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| Issue 12, 2024 |
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Extracts from the Internet | ||
Efimov states1 April 2006H.C.Nagerl of the University of Innsbruck, Austria, and his colleagues have for the first time observed three-particle bound states, whose existence was predicted theoretically by V.I.Efimov in 1970 (Yadernaya fizika 12 1080 (1970)) - in a rare example of the complex three-body problem allowing an exact analytical solution. A family of low-energy bound states arises for a system of three identical particles if there is a strong enough resonance between the pair forces involved. Interestingly, because of the quantum-mechanical nature of this effect, three-particle states can even exist in the absence of two-particle bound states. The size of the three-particle system is much less than the absolute scattering length |a|, and the three-particle interactions are universal in character and independent of precisely how two particles interact when close together. The researchers studied a degenerate gas of cesium atoms held in an optical dipole trap at temperatures between 19.0 and 120.0nK. The scattering length was controlled using a Feshbach resonance and could be varied over a wide range by varying the external magnetic field. For a < 0, the Efimov states were identified by a sharp increase in electron recombination losses in the trap - presumably due to the three-atom system rapidly decaying from the three-particle state to one involving a strongly bound dimer plus an individual atom. At a temperature of 10 nK, Efimov states appeared when the scattering length was -850 Bohr radii. Another type of Efimov state arose for a > 0. In this case recombination losses are much lower and exhibit in their spectrum a recombination minimum, due to the destructive interference of the three-particle system's quantum decay channels. The position of the minimum agrees well with theoretical predictions. At a > 0, Efimov states arise due to a Feshbach resonance between single atoms and dimers and can be interpreted as a three-particle generalization of the Feshbach resonance concept. Source: Nature 440 315 (2006) Quantum telecloning1 April 2006A experiment on `quantum telecloning' has been conducted for the first time by a research team from Japan and the UK. Unlike `quantum teleportation,' in this case information about a state is sent to two, rather than one, receivers, and the input state is not reproduced perfectly but with some errors due to Heisenberg's uncertainty principle. Whereas quantum teleportation transfers the state of a particle, what was transferred in the new experiment was information about the amplitude and phase of a laser beam wave. The beam was reproduced at two remote locations to within 58% - a rather good result considering the theoretical limit of 66% for the reproduction accuracy in such an experiment. Telecloning may have applications in quantum communications and possibly in future quantum computers. Source: Phys. Rev. Lett. 96 060504 (2006) A nanomotor1 April 2006An experiment involving the controlled rotation of a single molecule has been conducted by a team of researchers from the Netherlands. In this experiment, an asymmetric organic molecule placed on the surface of - and forming carbon-carbon double bonds with - a liquid-crystalline film underwent photo-isomerization from the left-handed form to the right-handed form when irradiated the with 365nm ultraviolet light and became left-handed again when the light was switched off. Repeating this twice made the molecule rotate through 360o in the plane of the film. The change in the shape of the molecule produced rotation-induced structural changes in the liquid crystal's surface profile. Importantly, small objects placed on the crystal's surface, in particular a glass rod 10,000 times the size of the molecule itself, were forced into rotation as a result. Source: Nature 440 163 (2006) Polarization of the cosmic microwave background1 April 2006Analysis of three years of WMAP satellite data has provided improved values of cosmological parameters and the first reliable information on the polarization of the cosmic microwave background radiation. The WMAP observations covered the entire celestial sphere and were performed with polarization-sensitive radiometers at five frequencies between 23.0 and 94.0 MHz. To filter out radio noises from galactic objects, an analysis of their sources was performed in detail. Microwave background polarization may be due both to primary gravitational waves and to scattering from gas clouds at a later epoch of the first stars reionizing the universe. The new observations have allowed a tight constraint on the gravitational wave contribution and showed that reionization occurred at higher redshifts than previously thought (implying early star formation at an epoch when the Universe was about 400 million years old). The way the background radiation fluctuates is best described by a model in which the Hubble constant is 73km/(s Mpc), baryonic matter accounts for 4% of the total density, and dark matter and dark matter, for 22% and 74%, respectively. It is also shown that the spectral power exponent of the primordial density perturbations that gave rise to galaxies is n=0.951, which is somewhat less than the 1 of Harrison-Zeldovich's flat spectrum. Thus, the WMAP observations have provided further support for those inflation models predicting n < 1, while ruling out models with a large contribution from the tensor perturbation mode (gravitational waves). Sources: astro-ph/0603449; astro-ph/0603450 |
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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