Entropy production upon continuous measurement
1 October 2020
Investigation of the relation between entropy and information is of importance both in fundamental physics and for applications in nanotechnology. On small scales, the role of fluctuations and quantum effects is important, which brings about the appearance of new features in the system’s behavior. As distinct from single measurements, the effect of continuous quantum measurements (responsible for a further system’s evolution at every instant of time) on the general entropy balance has not yet been examined. M. Rossi (University of Copenhagen, Denmark) with co-authors carries out such a study  for the first time. Using a test laser beam in the homodyne method, the position of an oscillating membrane in an optical resonator was observed, which allowed recording individual quantum trajectories of a given optomechanical system. The entropy production due to a continuous measurement both in the stationary case and upon relaxation after a short impact on the system was found by the shape of quantum trajectories, and a great influence of fluctuations on this process was noticed. For quantum measurements see [2, 3].
 Rossi M et al. Phys. Rev. Lett. 125 080601 (2020)
 Kadomtsev B B Phys. Usp. 38 923 (1995); UFN 165 967 (1995)
 Zheltikov A M Phys. Usp. 61 1016 (2018); UFN 188 1119 (2018)
Interaction of two time crystals
1 October 2020
The time crystals predicted by F. Wilczek  have already been demonstrated experimentally. Their properties are repeated in time similarly to the periodic arrangement of atoms in an ordinary crystal. S. Autti (Aalto University, Finland and Lancaster University, Great Britain) and co-authors were the first to investigate the interaction between two time crystals . Time crystals were created in Bose-Einstein condensate of magnons in the superfluid phase “B” of liquid helium-3 at a temperature of 130 µK in a magnetic field. One of the time crystals occurred in the bulk condensate and the second on the surface. Nuclear magnetic resonance was used to observe characteristic periodic time crystal motions associated with coherent spin precession. Exchange of magnons proceeded along the isthmus between the crystals. This induced Josephson oscillations with frequency equal to the difference of frequencies of two time crystals. An additional interaction of crystals was due to the fact that the bulk crystal introduced perturbations to the potential retaining the surface crystal. The observed behavior of interacting time crystals was successfully reproduced in numerical simulations. In future it will be possible to examine also more complicated interactions of time crystals, including their collisions. For Bose-Einstein condensation of magnons see .
 Wilczek F Phys. Rev. Lett. 109 160401 (2012)
 Autti S, Heikkinen P J, Mäkinen J T, Volovik G E, Zavjalov V V, Eltsov V B
Nature Materialsonline publication on August 17, 2020
 Kaganov M I, Pustyl’nik N B, Shalaeva T I Phys. Usp. 40 181 (1997); UFN 167 191 (1997)
Investigation of molecular quantum states
1 October 2020
K. Najafian (the University of Basel, Switzerland) with co-authors developed a new method of phase-sensitive measurements of the structure of quantum levels in molecules under their interaction with other molecules in an optical field . The investigated molecular ions 14N2+ and the auxiliary ions 40Ca+ were placed in an optical lattice formed by laser beams. 14N2+ were affected by two variable forces whose phase difference was changed in the experiment, and at the same time the Stark shift of the levels was registered. The investigations carried out for different rotational states of 14N2+ molecules allowed classification of their electronic and vibrational levels. The new method may prove beneficial for the study of molecules with a dense quantum-level structure when the use of ordinary spectroscopic methods is difficult. For the interaction of molecules with lasing see .
 Najafian K et al. Nature Communications 11 4470 (2020)
 Isaev T A Phys. Usp. 63 289 (2020); UFN 190 313 (2020)
1 October 2020
Experiments on quantum teleportation of particle states over various distances, including thousands of km from a satellite to the Earth, have been performed in recent years. A. Berera (the University of Edinburgh, Great Britain) considered theoretically the question of what maximum distance photons can cover in the interstellar space sustaining quantum coherence , e.g., as photons of a quantum-entangled pair. On their way in the Galaxy, photons interact with free electrons, with atoms and molecules, and with photons of the interstellar background. The consideration of these elementary processes suggests a conclusion that radio-frequency range photons can cover without scattering the distances from ≈ 100 to ≈ 106 pc exceeding in the latter case the size of the Galaxy. An additional obstacle leading to a polarization plane rotation (destruction of coherence in spin states) may be interstellar magnetic fields. Still longer cosmological distances can be covered by X-ray photons. Not excluded is the existence of cosmological objects whose radiation will show quantum coherence, for instance, hypothetical cosmic strings or evaporating primordial black holes.
 Berera A arXiv:2009.00356 [hep-ph]
Gravitational waves from merger of record massive black holes
1 October 2020
Two gravitational wave detectors LIGO and Virgo registered  a gravitational burst GW190521 from the merger of black holes (BH) with masses 85+21−14M☉ and 66+17−18M☉ at the red shift z≈ 0.8. The signal-to-noise ratio in this observation is equal to 14.7. The mass of the larger BH and the total mass of two BHs ≈ 150M☉ are record large among the LIGO/Virgo events registered hitherto. According to the current classification, the ultimate BH of mass ≈ 142M☉ that was due to the merger and radiation of gravitational waves is a BH of intermediate mass – between the masses of BH of stellar origin and of supermassive BH in galactic nuclei. The origin of the more massive of the two merged BHs is not yet clear since the production of such a massive BH during the star evolution is hardly probable because of the pair instability effect. This BH itself may have resulted from the merge of two BHs with smaller masses. For the properties of binary BHs as given by LIGO/Virgo observations see .
 Abbott R et al. Phys. Rev. Lett. 125 101102 (2020)
 Postnov K A, Kuranov A G, Mitichkin N A Phys. Usp. 62 1153 (2019); UFN 189 1230 (2019)
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.
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