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| Issue 12, 2024 |
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Extracts from the Internet | ||
New measurements of the fine structure constant1 January 2009Using a combined method based on the Bloch oscillations effect and atom interferometry, Ì. Cadoret and his coworkers in France were able to measure the fine structure constant with a relative accuracy of 4,6×10-9. Other recent superhigh-precision experiments for α directly measured only the the anomalous magnetic moment of the electron after which α was calculated using the formulas of quantum electrodynamics. In the experiment of Ì. Cadoret and his colleagues, measurements of α were more direct (did not need the assumption of validity of QED formulas) as they used recoil pulses from atoms in periodic potential. Rubidium atoms were illuminated by two oppositely directed laser beams with slightly different frequencies; atoms absorbed photons from one beam and then re-emitted them into the other beam. The frequency difference was compensated for by the Doppler effect on moving atoms, and its measurement yielded the value of α. The agreement with the results obtained in other experiments and with theoretical QED achieved the test of this theory at currently the highest accuracy. Source: Phys. Rev. Lett. 101 230801 (2008) Light pulse in optical filament1 January 2009The controversy concerning the momentum of light in a transparent medium remains the object of debate for nearly a hundred years. The problem of choosing between the expressions given by H. Minkowski and M. Abraham lies in the ambiguity of dividing the total momentum into that of the field and that of the medium, and in the need to take into account the action exerted by the electromagnetic field on the medium when light is emitted or absorbed (see Uspekhi Fiz. Nauk 118 175 (1976) (in Russian)). Chinese researchers W. She, J. Yu è R. Feng carried out a new experiment which confirmed Abraham's expression. Optical filament 1.5 mm in length and half a micron in diameter was suspended vertically in a hermetically sealed vessel. Light from two lasers was sent downward through the filament. The first laser, at wavelength 650 nm and power output 0.5 mW served to illuminate the fiber and facilitate observing its motion which was photographed 10 times/min through a lens installed in the wall of the vessel. When a light pulse from the second laser at wavelength 980 nm and variable power output of 0 to 79 mW emerged from the lower end of the filament, it imparted to it a momentum and the upward-directed recoil caused filament bending. This behavior confirmed Abraham's expression for momentum: if Minkowski's approach were correct, there would be a downward stretching load on the filament. The experiment was successful owing to the small weight of the filament: the recoil momentum compensated for the weight of the free end segment of the filament. The experiment confirmed the theoretical evaluation which predicted this compensation to occur at laser power output of about 4 mW. A not very different result was observed when the second laser worked in continuous, not pulsed, mode. Source: Phys. Rev. Lett. 101 243601 (2008) The Magnus effect for light1 January 2009Å. Hasman and his colleagues at the Technion-Israel Institute of Technology have been the first to observe in the adiabatic mode the spin Hall effect for photons, also known as the optical Magnus effect. This effect was observed earlier but only in the nonadiabatic case of strong nonuniformity when a particle's trajectory is stopped abruptly. The spin Hall effect for photons consists in the interaction between the spin of a particle and the curvature of its trajectory, resulting in an additional force affecting the trajectory of motion. Hasman et al studied propagation of laser light along a glass cylinder. The beam went through total internal reflections and its trajectory was twisted into a helix along the surface. Measured at the exit face of the cylinder were the beam direction and the Stokes parameters. The experiment carried out in the Technion confirmed the detailed theory of the optical Magnus effect, based on the dynamic effect of the geometric Berry phase. Source: Nature Photonics 2 748 (2008) The Lamb shift in solids1 January 2009The Lamb shift of atomic energy levels stems from the interaction between electrons and virtual electron-positron pairs created in the vacuum. Typically it is not possible to observe the Lamb shift in solids since energy levels in them form broad bands. However, A. Wallraff and his coworkers from Switzerland and Canada were able to measure the Lamb shift of the microscopic quantum bit (qubit) in a resonator. The qubit consisted of two tiny pieces of superconductor connected by two tunnel junctions. This system is known as transmon. The energy levels of the transmon are dictated by the distribution of Cooper pairs in superconductors. A transmon was placed in a microwave resonator where it could absorb and emit photons of certain frequencies. By virtue of its shape, the transmon possessed a large dipole moment; also, a special resonator configuration was chosen so as to enhance the effect of interaction with virtual photons. The Stark effect contributed only negligible corrections because it it was felt only outside the area of resonance with virtual photons. As a result, the observed Lamb shift of transmon's energy levels was approximately 1.4% of the energy difference between the neighboring levels, which is 10,000 times greater than the Lamb shift in the hydrogen atom outside the resonator. The Lamb shift results in decoherence of the qubit state. The experiment conducted by A. Wallraff and his colleagues provides a recipe for avoiding undesirable decoherence in future quantum computers: choose device configurations that are not in resonance with virtual photons. Source: Science 322 1357 (2008) Stimulated emission of surface plasmon polaritons1 January 2009Surface plasmons and plasmon polaritons constitute electromagnetic pulses in the electron gas, localized or moving along the metal-dielectric interface, respectively. These quasiparticles are strongly absorbed in the range of optical frequencies and have short propagation length, which create problems for possible practical applications. It was suggested that the problem may be solved by using optically active impurities. Ì.À. Noginov (Norfolk University, USA) and his coworkers were able for the first time to use this technique and achieve both the compensation of losses of surface plasmon polaritons and the observation of their stimulated emission which is similar to the stimulated emission of photons in lasers. A 32 to 82 nm thick silver layer was deposited onto a face of a grass prism. The silver layer was coated with a polymer film doped with dye molecules. Excitation of surface plasmon polaritons was produced by light pulses first on the side of the prism (for the sake of calibration needed to measure the reflection profile R(θ)) and then on the side of the polymer film. The dye molecules absorbed photons and emitted surface plasmon polaritons. The threshold for polariton emission and the spectrum of polaritons agreed with theoretical predictions for a laser-like radiation. This experimental study may lead to useful applications in creating novel metamaterials and plasmon nanodevices. Source: Phys. Rev. Lett. 101 226806 (2008) A very hot white dwarf1 January 2009The space telescope FUSE detected a white dwarf KPD 0005+5106 with record-high surface temperature of 200,000°Ñ. At this temperature, and object is visible in the UV range of spectrum. White dwarfs (their internal pressure is sustained by the degenerate electron gas) evolve from massive stars after the thermonuclear fuel inside them is exhausted. High temperatures can be produced only immediately after the white dwarf is formed, before it starts to cool down, so the observation of a white dwarf with a temperature of 200,000°Ñ is a very rare event. Source: http://www.space.com/scienceastronomy/081212-hot-star.html |
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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