Extracts from the Internet


Quantum dot qubits in germanium

N.W. Hendrickx (Delft University of Technology, Delft, the Netherlands) with co-authors created quantum bits – qubits in germanium that outbid qubits in silicon in a number of characteristics. Instead of electronic states in quantum dots in silicon, hole states were used in germanium. Quantum levels of holes were considerably different in energy, which improved the qubit operation accuracy. Two quantum dots in a flat germanium sample were between the control electrodes. A strong spin-orbit coupling and the Coulomb blockade effect allowed the electric field to control qubits without additional devices. Both a single qubit and a two-qubit logic gate were demonstrated. In the former case, the quantum fidelity was 99.3 %, and operations could be performed with the gate time of 20 ns. In the two-qubit case, the tunnel effect was used to couple qubits, and a logic gate “control negotiation” (CNOT) executed within 75 ns. Thus, quantum dots in germanium proved to be one of the most promising platforms for quantum calculations. Source: Nature, online publication of January 13, 2020

Antiferromagnetic topological insulator

An international group of scientists including Russian researchers from St. Petersburg and Tomsk State Universities, the Institute of Strength Physics and Materials Science SB RAS (Tomsk) and the Institute of Solid State Physics RAS (Chernogolovka) showed that the compound MnBi2Te4 is a combination of topological insulator and antiferromagnet. These properties of MnBi2Te4 had been predicted by theoretical ab initio calculaitons and then confirmed experimentally by photoemission spectroscopy with angular resolution. A topological insulator is material with an electrically conducting surface and a nonconducting bulk. Magnetization measurements at different temperatures and magnetic field strengths showed the presence in MnBi2Te4 of three-dimensional antiferromagnetic ordering at a temperature of 24.2(5) K at the Neel point. Earlier, magnetic topological insulators were only obtained through doping of nonmagnetic topological insulators with metals, which resulted in strong inhomogeneity. On the contrary, MnBi2Te4 has its own magnetic properties even without doping, and so its characteristics are homogeneous. Antiferromagnetic topological insulators can find important applications in both fundamental research and spintronics. For spintronics see UFN 189 849 (2019) [Phys. Usp. 62 795 (2019)] and for topological insulators see UFN 187 411 (2017) [Phys. Usp. 60 385 (2017)], UFN 188 1226 (2018) [Phys. Usp. 61 1116 (2018)], and UFN 188 1129 (2018) [Phys. Usp. 61 1026 (2018)]. Source: Nature 576 416 (2019)

Dissipation-induced instability in a quantum gas

It is a known fact that a mutual influence of the process of coherent evolution and dissipation can be the cause of nontrivial effects such as dissipative phase transitions and others. T. Esslinger (Swiss Federal Institute of Technology in Zurich) with colleagues investigated the interaction of the coherent quantum evolution and dissipation in a Bose – Einstein condensate of 87Rb atoms. The condensate atoms could be in two Zeeman states, and two perturbation modes - the atomic number density modulation mode and the spin density modulation mode — could be excited in the condensate. The modes were coupled by scattering of standing wave photons of laser radiation in the cavity. The system turned out to evolve following a circular path in the phase space of the two modes. This behavior was understood with the help of mechanical analogy with a position-depending nonconservative force. A phase transition was observed between the regime in which only one mode was excited and the regime with two excited modes. An instability region was revealed in which both modes were synchronized owing to strong dissipative coupling between them. For nonequilibrium dynamics see UFN 187 817 (2017) [Phys. Usp. 60 762 (2017)]. Source: Science 366 1496 (2019)

Rotation of a superfluid liquid

Rotation of quantum superfluid liquids was investigated in many works both theoretical and experimental (see, e.g., UFN 143 73 (1984) [Phys. Usp. 27 363 (1984)], UFN 165 829 (1995) [Phys. Usp. 38 791 (1995)], UFN 185 970 (2015) [Phys. Usp. 58 897 (2015)], and UFN 189 1104 (2019) [Phys. Usp. 62 1031 (2019)]). Of interest is the case where the rotation frequency approaches or exceeds the retentive frequency of the atomic trap potential. In this case, according to calculations circular structures must appear that can be represented by a combination of many quantum vortices in one giant vortex. Such structures were actually observed, but they swiftly decayed or the liquid density in the center was not low. Researchers from University Paris 13 and French National Center for Scientific Research experimentally obtained for the first time a circular structure that was stable for over one minute. Through rotation of the nonspherical trap potential the angular momentum was imparted to the Bose – Einstein condensate of 87Rb atoms. In the course of selective evaporation, it increased to 350×h/2π per atom. In the condensate structure, a ring ≈ 30 µm in radius with a hole in the center appeared which rotated at the supersonic linear velocity reaching Mach 18. In the ring, a quadrupole deformation mode was excited, for the description of which the existing hydrodynamic models appeared to be insufficient requiring a more detailed theory. Source: Phys. Rev. Lett. 124 025301 (2020)

A group of distant galaxies and Universe reionization

Hydrogen reionization in the Universe was presumably due to radiation of quasars and first stars in young galaxies. V. Tilvi (University of Arizona, USA) with colleagues discovered a group of galaxies at the red shift z ≈ 7.7 surrounded by mutually overlapping ionized gas bubbles. Three galaxies radiating strongly in Lyα line were discovered in photometric observations on the 4-meter telescope of the Kitt Peak Observatory and then a spectroscopic Keck I confirmation was obtained. One of the three galaxies was known from earlier observations. The intergalactic spacing along the line of sight is 0.7 Mpc and in the transverse direction 0.05-0.18 Mpc. The possibility of observing Lyα radiation lines implies that the galaxies are surrounded by rather lengthy ionized hydrogen bubbles, for in neutral hydrogen, the radiation would have been absorbed. During the time of their motion, photons inside the bubbles go beyond the absorption line owing to the cosmological red shift. According to the estimates, the bubble size is 0.55-1.02 Mpc, and therefore they must overlap with each other partly. Thus, it was proved for the first time that the group of galaxies might be responsible for the inhomogeneous Universe reionization. It is expected that a substantial advance in the study of Universe reionization will be possible with the now planned IR JWST (James Webb Space Telescope) and SKA-2 radio interferometer. Source: arXiv:2001.00873 [astro-ph.GA]

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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.

It is compiled from a multitude of Internet sources.

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