Reliable quantum computing
1 October 2011
For 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 metals
1 October 2011
Researchers 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)
1 October 2011
A 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 instability
1 October 2011
Researchers 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 horizon
1 October 2011
Measurements 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.