
Magnetoresistance of a superconductorcuprate in the normal phase
1 September 2018
The properties of hightemperature superconductors  cuprates in their normal nonsuperconducting phase near the superconducting transition point may cast light to the mechanism of hightemperature superconductivity and are therefore of great interest. To investigate this state, cuprate superconductivity is artificially suppressed by a strong magnetic field, thus giving rise to the socalled “strange metal phase” with a linear temperature dependence of magnetoresistance. The effect of the employed magnetic field on this phase is however not yet enough studied. P. GiraldoGallo (the University of Florida, USA) et al. examined thin La_{2x}Sr_{x}CuO_{4} films in magnetic fields up to 80 T to find that their specific resistance increases linearly with field as distinct from the quadratic dependence observed in usual metals. The described behavior takes place for the doping less than its critical value p ≈ 0.19. The observed double linear dependence on the temperature and magnetic field has not yet been completely explained theoretically but can testify to the fact that electric current in the “strange metal phase” is transported not by free quasiparticles, but in some other way.
Source: Science 361 479 (2018)
Qubits based on geometric phase
1 September 2018
The geometric phase, also referred to as Berry phase (see
Phys.Usp. 33 1 (1990)
and Phys.Usp. 36 1 (1993)), is a promising effect which can be used to create an elementary base for quantum calculations and quantum communication. Qubits based on the geometric phase have already been demonstrated, but they had however a low quantum fidelity. K. Nagata (Yokohama National University, Japan) with colleagues managed to heighten the quantum fidelity by realizing qubit on a degenerate subspace in the Gilbert space of the triplet state. Their system is an NV center in a diamond in which the spins of the nucleus and electron interact with the electromagnetic field generated by two crossed wires. The device operates at room temperature under a zero external magnetic field. Polarized radio emission generated by the wires interacts in a certain way with the spin states of the NV center, which are then read out by registering fluorescence radiation photons. The experiment demonstrated both a single qubit and a twoqubit system produced by quantumentangled spins of the electron and nucleus in the NV center, the quantum fidelity reaching 90%.
Source: Nature Communications 9 3227 (2018)
Statistics of quasiparticles in an excitonpolariton condensate
1 September 2018
The statistics of particles in quantum condensates was earlier studied experimentally for photons in laser light and in superconducting qubits and also for massive particles (atoms) in a BoseEinstein condensate. In the latter case, the form of distribution is considerably influenced by atomic interactions. However, such studies have not been performed for quasiparticles that are a combination of photons and massive particles. M. Klass (the University of Wurzburg, Germany) with colleagues measured for the first time the distribution of the number of photons spontaneously emitted by the BoseEinstein condensate of exciton polaritons. These quasiparticles consisting of photons and electronhole pairs were generated in a micronsize cavity in a semiconductor under the action of pump light. Transition edge sensors were used that allowed detecting unit photons, and the photon distribution suggested the conclusion concerning the statistics of condensate quasiparticles. After the condensate was formed, with a further increase of the pump power the condensate changed from the thermal state with an exponential distribution of the particle number to the coherent state with Poisson distribution. In their statistical properties, the exciton polariton condensate turned out to be closer to laser light than atomic condensate.
Source: Phys. Rev. Lett. 121 047401 (2018)
Quantum synchronization
1 September 2018
Synchronization of periodic processes is most often realized through an oscillation phase lock using an external signal. In recent years, this conception of synchronization has been reformulated also for a quantum region. In their theoretical work, the researchers from the University of Basel (Switzerland) A. Roulet and C. Bruder investigated conditions necessary for quantum system synchronization. On the symmetry grounds it was shown that the minimal systems  qubits that have two energy levels cannot be synchronized with an external signal for lack of the limit cycle on the Bloch sphere on which the Hilbert space of a qubit is plotted. The next in complexity is a threelevel system which can be represented by a particle with spin S=1. To examine it, a set of coherent spin states providing an extension of the Bloch sphere to the case S>1/2 was introduced. A. Roulet and C. Bruder considered the case where there is energy dissipation from states with projections of spin S_{z}=±1 to the state S_{z}=0. The study of the phase portrait of such a system showed that the phase capture and synchronization are possible if the dissipation coefficients in states S_{z}=±1 differ in values. Thus the quantum oscillator requires at least three energy levels for synchronization with a periodic signal.
Source: Phys. Rev. Lett. 121 053601 (2018)
Relativistic effects in the motion of star S2
1 September 2018
In the center of Galaxy, several stars are observed that speeding up to high velocities in the pericenter come closely to a supermassive black hole Sgr A^{*}. Since the 1990s, star S2 has been monitored which revolved more than once around Sgr A^{*} during this time. Using the observational VLT data obtained in Chili, in particular, during the last passage through the pericenter, GRAVITY collaboration revealed for the first time the effects of the Relativity Theory in the motion of star S2, namely, the gravitational red shift and the relativistic transverse Doppler effect. These observational data cannot be explained by the Newtonian theory alone. The relativistic precession of the orbit of star S2 predicted by General Relativity is expected to be detected in the nearest future.
Source: Astron. & Astrophys. 615 L15 (2018)

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.
