Testing the black-hole area law
1 July 2021
S. Hawking showed in 1971 that in the course of any classical processes, the sum of black hole (BH) event horizon areas does not decrease with time. This statement is a consequence of the main principles of the General Theory of Relativity. The Hawking’s theorem implies that after merging of two black holes the horizon area of the new black hole will be no less that the sum of horizon areas of the original BHs. Using the characteristics of the first gravitational-wave signal of GW150914, registered by LIGO interferometer in 2015 , Isi (Massachusetts Institute of Technology, USA) and his co-authors obtained the first observational confirmation of the area law . The observed shape of the GW150914 signal was compared with the theoretically calculated curves of signal pulsations before and after merging of two BHs. This gave the masses, angular momenta and areas of BH horizons before and after merging. It was obtained that the sum of the initial horizon areas is less than the final area with 95-97-% probability. This conclusion is another successful testing of General Theory of Relativity. For BHs in binary systems, see .
 Reitze D H Phys. Usp. 60 823 (2017); UFN 187 884 (2017)
 Isi M. et al. Phys. Rev. Lett., in press
 Postnov K A, Kuranov A G, Mitichkin N A Phys. Usp. 62 1153 (2019); UFN 189 1230 (2019)
1 July 2021
The Stern-Gerlach experiment performed in 1922 showed that projections of atomic magnetic moments onto the magnetic field direction acquire discrete values . Since then, the idea of designing interferometer on the basis of the Stern-Gerlach effect has been repeatedly discussed, but it has been concluded that it is an exceedingly difficult task because the magnetic field needs an extremely exacting control. Nevertheless, the Stern-Gerlach interferometer was realized by Y. Margalit (Ben-Gurion University, Israel) and their co-authors first in the half-loop version and then with a full loop. The experiment  was carried out “on a chip” with atoms of Bose-Einstein condensate transferred into superposition of two spin states. The atoms were subjected to a pulsed action of the magnetic field gradient with the result that the wave packets of atoms in different states were first separated and then were coming closer to each other again to form a complete interferometer loop on the space-time diagram. At the output, the absorption method was used to examine the spin level populations that explicitly showed interference. The new interferometer may be used in fundamental studies. Its realization employing macroscopic objects, e.g., diamond nanoparticles, would allow testing even quantum gravity effects.
 Landsberg G S UFN 7 494 (1927), in russian
 Margalit Y et al. Science Advances 7 eabg2879 (2021)
Andreev reflection and fractional quantum Hall effect
1 July 2021
An electron incident on a normal-metal superconductor can be reflected as a hole (the Andreev reflection effect predicted by A.F. Andreev in 1964), while a Cooper pair of electrons appears in the superconductor . It was predicted theoretically that such an effect must also be observed at the interface of two substances in which a fractional quantum Hall effect occurs provided that the substances have different Landau level filling factors ν. M. Hashisaka (NTT Fundamental Research Laboratory, Japan) and their co-authors were the first to confirm this prediction in their experiment with a GaAs quantum wall in a magnetic field . Andreev reflection showed up in oscillations of conductivity of the narrow junction between regions ν=1/3 and ν=1 under variation of its width caused by the electrode potential variation. Thus, Andreev reflection was first revealed in a topological system without superconductivity.
 Andreev A F Sov. Phys. JETP. 19 1228 (1964)
 Hashisaka M et al. Nature Communications 12 2794 (2021)
Exciton-polariton room-temperature Bose-Einstein condensate
1 July 2021
Exciton-polaritons (EP) are strongly coupled systems of excitons and photons (see, e.g., ). Obtaining EP Bose-Einstein condensates is important for creation of lasers with unique properties and optical logical elements. EP room-temperature condensates have already been demonstrated earlier using organic systems located between two flat reflectors. J. Tang (the Institute of Chemistry and the University of the Chinese Academy of Sciences, China) and their co-authors obtained an EP room-temperature Bose-Einstein condensate by a new method – in extended microcavities inside an organic crystal . Microcavities are used as Fabry-Perot resonators, in which a strong coupling between Frenkel excitons and photons results in EP condensate generation. The low microresonator Q-factor is compensated by a high exciton density. A controlled flux of coherent light generated in the given device was demonstrated. The results of measurements agree well with the calculations based on the Gross-Pitaevskii equation. For exciton condensation, see .
 Gavrilov S S Phys. Usp. 63 123 (2020); UFN 190 137 (2020)
 Tang J et al. Nature Communications 12 3265 (2021)
 Glazov M M, Suris R A Phys. Usp. 63 (11) (2020); UFN 190 1121 (2020)
1 July 2021
The Lee-Huang-Yang correction to the ground-state energy of the Boson gas describes the of quantum fluctuation effect. The influence of this correction was observed in several experiments. T.G. Skov (Aarhus University, Denmark) and his co-authors performed a new experiment with a mixture of Bose-Einstein condensates where the effect of mean-field interactions was cancelled and the Lee-Huang-Yang correction became determinant . It created force generating monopole condensate oscillations. Investigated was a two-component condensate of 39K atoms in spin states of hyperfine splitting |F=1,mF=-1〉 and |F=1,mF=0〉. The condensate was trapped in a spherically symmetric harmonic potential created by laser beams. Condensate oscillations were absorbed by the absorption method at the stage of free expansion after the potential was off. Good agreement was obtained with numerical simulations of condensate dynamics with allowance for the dominant role of Lee-Huang-Yang correction.
 Skov T. G. et al. Phys. Rev. Lett. 126 230404 (2021)
1 July 2021
Topological insulator lasers
In topological insulator lasers, optical edge mode excitation is used. Owing to topological stability of these modes against defects and perturbations, such lasers are highly reliable and effective, but their operation typically requires cryogen temperatures or optical pumping. J.-H. Choi (University of South California, USA) and their co-authors demonstrated for the first time a room-temperature electrically pumped topological insulator laser . The device is a periodic array of micro-ring resonators on a semiconducting substrate, coupled by a nonperiodic set of auxiliary elements. Synthetic gauge fields imitated quantum spin Hall effect for photons. In the array, the electric field excited coherent optical edge modes generating telecommunication-range room-temperature lasing.
 Choi J.-H. et al. Nature Communications 12 3434 (2021)
1 July 2021
A.V. Akimov (P.N. Lebedev Physical Institute) and his colleagues designed a gyroscope based on a hyperpolarized ensemble of nuclear spins of # in NV (nitrogen vacancy) centers in diamond . A gyroscopic effect was due to a stable direction of spins not subjected to external action. The states of nuclear spins were polarized and readout in a standard method using electron spins of NV centers. The new gyroscope detected rotations with angular velocity of several ten degrees a second, and its work was verified using now existing microelectromechanical gyroscopes. The idea of nuclear gyroscopes was discussed as far back as the 1960s, and it was at that time that the first prototypes were created . The spin gyroscope can be much more compact than the circular laser gyroscopes based on the Sagnac effect since the precision of the latter depends on the ring area. Therefore, the gyroscope based on NV centers paves the way to practical applications in various navigation facilities.
 Soshenko V V et al. Phys. Rev. Lett. 126 197702 (2021)
 Kolpakov N M UFN 87 732 (1965), in russian
Coherence of scattered photons
1 July 2021
S. Sajeed and T. Jennewein (University of Waterloo, Canada) worked out a method of transferring quantum coherent photon pairs between the source and detector situated out of each other’s field of view . The state of photon polarization, which is normally used to create quantum entanglement, is frequently lost upon photon scattering. To overcome this difficulty, the authors used quantum coherence coded in time intervals, which is stable under scattering. A multimode interferometer and an array of single-photon detectors with time resolution of ≈ 120 ps were employed in the experiment. A phase converter based on Maikleson interferometer transferred laser pulses into pairs of successive coherent pulses, and their reflection from a sheet of white paper was examined. Scattered pulses were analyzed with a second interferometer. Although the scattering angle lay in the range of -45° to +45°, the photon visibility remained at the level of 95%, and the signal above the incoherent noise level was singled out owing to quantum coherence. Quantum coherence of photons is of importance for quantum communication devices and for quantum probingin different fields, including biomicroscopy .
 Sajeed S, Jennewein T Light: Science & Applications 10 121 (2021)
 Zheltikov A M, Scully M O Phys. Usp. 63 (7) (2020); UFN 190 749 (2020)
A recurrent fast radio burst source in a globular star cluster
1 July 2021
From observations on radio telescopes-interferometers with a very long base, F. Kirsten (Chalmers University of Technology, Sweden) and his co-authors have found that the source of recurrent fast radio bursts (FRB) FRB 20200120E is situated in the globular cluster located in the tidal bridge between galaxies M81 and NGC 3077 . The probability of occasional association with a globular cluster is <1,7 × 10−4. This observation is unusual because according to the most popular model, FRB are generated in magnetars (young magnetized neutron stars), which is confirmed by the recent observation of bright bursts from a galactic magnetar. And these objects typically find themselves among young stars in galactic discs. They have not been expected in old globular clusters. FRB 20200120E lies much closer than the other known sources of extragalactic FRBs. This provided constraints on the flow of a constant radio, X-ray, and gamma-ray emission from this object, which downranges the possible classes of FRB generation models. The authors assume that the magnetar FRB 20200120E resulted either from a collapse of an accreting white dwarf or from white-dwarf and/or neutron-star merging in a binary system, whereas its birth via explosion of a core-collapse supernova in a globular cluster is hardly probable. For FRB, see .
 Kirsten F et al. arXiv:2105.11445 [astro-ph.HE]
 Popov S B, Postnov K A, Pshirkov M S Phys. Usp. 61 965 (2018); UFN 188 1063 (2018)
FAST pulsar survey
1 July 2021
A study of pulsars is of importance for an analysis of the state of matter under extreme conditions, for the study of star evolution and verification of gravitation theories. The new survey obtained from FAST radio telescope presents data on pulsars at an angular distance of ±10° from the galactic disc plane . With an aperture of 300 m, FAST is the most sensitive radio telescope aimed at pulsar research. A binary millisecond pulsar was revealed in the globular cluster M13, an eclipse millisecond pulsar was found in M92, and several pulsars with a high dispersion measure, including pulsar PSR J1901 + 0435 with an inverted spectrum were investigated. Several pulsars were observed coincident with supernova remnants, 40 millisecond pulsars, 16 binary pulsers and rotating radio transients. The released survey of pulsars will allow a more thorough study of newly found interesting objects using alternative facilities. For neutron stars, see [20-22].
 Han J L et al. Research in Astronomy and Astrophysics 21 107 (2021)
 Beskin V S Phys. Usp. 61 353 (2018); UFN 188 377 (2018)
 Shakura N I et al. Phys. Usp. 62 1126 (2019); UFN 189 1202 (2019)
 Tutukov A V, Cherepashchuk A M Phys. Usp. 63 209 (2020); UFN 190 225 (2020)
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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