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| Issue 12, 2024 |
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Extracts from the Internet | ||
t-Quark electric charge1 March 2007Heavy quarks discovered in 1995 on the proton-antiproton Fermilab Tevatron collider were identified as the t-quarks of the Standard Model of elementary particles in which their electric charge is +2e/3. In fact, the charge as such was not measured in these experiments, which left possible an alternative theoretical scenario of the new quark having the charge -4e/3 and forming a doublet with another unknown quark having the charge -1e/3. In this case the t-quark predicted by the Standard Model may have remained unobserved until now. The D0 collaboration ran a new experiment on the Tevatron collider which excludes this hypothetical scenario at 92% confidence level. The object of study was decays of particles, discovered in 1995, to W-bosons and b-quarks. Their charge was established to be +2e/3 and hence they are indeed the t-quarks of the Standard Model. Sources: Phys. Rev. Lett. 98 041801 (2007) John Wheeler’s “delayed decision” experiment1 March 2007The result of the famous two-slit experiment (formation or absence of an interference pattern) depends on whether measurements are carried out that are capable of determining through which of the slits has the photon has traveled. In order to clarify the conceptual foundations of quantum mechanics, John Wheeler had proposed in 1984 an experiment with “delayed decision” in which the decision on measuring or not measuring is taken after the photon has traveled through a slit but still prior to the moment by which the information on the decision, traveling at the speed of light, could have reached the photon. In this case the behavior of the photon (traversing through both slits at the same time as a wave or through a single slit as a particle) and the act of taking a decision on conducting the measurements are not causally connected: these events are separated by a spacelike relativistic interval and therefore no “hidden parameters” could have been passed on to the photon in advance. Speaking metaphorically, the photon “is unaware” of whether its trajectory is to be measured. V.Jacques and co-workers in France and China carried out an experiment with delayed decision using an arrangement that is closest to the original proposal of John Wheeler. Having passed through the splitter at the entrance to a 48m long Mach-Zehnder interferometer, photons could travel along two different paths. Impurity atoms in a diamond nanocrystal excited by laser pulses were used as sources of single photons. An electro-optical modulator at the other end of the interferometer was controlled by a quantum random-number generator. If a signal was sent, the modulator rotated the photon polarization plane. Depending on the state of the modulator, a photon that traveled along any of the two paths of the interferometer either had the same polarization in both and underwent interference when traveling through a prism, or acquired different polarizations and reached the detectors on emerging from the two paths without undergoing interference. In this last case it is possible to establish which path the photon went through, that is, the measurement was carried out. The photon source and the modulator were synchronized with time delay in such a way that passage of the photon through the splitter at the entrance to the interferometer and the random selection of the state of the modulator were causally unrelated. Therefore, in contrast to earlier “delayed decision” experiments, all conditions of the John Wheeler experiment were fully met. In those cases where no trajectory measurement was carried out, the interference pattern with 94% sharpness was recorded (this deviation from 100% was caused by experimental errors), but if the trajectory was measured, interference was absent with 1% accuracy. The experiment thus confirmed once again the prediction of quantum mechanics and demonstrated that the wave-particle dualism is the profound fundamental property of the theory and cannot be reduced to “hidden parameters”. Sources: Science 315 966 (2007) The Casimir-Polder force as a function of temperature1 March 2007The force of interaction between atoms and the surface due to electromagnetic quantum fluctuations is known as the Casimir-Polder force. This phenomenon is one of the manifestations of the Casimir effect, that is, of the attraction between two surfaces. In 1955 E.M.Lifshitz predicted temperature to affect the Casimir force, resulting from the effect of temperature on thermal fluctuations and on the spectrum of electromagnetic oscillations in the space between the bodies. J. M. Obrecht and co-workers at the JILA (in Colorado) were the first to study experimentally the effect of temperature on the Casimir-Polder force. They measured the frequency of mechanical vibrations of a Bose-Einstein condensate cloud of 87Rb atoms at a distance of several microns from a dielectric plate and calculated the attraction force from the measured frequency. The thermal background temperature outside the plate on the side of the condensate was kept constant (at -310Ê), while the plate temperature and correspondingly that of the thermal radiation inside the plate were varied by laser-heating the side of the plate opposite to the condensate. The theoretical foundation of this method was given by P.Antezza, L.P.Pitaevsky and S.Stringari in 2004-2005; the results of measurements are in good agreement with these calculations. Thus heating a plate from 310 to 605K increased the Casimir-Polder force by a factor of three. This temperature effect should be taken into account when designing nanoscale devices. Sources: Phys. Rev. Lett. 98 063201 (2007) The Henbury Brown-Twiss effect for bosons and fermions1 March 2007The Henbury Brown-Twiss effect, initially discovered with photons, manifested itself in the tendency of photons to reach a detector bunched into groups; this is a consequence of the Bose-Einstein statistics that dictates the behavior of photons. Contrary to this, fermions in a similar experiment should produce anticorrelation. Ò.Jeltes and co-workers in The Netherlands and France were able for the first time to study the Henbury Brown-Twiss effect for bosons and fermions in one and the same experimental setup, which made it possible to directly compare the results. Ultracold 4He atoms (bosons) and 3He atoms (fermions) were falling due to gravitation from an atomic trap onto the detector. The temperature and cloud size in the trap were identical for both types of particles. The experiment measured spatial and temporal correlations of the detected atoms. In agreement with the expectations, correlation (bunching) was observed with bosons and anticorrelation (antibunching) was observed with fermions. The measured ratio of correlation lengths, determined by the ratio of particle masses, was found to be 1.3(+-0.2), in agreement with the theoretical value 4/3. Sources: Nature 445 402 (2007) Nanopolymers1 March 2007Researchers at the Massachusetts Institute of Technology created a new class of material called “nanopolymers”. Ligands, such as thiol molecules incorporating a sulfohydrilic group SH, were bonded to the poles of spherical metallic nanoparticles. These groups are capable of bonding to one another and thus building chains of nanoparticles and also creating films by chain-to-chain bonding. The film structure resembled that of ordinary polymers formed of filament-shaped organic molecules. The process of formation of a nanopolymer is also very similar to ordinary polymerization reactions. Up to 50,000 nanoparticles could be bound into a single chain, and film area reached up to 1cm2. Nanopolymers may lead to useful practical applications, for instance, to designing materials with controlled porosity on the nanoscale. Sources: Science 315 358 (2007) |
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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