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| Issue 11, 2024 |
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Extracts from the Internet | ||
Reliable quantum computing1 October 2011For the quantum computer to operate, it needs quantum logic cells in which the error-rate due to decoherence should be not higher than ≈ 10-4. Calculations show that this low rate makes possible efficient functioning of error-correcting protocols. K.R. Brown (National Institute of Standards and Technology — NIST, USA) and his colleagues were able to overcome this barrier for the first time in their experiment by designing a qubit (quantum bit), whose state could be controlled at error probability level (2.0 ± 0.2) × 10-5. The qubit was implemented at rf transitions between hyperfine splitting levels of the ion 9Be+. The key factor for the effectiveness of this technique was the use of microwave pulses. By contrast, experiments using lasers involve fluctuations of beam intensity and instability of its focusing on the ion, and hence decoherence of the quantum state of the ion. Unlike lasers, the stability of microwave radiation intensity is easy to maintain; in addition, nothing in the experiment caused displacements of the emitting micro antenna mounted inside the trap with the ion. Furthermore, the trap was shielded from fluctuations of the external magnetic field by copper walls cooled to 4.2 K. The state of the ion was determined from its photoluminescence. In this experiment, 1000 series of quantum computations with lengths from 1 to 987 operations each were carried out with the qubit. The next urgent task is to combine multiple qubits into a single computing system. Source: arXiv:1104.2552v1 [quant-ph] Re(n) < 0 in ordinary metals1 October 2011Researchers at the Wurzburg and Gottingen Universities (Germany) found that pure cobalt Ñî and the alloy Fe/Co in strong magnetic field at room temperature manifest negative real part of refractive index Re(n) < 0 for radio emission in the millimeter wavelength range. The scenario with n<0 that was studied theoretically in V.G. Veselago's papers, can perhaps be implemented only in man-made metamaterials — in which it has already been observed. However, the weaker condition Re(n) < 0 can be achieved in ordinary metals with complex refractive index (this corresponds to the attenuation of waves in matter) in the vicinity of the ferromagnetic resonance. This experiment had the configuration of the Mach – Zehnder interferometer and measured phase shift of the emission passing through thin polycrystalline Ñî and Fe/Co films on MgO substrate depending on radiation frequency, temperature and magnitude of external magnetic field. The range of parameters was found in the vicinity of the ferromagnetic resonance in which Re(n) < 0. For example, external magnetic field ≈ 3.1-3.9 T is necessary for Co at room temperature at frequency 140 GHz. Source: Europhysics Letters 95 37005 (2011) Superdense aluminum1 October 2011A new superdense modification of aluminum has been obtained by producing microimplosions caused by focused laser pulses at a depth of several micrometers beneath the surface of the corundum crystal α-Al2O3. Laser pulses ≈ 150 fs long created energy density ≈ MJ cm-3 in submicron volume. Evaporation of the crystal in this volume created plasma at a temperature of ≈ 105 K, and the expanding shock wave produced pressure on the order of 1 TPa on the inner surface of the resulting microvoid. The crystal lattice of aluminum placed in these conditions typical of planetary cores acquired cubic bulk-centered bcc-Al structure. An entire array of microvoids of radius ≈ 360 nm has been created inside one crystal in the experiment carried out in the Shizuoka University (Japan) by successive microimplosions at many points. The new modification of Àl was deposited on the inner surfaces of microvoids and could be studied for a considerable time after the compression. The bcc-Al modification with lattice constant a=2.866 A was identified in this crystal by x-ray diffraction at the Argonne National Laboratory. The authors of the experiment suggest that due to different rates of diffusion of ions with different masses, aluminum and oxygen ions get spatially separated in the hot hot plasma by an average distance ≈ 18 nm and and this results in the crystallization of pure bcc-Al. It was also found that after a microexplosion the inner surface of the microvoid remains under stress at pressure ≈ 60-100 GPa, which stabilizes the bcc-Al phase. Source: Nature Communications 2 445 (2011) Vibrational modes and structural instability1 October 2011Researchers from the USA and China experimentally confirmed a model in which the greatest strain of the granular medium develops in areas where elastic vibration modes have minimum frequencies. Since in the real world oscillations are very strongly damped, the frequencies in the model should be determined when the damping is turned off, while maintaining the other mechanical properties of the medium and its geometry unchanged. In contrast to the continuum, the spectrum of elastic waves in granular media varies greatly over space. In theoretical papers the areas with minimum frequency were referred to as “soft spots” and the greatest displacements in the medium were predicted in these areas; indeed, here the interaction between particles is the weakest and the energy barrier for shear is minimal. In practical situations such areas may be considered as defects in the medium. The experiment of K. Chen et al was carried out for the quasi-two-dimensional case. The colloidal glass, which was an aqueous suspension of elastic microgel granules, was poured between two glass plates through which microphotography of location and motion of the granules was conducted. The restructuring of the medium was produced by heating it. Heating expanded the granules and displaced them by 1/30 of their size on average. As expected, the greatest collective displacement of granules occurred where the vibrational frequencies of the elastic motion, evaluated by comparing to the initial positions, were minimal; displacements occurred in the direction of polarization vectors of the vibrational modes. These studies may prove useful for predicting earthquakes, for flaw detection and other applications. Sources: Phys. Rev. Lett. 107 108301 (2011), Phys. Rev. Lett. 107 108302 (2011) X-ray emission close to the black hole horizon1 October 2011Measurements of x-ray emission from the center of the Seyfert 1 Galaxy 1Í0707-495, carried out using the telescopes XMM-Newton and Swift, made it possible to conclude that this radiation arrives to us from a distance less than one gravitational radius Rg from the horizon of the central supermassive black hole. In the period from December 2010 to February 2011 the intensity of x-ray emission from the core 1N0707-495 weakened considerably, and the galaxy was in a relatively quiet phase. Certain features of the emission spectrum during this period permitted modeling the source structure. The best agreement with the observations was provided by a model in which about 80% of the detected flux is the radiation reflected from the inner part of the accretion disk which imparts gravitational redshift to it. Region of reflection lies at a distance < 1Rg from the horizon of a rapidly rotating black hole (rotation parameter a > 0.997), i.e. it lies in the inner part of the thin accretion disk near the last stable orbit. It is likely that the source of emission was a strongly magnetized plasma structure and thus would not lie higher than 1.5 Rg above the the plane of the accretion disk near its axis, or with some lateral offset. Source: arXiv:1108.5988v1 [astro-ph.HE] |
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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