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Issue 12, 2024
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Extracts from the Internet


Quantum H theorem

Entropy non-diminishing theorems resembling Boltzmann’s H theorem in classical statistics were formulated in the quantum information theory. In particular, it was shown that the entropy of a quantum system does not diminish if its evolution proceeds in the so-called unital quantum channel. On the basis of this result the team of researchers from L.D. Landau Institute of Theoretical Physics (1), the Swiss Federal Institute of Technology, Zurich (2), the Argon National Laboratory (3), and Moscow Institute for Physics and Technology (3) G.B. Lesovik(1,2), A.V. Lebedev(2), I.A. Sadovskii(3), M.V. Suslov(4), and V.M. Vinokur(3) formulated the quantum H theorem in terms of physical observables referring to the system interactions with its surrounding (reservoir). It was assumed that the considered system is quasi-isolated in energy, but its interaction with the reservoir changes the quantum state phases. Under the conditions formulated in the theorem that restrict the character of admissible interactions with the reservoir the entropy of the system must not diminish, which was demonstrated on a number of examples in the paper of G.B. Lesovik et al. The models of electron scattering by potential barriers and spins and by a two-level system as well as the electron-phonon interaction at energies exceeding the Debye energy were considered. It was shown that in these cases the conditions of the theorem are met and the entropy does not diminish. It was also claimed that if the conditions of the theorem are not fulfilled, the entropy of an energy-isolated quantum system diminishes under certain conditions, i.e., the second law of thermodynamics may be locally violated. (see also G.B. Lesovik, JETP Letters 98 184 (2013)) Source: Scientific Reports 6 32815 (2016)

Long-lived quantum coherence

Retention of quantum coherence during a long time is essentially important in fabrication of setups processing and transferring quantum information, but the external action typically provokes fast decohering. In 2015 T.R. Bromley, M. Cianciaruso and G. Adesso showed in their theoretical work that in a compound quantum system coherence can be stored unlimitedly long if the initial state of the system was specially prepared so that the decoherence act on the space of states in the orthogonal direction. No algorithms for correction of mistakes or another external system control are needed. This effect was demonstrated experimentally for the first time in the new work by G. Adesso (the University of Nottingham, Great Britain) and his colleagues. A ”frozen” quantum coherence occurred for two- and four-qubit systems realized on spins of atomic nuclei in room-temperature molecules. Radio frequency pulses were used to transfer the systems to the diagonal Bell state, and after a certain evolution their state was measured. The experiment showed that the coherence is retained on the time scale of the order of a second. Source: Phys. Rev. Lett. 117 160402 (2016)

Atomic gravimeter combined with magnetometer

K.S. Hardman (Australian National University) with colleagues designed a high-precision atomic interferometer capable of measuring simultaneously the free fall acceleration g and the magnetic field gradient. In the interferometer, Bose-Einstein condensate of 87Rb atoms is used. The atoms fell from the height of several meters, and g was measured using the method of atomic interferometer. Furthermore, the atoms were in a superposition of three spin states mf=1,0,-1 interacting differently with the magnetic field thus inducing an additional phase difference depending on the magnetic field gradient. This device was tested by measuring the gravitational field variations due to tide-depending rises and falls of the Earth crust. The relative measurement error made up Δ g/g=1.45×10-9, this accuracy being restricted by the laser system noises. The measurement precision of the magnetic field gradient 1.2×10-10 T m-1 is close to the level of sensitivity of solid-state magnetometers and SQUIDs. The new combined device may appear to be applicable in prospecting for minerals through a simultaneous registration of gravitational and magnetic anomalies in the Earth crust. Source: Phys. Rev. Lett. 117 138501 (2016)

X-ray source with optical illumination of the X-ray focus

In the laboratory of X-ray methods of nanostructure diagnostics of P.N. Lebedev Physical Institute of RAS, a new compact X-ray tube was elaborated that has 10 mm in radius, 75 mm in length and 20 g in mass and has electrostatic focusing of electrons in which one and the same electron beam generates both X-ray and optical radiation. Used in the tube was a thin-film metallic anode layered onto an optically activated transparent diamond substrate. X ray radiation is generated in the metal film and optical radiation in the diamond substrate. The size of the X-ray focus is ≈10 µm. The X-ray and optical foci appear to be superposed, which is very convenient in the practical use of the device because the optical focus can readily be directly observed. Source: SIA “FIAN-inform”

Isotropy of the Universe

On large scales, the Universe looks on the average the same in all directions, however, some small anisotropy cannot be excluded. D. Saadeh (University College London, Great Britain) and her colleagues were the first to allow for all possible modes of anisotropic perturbations of the Universe metric and obtained the most general (without additional assumptions) restriction on the degree of the Universe anisotropy. The theoretical calculation of the anisotropic mode evolution was compared with the Planck telescope data on relic radiation fluctuations. No statistically significant deviation from the isotropic model of the Universe was found. To the vector mode corresponding to rotation the relative anisotropy does not exceed 4.7×10-11 at the confidence level of 95 %. This restriction is better by an order of magnitude than that obtained in the previous works performed with disregard of the Planck data on radiation polarization which is very sensitive to anisotropy. Restrictions from above on the magnitude of scalar and tensor anisotropic perturbations were also obtained in the new work. The restrictions on anisotropy reduce the amount of models of the Early Universe. Source: Phys. Rev. Lett. 117 131302 (2016)

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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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