Extracts from the Internet

Quantum entanglement of the order of events in a gravitational field

In the classical General Relativity Theory, the sequence of possible causes and effects depends on the mass distribution on the initial spacelike hypersurface because of the effect of gravitational time dilation. The question of accordance of quantum superposition of states and the effect of massive-body gravitation on time flow has not yet been completely answered. M. Zych (the University of Queensland, Australia) with colleagues showed a possible approach to this problem. They considered a thought experiment with two events and a massive body and constructed the probability amplitude describing a quantum superposition of states with different metrics (different positions of the massive body) and simultaneously a quantum superposition of the temporal order of events separated by a timelike interval. It was shown that such systems can be quantum entangled in the sequence of events. The authors also formulated analogues of Bell inequalities that might be used to check the quantum character of the indicated entanglement. Source: Nature communications 10 3772 (2019)

Quantum mechanics in a noninertial reference frame

An experimental study of quantum phenomena in noninertial reference frames is of great interest, as it cannot be excluded that new fundamental effects will be revealed. M.J. Padgett (the University of Glasgow, Great Britain) and his colleagues performed an experiment examining optical quantum phenomena on a rotating laboratory table. Investigated was a combination of the Sagnac effect and quantum mechanical interference in the Hong – Ou – -Mandel interferometer. As distinct from experiments with the pure Sagnac effect, at the input it is not a splitter but a nonlinear crystal with down-conversion of laser radiation photons and photon pair production in a quantum-entangled state. One of the two photons of the pair described a circle clockwise along the optical fiber and the second photon moved in the backward direction. At the output, a splitter was located and two single-photon detectors operating in the coincidence circuit. As was expected, a uniform rotation leads only to a path-length difference in the two directions. This diminishes the degree of indistinguishability of the photons from the pair and modifies the picture of quantum interference in accord with the predictions of quantum mechanics. At the current level of precision no new effects were found. The authors suggest performance of such an experiment using satellites at Earth’s orbit. Source: Phys. Rev. Lett. 123 110401 (2019)

Demonstration of quantum Darwinism in an experiment

It is not yet known why the quantum state of a system passes over in the classical in the course of measurement or decoherence. One of the proposed versions is “quantum Darwinism”. According to this conception, classical are the states that could proliferate themselves most widely in the environment, which resembles biological evolution. T. Unden (the University of Ulm, Germany) and co-authors observed experimentally such a change-over to the classical description with proliferation of quantum information. An NV-center (a nitrogen-substituted vacancy) in a diamond surrounded by carbon 13C nuclei that are present as an impurity among 12Cnuclei was under study. A set of nonzero-spin 13C nuclei modeled the environment. After the action of a laser pulse on the NV center, the state of the electron changed, and its magnetic interaction with the surrounding 13C nuclei, registered by detectors, generated their emission in the microwave range. Thus, observing the set of 13C nuclei one could investigate proliferation of quantum information from the NV center to the environment without perturbing the quantum system itself. The results of the experiment correspond to the quantum Darwinism conception. Indeed, the process was observed in which beginning with some moment a set of measurements gave one and the same answer concerning the state of the quantum system, which corresponded to the change-over to the classical picture as information proliferated in the medium. Source: arXiv:1809.10456 [quant-ph]

Fermion excitations in YbB12

The topological insulators SmB6 and YbB12 have recently attracted increased attention, as the first of them showed quantum oscillations of magnetization (de Haas – van Alphen effect) and the second exhibited quantum oscillations of conductivity (Shubnikov – de Haas effect). These nontrivial properties testifying to the presence of Fermi surface in strong magnetic fields caused discussion and the appearance of a number of theoretical models based on different effects. In their new experiment, Y. Sato (Kyoto University, Japan) and co-authors measured electric conductivity, heat capacity and thermal conductivity of low-temperature (up to ≈ 0.6 K) YbB12 crystals without a magnetic field and in magnetic fields. The analysis of the obtained data is indicative of a probable presence in YbB12 of new quasi-particles – moving gapless fermionic excitations interacting with the magnetic field in spite of the absence in them of an electric charge. Possibly, it is these neutral fermions that are responsible for the unconventional properties of SmB6 and YbB12. Source: Nature Physics 15 954 (2019)

Investigation of microprocesses in biophysics

Y. Wang (Nanjing University, China) with colleagues developed a new method of biophysical studies of microprocesses allowing, among other things, identification of single molecules. In the method called DiffusiOptoPhysiology, processes are investigated by optical technique in nanometer-scale pores without the use of measuring electrodes. Fluorescent radiation of Fluo-8 dye molecules was observed. It was bonded with Ca2+ ions, which made it possible to observe an ion flux moving through nanopores. In this method, electrodes are only applied to produce an effect of electrophoresis - the total flux of matter. Registration of single molecules of cyclodextrin, PEG1500 and dsDNA in an ion flux was demonstrated. Nanopores are widespread, they are present in biological membranes for substance transport. The new method allows a simultaneous observation of thousands of nanopores, which makes it highly efficient. The device for DiffusiOptoPhysiology can be assembled on a small chip and can find wide application in different fields of clinical diagnostics and in scientific research. Source: Science Advances 5 eaar3309 (2019)

Quantum dot energy generator

G. Jaliel (University of Cambridge, United Kingdom) with co-authors constructed a microscopic thermo-electric generator on two quantum dots using the effect of resonant electron tunneling. For the device operation, the presence of temperature gradient will suffice. Thermo-electric generators on quantum dots have already been designed earlier, but they had low efficiency. In the new device constructed using the scheme proposed by A.N. Jordan (the University of Rochester, USA) with co-authors, two quantum dots, 310 nm in diameter each, were realized on the heterostructure GaAs/AlGaAs on either side of the hot-electron reservoir. The energy levels of the quantum dots were different – they corresponded to low-energy electrons on one side and to higher-energy electrons on the other side. Thus, selective electron transport between two dotstook place, and the temperature difference was converted to electric energy. The new device can generate a thermal power of 0.13 fW for a temperature difference across each dot of about 67 mK. Its efficiency amounts at least to 10 % of the ideal Carnot heat engine. Source: Phys. Rev. Lett. 123 117701 (2019)

Unidentified gamma-ray sources

The gamma-ray survey of the sky performed by the space telescope Fermi-LAT revealed a lot of unidentified sources of unknown origin. In other ranges, including optical ones, these objects are invisible. The H.E.S.S. complex of Cherenkov gamma-ray telescopes located in Namibia was used to carry out new observations of four of the unidentified sources at energies ≥ 100 GeV. The observations of ground-based Cherenkov detectors supplement the Fermi-LAT observations because they allow a long-exposition survey of the high-energy part of the spectrum. These sources turned out to be invariable in gamma-rays, and their hard spectrum is close to the one to be generated in annihilation of dark-matter particles with masses less than or equal to ≤ 0.4 TeV/c2. In these features the sources are very similar to dark matter clumps in which annihilation takes place. The existence of such clumps (subhalo) is predicted in the hierarchical picture of galaxy formation where small objects merge to form larger ones up to the formation of galactic halos and galactic clusters. Nevertheless, it cannot be stated confidently that unidentified sources are dark matter clumps, and therefore further studies are needed. Source: arXiv:1909.01072 [astro-ph.HE]

New constraints on primordial black holes

The possibility of primordial black hole (PBH) formation in the early Universe was predicted by Ya.B. Zeldovich and I.D. Novikov in 1967. The interest in PBH has recently been considerably renewed because the merging of PBH pairs might explain some of the gravitational wave bursts observed by LIGO/Virgo detectors. A lot of constraints were obtained on PBH distribution in the Universe, which almost excluded the possibility for PBH to constitute the whole dark matter. However, part of these constraints are model-dependent, and the limits become weaker when PBHshow a wide mass distribution. In particular, not finally excluded is now the PBH mass interval MPBH ≈ 20-80M and the region near ≈ 10-10M. A group of astrophysicists from Italy and Switzerland obtained new constraints on PBH. If PBH exist, their gravitation would create additional inhomogeneities in the intergalactic gas, and these inhomogeneities must affect the Lyman-α forest – the set of absorption lines observed in the quasar spectra. R. Murgia with colleagues used MIKE and HIRES spectrometers data, and gas hydrodynamics was simulated numerically with allowance for PBH. The absence of a considerable influence on the Lyman-α forest gives a constraint excluding a notable part of the interval ≈ 20-80M on the side of large masses. Source: Phys. Rev. Lett. 123 071102 (2019)

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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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