lattice (more precisely, its electronic density, electronic density) is generated, in fact, a multidimensional array (QLT image) special integrals on contours of increasing size. This allows you to cover the area of the grille is sufficient for understanding topological properties. After that, a multidimensional array is input to a previously trained single layer neural network, which concludes, if the condition is topological or not. Compared with traditional methods, this method proved to be very productive, and the authors plan to develop a machine learning application to condensed matter physics.

Source: https://geektimes.ru/post/296945/

Tags: science


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