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| Issue 12, 2024 |
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Extracts from the Internet | ||
The average trajectory of photons in the two-slit experiment1 July 2011A. Steinberg (University of Toronto, Canada) and his colleagues have determined by way of weak quantum measurements the average trajectories of photons in an interference experiment similar to Young's two-slit experiment. An experiment is referred to as weak if it yields only partial information about the quantum state of the system in a way that does not destroy this state. For the light source A. Steinberg et al used a quantum dot of low luminosity; only one photon was in the beam at any moment of time. Photons propagated through the optical splitter and entered two arms of the Brown – Twiss interferometer. A calcite crystal placed in the path of the two beams generated phase shift and rotation of the polarization vector by the amount dependent on the direction of the photon momentum. Therefore selection of photons with a specific polarization allowed conduction of weak quantum measurements of photons' transverse momenta at different distances from the screen (the CCD matrix). After a large number of measurements of information on the average momenta and coordinates of interactions between photons and the screen, it was possible to calculate the set of average photon trajectories. The concept of average photon trajectories was introduced operationally; they are not real trajectories, and carry only a limited meaning in quantum mechanics. Compressions and rarefactions of the bunch of average trajectories was found to correspond to maxima and minima of the interference pattern on the screen; the pattern was not destroyed by weak quantum measurements of pulses. The results of the experiment fully comply with the standard Copenhagen interpretation of quantum mechanics. Source: Science 332 1170 (2011) Dynamic Casimir effect1 July 2011An experiment has been carried out at the Chalmers University of Technology in Gothenburg, Sweden in which C.M. Wilson and his colleagues observed for the first time the dynamic Casimir effect which was theoretically predicted by G. Moore in 1970. The dynamic Casimir effect, in contrast to the usual static Casimir effect with two plates, occurs for a single mirror moving at relativistic speed. It changes the spectral composition of zero-point quantum fluctuations in the vicinity of the mirror, and the pairs of virtual particles arising from the vacuum at sufficiently high velocity can separate and convert into real particles at the expense of energy obtained from the mirror. Ñ.Ì. Wilson and his colleagues built an aluminum waveguide approximately 100 micrometers long, plugged by a superconducting contact — a SQUID. The SQUID's inductance was modulated within 10 % by the microwave field frequency of ≈ 11 GHz. The modulation produced periodic changes in boundary conditions at the end of the waveguide whose effective electric length correspondingly oscillated with the amplitude of ≈ 1 nm. This produced an analog of a mirror oscillating at a speed of about ≈ 5 % of the speed of light. Observations recorded emerging microwave photons. This radiation was composed of two correlated modes with different frequencies, which pointed to its quantum origin from split pairs of virtual photons in agreement with the theoretical calculations for the dynamic Casimir effect. Source: arXiv:1105.4714v1 [quant-ph] “Friction” between holes and electrons in a semiconductor1 July 2011Researchers from the University of California, Berkeley, and the Sandia and Lawrence Berkeley National Laboratories investigated the motion of electron-hole excitations in a single quantum wall. Unlike the case with non-interacting electrons and holes, exchange of momentum between them reverses the direction of flow of charge waves, namely, the flow of electrons drags holes in the direction of its motion. A similar but weaker effect has already been observed earlier in multilayer systems in which the interaction was between electrons and holes from neighboring layers (parallel quantum walls). In this experiment the quantum wall was an n-doped 9 nm wide GaAs/AlGaAs structure. The wave of electron-hole density was excited by laser pulses and created variations of the refractive index in two-dimensional gas. These variations were recorded by optical measurements. These measurements by phase-resolved transient grating spectroscopy yielded the values of the ambipolar diffusion coefficient and charge mobility, and this data allowed calculation of the characteristic value of the interaction (“friction”) between the fluxes of electrons and holes in charge waves. Source: Phys. Rev. Lett. 106 247401 (2011) Helical structures in a magnetized plasma1 July 2011Ì. Schwabe (Max-Planck-Institute for Extraterrestrial Physics, Germany) and his colleagues found new ordered structures in the plasma in strong magnetic fields. Plasma from neon, argon, krypton, and from the air was studied at low pressure at room temperature. The plasma with ionization level 10-7-10-6 was created by radio frequency discharge at the center of a superconducting solenoid whose magnetic field (vertically oriented) was uniform to within 0.65 %. To achieve visualization, microscopic dust particles (2.55 micrometers in diameter) were injected into the plasma and illuminated by a horizontal laser beam; the observations were conducted through a transparent top electrode and through the side wall of the vessel. If the solenoid magnetic field was low, the plasma-dust crystal filling the vessel rotated in almost solid-body fashion. As the field grew above ≈ 1.3 T at pressures less than 20 Pa, plasma filaments emerged parallel to the magnetic field and their common rotation separated into individual vortices of about 1 mm radius around the axes of filaments. At the same time, microscopic particles in the plasma formed patterns of spirals and concentric circles in the transverse direction. No detailed theory of filament rotation and pattern formation has been developed so far. The authors of the experiment hypothesize that the phenomenon responsible for this effect is probably the motion of positive ions. In contrast to electrons, they are not trapped in the filaments and can travel through the vessel, are deflected by the magnetic field and affect the motion of dust microparticles. Source: Phys. Rev. Lett. 106 215004 (2011) Galactic cluster PLCK G266.6-27.31 July 2011Observations with the Planck Space Radio Telescope (based on the Sunyaev – Zeldovich effect) and the X-ray Observatory ÕÌÌ-Newton revealed that the cluster of galaxies PLCK G266.6-27.3 belongs to a rare type of distant massive clusters with high X-ray luminosity. The iron emission line in the spectrum yielded the red shift z = 0.94 ± 0.02. The mass of the cluster, based on its X-ray luminosity ≈ 1.4 × 1045 erg s-1 in the range 0.5-2 keV and on dynamic models, is found to be (7.8 ± 0,8) × 1014M☉. This cluster is thus a member of the three known clusters at redshift z > 0.5 with the largest X-ray luminosity, and has become one of the two most massive clusters with z ≈ 1. The cluster PLCK G266.6-27.3 is unique in that it appears to be very regular: spherically symmetric with a power-law density cusp at the center and a cold core. In other words, even though PLCK G266.6-27.3 is observed in a fairly early cosmological epoch when most of the clusters are only just beginning to form, the cluster PLCK G266.6-27.3 had already undergone dynamic relaxation. One possible explanation of these properties is that this cluster grew out of a very rare large density perturbation. Source: arXiv:1106.1376v2 [astro-ph.CO] |
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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