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| Issue 12, 2024 |
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Extracts from the Internet | ||
Nondestructive photon detection1 August 1999While a conventional photodetector measures a photon by converting it into an electrical signal after an absorption event, nondestructive photon observations are not ruled out by the fundamental laws of quantum mechanics, and indeed a number of nondestructive techniques have been developed over the years. These, however, have been too difficult to execute until now. Researchers from France have recently been successful in performing a nondestructive observation. In the experiment of Nogues and coworkers, an atom of rubidium passes through a cavity between two mirrors, whose wave function phase shift acquired in the presence of a photon can easily be detected. By sending additional rubidium atoms through the cavity, one can measure the photon repeatedly without destroying it. Although a photon is not destroyed as a particle, its quantum state is of course altered in accord with Heisenberg's Uncertainty Principle. The method may be used in quantum logic gates design, according to the authors. Source:http://www.nature.com/ Unusual crystals1 August 1999In usual diffraction experiments, x-ray radiation passing through an atomic crystalline lattice forms a characteristic interference pattern after being scattered on the atoms. Austrian physicist Zeilinger and his colleagues performed what may be called an inverse experiment by making a standing electromagnetic wave (`crystal') scatter an atomic beam. The wave was produced by reflecting a laser beam from a mirror, and the beam was formed in the so-called `atomic laser.' Having passed the standing wave, atoms produce an interference pattern similar to that obtained on a crystal lattice. While experiments of this type were started by Zeilinger' group back in 1996, the most complete analog of Bragg diffraction has only recently was obtained. Source:http://www.nature.com Superhigh precision measurement of light frequency1 August 1999A technique for measuring the frequency of visible light to a precision of 3×10-17 (to compare with the 2×10-15 accuracy of the best atomic transition clocks) has been developed at the Max Planck Institute for Quantum Optics using femtosecond laser pulses with a regular set of different frequencies forming their spectra. By comparing the laser emission, light signal, and reference wave frequencies, first the frequency difference between the signal and the reference wave, and then the signal frequency itself can be determined to high accuracy. In this way, the atomic transition frequencies of caesium (D1 line) were measured with 100 times the accuracy of previous measurements. With this technique, it is hoped that a more effective optical version of atomic clocks can be developed and more accurate values for some of the fundamental constants will be obtained. Source: Physics News Update, Number 434 Neutrino oscillation1 August 1999New evidence for neutrino oscillation (mutual transformations of different neutrino types) has been found in the K2K (KEK + Kamiokande) experiment in Japan. In this, a narrow neutrino beam was generated by the proton accelerator at the KEK laboratory near Tokyo, whose muon and tau neutrino percentages were measured using a number of detectors, particularly a kiloton water Cherenkov detector, to record neutrino events. The neutrino beam then traveled 250 km in Earth and reached the underground Super- Kamiokande detector, a steel 50,000-ton water reservoir with thousands photomultipliers on its inside surface for detecting Cherenkov radiation. At this end, a greatly reduced number of muon neutrinos indicated oscillation processes to occur in the traveling beam. Evidence for neutrino oscillation was also found in the Super-Kamionde experiments about a year ago, when neutrinos from cosmic ray collisions with the upper atmospheric layers were examined. The phenomenon of neutrino oscillation requires non-zero-mass neutrinos, and these are predicted by most Grand Unification theories in which various interaction types (weak, electromagnetic, and strong) are united. The neutrino oscillation discovery may help explain the deficit of solar neutrinos, and massive neutrinos may account for a large part of dark matter (or hidden mass) in the Universe thus furthering our understanding of its large-scale structure. Source:http://unisci.com/ Star birth1 August 1999According to current views, stars are formed by the gravitational contraction of and subsequent thermonuclear reactions in the dense clouds of interstellar gas and dust. The details of these processes are not entirely clear, however, nor the conditions for the formation of particular star types are known. At present, while very old stars aged 13 billion years or more and also very young stars exist, the star formation process goes on so that in principle protostars at the very early evolution stage of cold cloud contraction can be present. This stage is normally difficult to see, however, because the dust component of the protostar material blocks light and so prevents the protostar interiors from observation. This difficulty has been overcome by E. Lada and her colleagues at the University of Florida, who developed an elegant near-infrared technique for observing stars. Specifically, the object they studied was the dark globule Barnard 68 (B68), 500 light years from Earth and located against a dense background of stars whose infrared light does penetrate the globule. Based on the change in the color of the background stars, the distribution of dust within the B68 globule was examined and some information on its inner structure obtained. It is found that B68 is currently at the very early contraction stage, and that it will take another 10,000 years or so for the contraction to complete. About 10 million years after that, thermonuclear reactions will start and a new star will form. About 4.5 billion years ago, our Sun must have undergone similar processes. Source: http://unisci.com/ |
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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