Nuclear interaction of nucleons
1 April 2020
In the Jefferson Lab experiment , pair interactions of nucleons (neutrons and protons) in 12C, 27Al, 56Fe and 208Pb atomic nuclei were investigated in the region of short inter-nucleon distances not studied earlier. The protons knocked out of nuclei in quasi-elastic scatterings of electrons by nuclei were registered. The ratio of the number of events with two emitted protons to the number of events with one emitted proton first increases linearly in the interval of 400 to 650 MeV s-1 with increasing relative momentum of two interacting nucleons (that is, with decreasing distance between them) and then remains unchanged up to 1000 MeV s-1. The inter-nucleon interaction is due to quark-gluon interactions within quantum chromodynamics. But because of complicacy of its equations, simplified models are often used. The present experiment confirmed that a good approximation for the electron scattering by nuclei under a large momentum transfer is the model where the nucleons are consideredto be point-like particles with a certain effective interaction force between them, and the momentum transfer from the electron is due to one virtual photon. The above-mentioned attainment of plateau corresponds to the transition from attracting spin-dependent (tensor) pair interaction of nucleons to spin-independent (scalar) interaction with repulsion. This theoretical approach may appear to be useful, in particular, for calculating the neutron star structure.
 Schmidt A et al. & The CLAS Collaboration, PNature 578 540 (2020)
Cooling of levitated nanoparticles
1 April 2020
U. Delic(the University of Vienna, Austria) with co-authors managed to cool a solid-state nanoparticle containing 108 atoms to the lowest quantum state, when on the average less than one phonon (quantum of thermal motion) was excited in a nanoparticle . A feedback cooling was applied when the influence of cooling laser pulses depended on the particle motion. This method was used earlier to cool macroscopic particles only to a state with four phonons. In the present experiment, spherical quantum nanoparticles 143 ± 4 nm in diameter were trapped by “optical pincers” (levitated) in an optical field between two concave mirrors in a vacuum chamber linked with a cavity. The photons scattered by a particle carried away the thermal energy thus resulting in cooling. The average number of phonons was 0.43 ± 0.03, which corresponds to a temperature of 12.2 ± 0.5 µK and to the probability of 70 ± 2 % for a particle to be in the ground quantum state. Cooling of nanoparticles is important for verification of the basic elements of quantum mechanics in the “macro-quantum” region and can also appear to be usefulin the design of new ultrasensitive sensors.
 Delic U et al., Science 367 892 (2020)
Non-demolition readout of the spin state
1 April 2020
In quantum calculations, it is sometimes necessary to apply the so-called non-demolition readout of qubit (quantum bit) state when after readout the qubit remains in the initial state. J. Yoneda (the Institute of Physical and Chemical research RIKEN, Japan) with co-authors experimentally demonstrated  the method of non-demolition single-shot readout of the electron spin state at a quantum dot in the magnetic field in silicon. Near such a dot, a second auxiliary quantum dot was placed. The electron at this dot was coupled by an exchange interaction with the electron at the first dot, which allowed the state of the first electron to be determined from the state of the second electron. The states of qubits were prepared using microwave radiation pulses, and the state of the second qubit was measured using the charge sensor, i.e., a one-electron transistor. Over 30 successive non-demolition readouts of the spin state were implemented that yielded similar results with quantum fidelity of nearly 99 %, and the overallmeasurement fidelity was 95 % within 1.2 ms.
 Yoneda J et al., Nature Communications 11 1144 (2020)
The Kondo screening cloud
1 April 2020
The Kondo effect consists in screening of the spin located in the metal of the magnetic impurity atom by a coherent spin cloud formed by conduction electrons, which causes the increase of electric resistance with lowering temperature. Although the Kondo effect has been experimentally detected long ago, the screening cloud itself has not been observed. I.V. Borzenets (the City University of Hong Cong) and co-authors were the first to register the Kondo cloud and to measure its length . To this end, a quasi-one-dimensional channel was created with three electric gates at different distances from impurity atoms at aquantum dot. By heightening the voltage at a certain gate, one could form barriers and observe the conductivity changes in the channel. The characteristic scale of the Kondo cloud measured by this method was several µm. The measurement results are well consistent with the theoretical calculations of the Kondo effect, thus confirming the conclusion concerning the existence of the screening spin cloud. The Kondo effect is supposed to play an important role in spin glasses, in high-temperature superconductivity and in other processes, and therefore a direct recording of a Kondo cloud can help in the corresponding investigation. For the Kondo problem see .
 Borzenets I V et al., Nature 579 210 (2020)
 Ovchinnikov Yu N and Dyugaev A M, UFN 171 565 (2001) [Phys. Usp. 44 541 (2001)]
A record far blazar
1 April 2020
Blazars are radio bright active galactic nuclei with relativistic jets directed towards the Earth. The jets are formed along the rotation axis of accretion discs around supermassive black holes in galactic centers. Only seven blazars at red shifts z=5.0-5.5 (with the maximum z=5.47) have been known till recently. Radio bright galaxies and quasars have also been observed at still larger z, but not blazars. When investigating several radio surveys of galaxies, S. Belladitta (Brera Astronomical Observatory and the University of Insubria, Italy) with co-authors discovered  a new blazar PSOJ030947.49+271757.31 at the red shift z ≈ 6. This blazar shows a record brightnessin the radio frequency band. The Swift (X-ray Observatory) observation of PSOJ030947.49+271757.31 also showed that it possesses the highest X-ray luminosity among all known active galaxies at z ≥ 6. The observation statistics implied the presence of ≈ 10-2 such blazars at z ≈ 6 in a cubic Mpc. The red shift z ≈ 6 corresponds to the Universe age of 900 mln years. The study of active objects in this epoch is important for understanding of the formation and growth of supermassive black holes. Blazars can be sources of neutrinos and high-energy cosmic rays (see ).
 Belladitta S et al., Astronomy & Astrophysics 635 L7 (2020)
 Ptitsyna K V, Troitskii S V, UFN 180 723 (2010) [Phys. Usp. 53 691 (2010)]
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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