Extracts from the Internet

Oscillations of atmospheric high-energy neutrinos

The effect of neutrino oscillations at energies up to 56 GeV, which is an order of magnitude higher than was reported earlier, was obtained in the IceCube experiment being carried out in the ice of Antarctica on the South Pole. Muon neutrinos were registered that had been born in the interactions of cosmic rays with air molecules in atmosphere. Not only downright flying neutrinos were observed but also those passing through the Earth. A deficit of upward flying muon neutrinos, which was referred to the effect of oscillations (transformations) of muon neutrinos to tau neutrinos, has already been observed earlier. A new analysis of the IceCube obtained for three years gives Δm232=2.31+0.11−0.13×10−3 eV2 and sin2θ23=0.51+0.07−0.09 for the neutrino parameters for a normal mass ordering. The neutrino mixing is close to the maximal, which corresponds to the data of the acceleration experiment T2K, but differs from the NOνA data. Source: Phys. Rev. Lett. 120 071801 (2018)

Observation of quantum energy-time entanglement

Researchers from the University of Waterloo (Canada) and the University of Paderborn (Germany) performed the experiment to observe for the first time the quantum energy entanglement of a photon with the time needed to detect the second photon, quantum-entangled with the first one. This effect could not yet been directly observed because of the high speed of the processes. In the experiment of J.-P. W. MacLean, J.M. Donohue and K.J. Resch, pairs of photons were born under the action of laser pulses on a nonlinear crystal. In one of the photons, an “optical comb” was used to determine the frequency (energy), and the time of the second photon detection was measured using the stroboscopic method. This method uses auxiliary laser pulses – time marks. Entering a nonlinear BiBO crystal almost simultaneously with such a pulse the second photon of the pair could be converted to a photon with a higher frequency whose observation served for counting the time with an accuracy to 10−12 s. Verification of certain relations between photon frequencies and times of their detection showed the presence of quantum entanglement. The effect can find application for key distribution protocols, for synchronization and for other purposes. Source: Phys. Rev. Lett. 120 053601 (2018)

Bell’s inequalities with continuous variables

Verification of Bell inequalities is a fundamental test of quantum mechanics excluding hidden parameters. Many experiments demonstrated a violation of these inequalities using discrete quantum variables. In their experiment, O. Thearle (Australian National University) et al. showed for the first time that Bell’s inequalities are also violated for a system characterized by continuous variables represented by four polarization modes obtained in the mixing of two photons in compressed states. The results of the measurements carried out through synchronous detection clearly showed a violation of Bell’s inequalities in the Clauser – Horne – Shimony – Holt version (CHSH inequalities). This experiment opens up new possibilities for using continuous variables in device-independent quantum protocols. Bell’s inequalities for photons were considered in detail in the review in Phys. Usp. 36 653 (1993). For the experimental studies of the basic principles of quantum mechanics see also Phys.-Usp. 42 481 (1999)and the paper by A.V. Belinsky and A.A. Klevtsov in the present Phys.-Usp. issue. Source: Phys. Rev. Lett. 120 040406 (2018)

Multidimensional Einstein – Podolsky – Rosen steering

The Einstein – Podolsky – Rosen (EPR) steering, the conception of which was introduced by E. Schrodinger in 1935, was demonstrated in recent qubit experiments. The steering means not only quantum entanglement of two systems, but also the possibility of steering the reduction of the wave function of a remote system through the choice of the measuring basis for the near system. Theoretical papers predicted the possibility of steering when the systems are quantum-entangled in a larger number of variables d>2 than in the qubit case. Q. Zeng, B. Wang, P. Li and X. Zhang (Beijing Institute of Technology, China) demonstrated a multidimensional steering for dimensions d=2-5. Information coding was realized in orbital angular momenta of photons set with the help of spatial modulator. Considering that the orbital angular momentum had many states a multidimensional steering was possible. A multidimensional steering was also investigated under artificial noise and its noise resistance was shown. The well-known paper by Einstein, Podolsky, and Rosen and the comments to it by V.A. Fock and N. Bohr were published in UFN 16 436 (1936). For a distant steering of wave function collapses see also the book Dynamics and Information by B.B. Kadomtsev and his paper in Phys.-Usp. 37 425 (1994). Source: Phys. Rev. Lett. 120 030401 (2018)

Double superconducting interferometer

Researchers from the Chernogolovka Institute of Microelectronics Technology and High-Purity Materials RAS, Moscow Institute of Physics and Technology (Dolgoprudny, Russia) and the Royal Holloway University of London (Great Britain) demonstrated a new type of superconducting interferometer which surpasses the traditional SQUIDs by more than three orders of magnitude in sensitivity to magnetic field variations.The interferometer designed by V.L. Gurtovoi with colleagues consists of two superconducting plane aluminum loops several µm in sizes uperimposed on each other and weakly coupled at two points by Josephson junctions. The device was cooled to 0.6 K. With external magnetic flux variation, the current circulating in the loops is much higher than the bias current running through the two loops. The high sensitivity of the new device is due to the high degree of discreteness of its energy spectrum. Upon variations of the external magnetic field, the periodic oscillations of current were observed in line with the theoretical calculationsand the high sensitivity of the device was confirmed. For the Josephson effect underlying the SQUID functioning see the paper by G.F. Zharkov in Phys.-Usp. 9 198 (1966) and for supersensitive SQUID magnetometry see Phys.-Usp. 42 209 (1999) Phys.-Usp. 42 209 (1999). Source: Nano Lett. 17 6516 (2018)

Friction in superfluid 3He

The researchers from Aalto University (Finland) J.T. Makinen and V.B. Eltsov studied the effect of friction in superfluid helium at low temperatures T=(0.13-0.22)Tc, where Tc is the superfluid transition temperature. Earlier, friction was only measured at T≥0.3Tc. Helium 3He in the low-temperature phase B was in a rotating quartz glass vessel and a vortex-filament grating appeared in it. At a certain moment the vesselbroke sharply. The turbulence caused by braking was quickly suppressed, and then observed during several hours was a laminar motion with oscillations due to vortex cluster rotation and braking due to friction. The temperature- and pressure-dependences of the measured friction parameter agree with the theoretical predictions although its value is somewhat lower than the predicted one. The friction parameter proved to remain finite with extrapolation to zero temperature. According to the calculations of the authors of the experiment the decisive role in the low-temperature friction is played by Kelvin waves that penetrate from the surface of the container into the bulk and interact with the fermions trapped in the cores of superfluid filaments. This interaction had been predicted by M.A. Silaev (the Institute of Microstructure Physics of RAS, Nizhny Novgorod and the The Royal Institute of Technology, Sweden) in Phys. Rev. Lett. 108 045303 (2012). Source: Phys. Rev. B 97 014527 (2018)

Bethe strings

In 1931, H. Bethe predicted that one-dimensional magnets may have bound states of two quasi-particles – magnons (for the magnons and spintronics see Phys.-Usp. 58 1002 (2015)). This approach was later extended to the bound states of a large number of magnons, and such excitations in the form of chains were called Bethe strings. However, the string states in solids could not be identified till recently. Z. Wang (the University of Augsburg and the Helmholtz Center Dresden-Rossendorf, Germany) with colleagues were the first to discover Bethe strings in the compound SrCo2V2O2 in a magnetic field. In this substance, CoO6 octahedra form a chain of magnetic moments. High-resolution spectroscopy was used in the terahertz range of electromagnetic waves. The spectrum of SrCo2V2O2 showed singularities that correspond to Bethe strings of two and three magnons. The dependence of their properties on the magnetic field is well consistent with the theoretical calculations based on the ansatz proposed by H. Bethe. Source: Nature 554 219 (2018)

Dyakonov-Perel spin relaxation in platinum

Platina # belongs to metals with a high value of spin-orbital interaction of electrons that must lead to spin relaxation. A group of scientists from the Emory University and the University of Tennessee (USA) carried out new Pt examination through measuring giant magnetoresistance at different temperatures. Studied was a Pt sample several nm thick clamped between copper layers. Such a configuration allows separation of surface and bulk effects. Measurements showed that at room temperature Elliott – Yafet spin scattering dominates in the spin relaxation effect, while at cryogenic temperatures Dyakonov – Perel spin relaxation is predominant. This effect was predictedby M.I. Dyakonov and V.I. Perel in 1971, its mechanism is based on electron spin precession around the magnetic field direction in the absence of the center of symmetry. Source: Phys. Rev. Lett. 120 067204 (2018)

White dwarf with unusual variability

A. Scholtz (St Andrews University, United Kingdom) et al. discovered a magnetic white dwarf SDSS 160357.93+140929.97, a star kept in equilibrium by the pressure of a degenerate electron gas. The radiation of this dwarf varies with period P=110 ± 3 min. The color filters on the optical telescopes used for observation were changed every several minutes. This method allowed revealing the variable phase shift between luminosities in the red and blue spectral regions, which is indicative of the earlier unknown type of white dwarf variability. Researchers assume that the variability may be due to an invisible companion which reflects part of the emission from the white dwarf. Or, more probable, an unusual magnetic spot with a combination of hot and cold regions exists on the star surface. In the latter case, SDSS 160357.93+140929.97 must have the most rapid rotation among the known white dwarfs. Source: arXiv:1802.02636 [astro-ph.SR]

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The Extracts from the Internet is a section of Uspekhi Fizicheskih Nauk (Physics Uspekhi) — the monthly rewiew journal of the current state of the most topical problems in physics and in associated fields. The presented News is devoted to the fundamental discoveries of physics and astrophysics.

Permanent editor is Yu.N. Eroshenko.

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