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| Issue 12, 2024 |
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Extracts from the Internet | ||
Polarized degenerate Fermi gas1 January 2007G.B.Partridge and his colleagues from the US and the Netherlands have studied the properties of a cloud of polarized degenerate fermionic 6Li atoms in a magneto-optical trap. Using microwave radiation to control the degree of polarization (i. e., the relative population of spin-up and spin-down atoms), the team looked at the shape and phase state which different regions of the cloud assumed at various polarizations and temperatures. As some portion of the atoms were converted into the paired state using the Feshbach resonance, the resulting pairs of atoms, now bosons, formed an unpolarized core of Bose-Einstein condensate at the center of a partially polarized gas halo (the elongated spheroidal shape of the halo being consistent with the geometry of the trap potential). At a sufficiently low temperature the core was observed to have a sharp boundary and a close-to-spherical shape due to surface tension forces at the boundary. Increasing temperature strongly deformed the core and gave rise to a transition layer of partially polarized gas between it and the halo. The experiment confirms the existence in the phase diagram of a triple point where the superfluid-to-normal transition changes from second order to first order as the temperature is lowered. Source: Phys. Rev. Lett. 97 190407 (2006) `Purifying' entangled quantum states1 January 2007There has been much recent activity in the study of entangled quantum states (see Phys. Usp. 44 597 (2001)). The focus of experimental interest in this area is decoherence, i. e., the perturbation-induced destruction of quantum correlations in the system. C.H.Bennett and colleagues in 1996 suggested an `entanglement purification' algorithm that first checks whether there is correlation in the system and then picks up the correlated pairs of particles. The basic idea underlying the algorithm is to make copies of a quantum state (i. e., additional pairs correlated with the original one) and then to use classical information channels to compare their quantum state with that of the first pair. Until recently, this algorithm has only been implemented in experiments with photons, but with low efficiency and in a destructive way, i.e., with the entangled state being destroyed in the measurement process. Now R.Reichle and his colleagues of the National Institute of Standards and Technology at Boulder, Colorado, USA, have developed a new technique which they used in an experiment on spin-entangled pairs of 9Be+ ions. After first entangling two such ions in an ion trap by using laser pulses, the researchers entangled each ion of the first pair with the ions of the second. Measuring the quantum correlation of one of the pairs revealed whether the ions of the second one were or were not successfully entangled, thus weeding out the failures. The `entanglement purification' level achieved was 35%, the important point being that the entangled state of the second pair remained undestroyed and could be used in further studies. The algorithm can in principle be repeated several times, leading to higher-fidelity entanglement. Source: Nature 443 838 (2006) Superconductivity in boron-doped silicon1 January 2007Superconducting silicon has previously only been observed at a very high pressure of about 10GPa in the hexagonal phase. Now E.Busterret and his colleagues from France and Slovakia have for the first time observed boron-doped silicon to become superconducting in the cubic phase at atmospheric pressure. The team used a method known as `gas immersion laser doping', in which boron atoms were made to penetrate a thin silicon film by repeatedly melting and solidifying the film. In this way silicon films with a very high (several percent) boron content were created. The superconducting transition temperature of the film was 0.35K, and the (critical) magnetic field necessary to suppress superconductivity was 0.4T. The superconducting transition was detected by electrical resistance and magnetic susceptibility measurements. Doped silicon turned out to be a second-kind superconductor. The superconductivity of the films is due to a phonon mechanism, as theoretical calculations predict and Raman spectroscopy confirms. Source: Nature 444 465 (2006) A gamma-ray source at the center of the Galaxy1 January 2007In 2004, a TeV gamma-ray source was detected at the center of our Galaxy, whose nature is not yet known. Suggested as possible candidates for emitting radiation have been the shell of a newly exploded supernova, matter near the central supermassive black, and dark matter particles annihilating at the center of the Galaxy. Theoretical models predict (see, for example, Phys. Usp. 38 687 (1995)) and galaxy formation simulations confirm that radiation due to annihilation appears as a pointlike gamma-ray source if dark matter increases in density toward the center of the Galaxy. Now astronomers have used the HESS telescope, an array of four Cherenkov detectors, to further study the source by detecting the particle showers high-energy photons initiate in the atmosphere. It is found that the source is less than 1.2' in angular size and that it has an approximately power-law energy spectrum with no falloff at high energies. Given the current limits on the parameters of neutralinos, the annihilation of these particles cannot explain this spectrum. The cutoff scale for the spectrum produced by a neutralino model should be the energy equivalent of the particle mass, so neutralino annihilation does not contribute more than about 10% to the source's gamma-ray radiation, and the bulk of the radiation flux should be of a different origin. Besides, the HESS observations did not reveal any variabilities or spectral lines in the radiation emitted by the source. Source: astro-ph/0610509 Dark energy at z=11 January 2007It is quite an established fact (see Phys. Usp. 47 747 (2004)) that about 70% of the total density of the Universe is dark energy. To determine the nature of dark energy, it is essential to know precisely how its density evolved in time. Astronomers using the Hubble space telescope have now carried out a new search for type Ia supernovae, thermonuclear explosions of white dwarf stars that serve as standard candles for estimating cosmological distances and the cosmological expansion rate. In addition to the two previously found z>1 supernovae, Hubble detected 23 more supernovae from 8 to 10 billion years ago. The analysis of the observations suggests that even that far back dark energy contributed noticeably to the total cosmological density, and the expansion of the universe was gradually changing from decelerating to accelerating. Unfortunately, with the precise dark energy equation of state still unknown from observations, quintessence models remain a possibility along with the cosmological constant. Source: astro-ph/0611572 |
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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