Brown-Twiss effect for atoms
1 October 2005
The Brown-Twiss intensity interferometer measures wave intensity correlation between detectors some distance apart. Two telescope interferometers are usually used in astronomy to measure star diameters. The first atomic optics application of such an instrument - specifically to the study of ultracold gases - was made at the University of Tokyo in 1996. Now, C.Westbrook and his colleagues from France and Portugal have improved measurement technology to the point where single atoms flying out of a Bose-Einstein condensate can be detected on a time scale of a few nanoseconds and at distances of about 200mkm. The team studied the interference properties of magnetically trapped helium atoms evaporatively cooled to about 0.5mkK. The way to eject atoms from the trap was by turning the magnetic field off. It is found that the interference pattern observed above the BEC transition temperature disappears when the gas is cooled and BEC forms. The apparent paradox of a coherent source failing to produce an interference pattern is explained by the quantum correlation of the BEC atoms and by the way the intensity interferometer operates. The experiment examined the dependence of the effect on the size of the helium gas cloud and provided a full 3D picture of the correlations observed.
Molecular Bose Einstein condensate
1 October 2005
In recent years, molecular Bose Einstein condensates have been created in which weak atomic binding in molecules occurs in the vicinity of the Feshbach resonance. Now, K.Winkler and his colleagues from Austria used the photoassociation technique to create a Bose-Einstein condensate of rubidium, whose molecules are in the ground state and bound strongly. The key difficulty with photoassociation is that the molecules turn out to be destroyed (dissociated) by the same radiation creating them. To get around this, the researchers used an approach akin to the induced transparency technique familiar from optics. Radiation from a sapphire laser caused condensate molecules to form in an excited quantum state, and a diode laser, of a different frequency and lower power, transferred them to the ground state, making them immune to the destructive action of the first laser. In this way, about 100 molecules of Rb2 were created in a condensate containing a total of 4x105 Rb atoms. The molecule detection technique relied on the absorption spectrum of the molecules. Changing the emission range of the diode laser led to a resonance at the transition frequency between the excited and ground states of the molecules.
Observation of the Casimir-Polder effect
1 October 2005
The Casimir-Polder effect is the attraction of an atom to a flat surface and, like the ordinary Casimir effect, is due to the change in the spectrum of zero-point fluctuations in vacuum. The Casimir-Polder effect has been observed in a number of experiments with ultracold gases. However, due to sensitivity limitations, measurements were only made at a very small distance (0.1mkm) from the surface - the region where van der Waals forces also are very strong. D.M.Harber and his colleagues from the University of Colorado have employed a new technique to investigate the Casimir-Polder effect at distance 5mkm from the surface of a dielectric. The team studied the mechanical oscillations of a cloud of Bose-Einstein condensed 87Rb atoms located close to the specially treated flat surface of sapphire or silicon dioxide. Metal surfaces, although more preferable, were of no use because of surface defects and their associated perturbing electric fields. By changing the magnetic field in the trap one can vary the cloud-to-surface distance, and by applying a short duration pulse, to cause the condensate to oscillate. The presence of a gradient in the Casimir-Polder force leads to a shift in the oscillation frequency, whose measured value was found to vary with the distance from the surface much in the way predicted by theoretical calculations (M.Antezza, L.P.Pitaevskii, and S.Stringari, Phys. Rev. A70 053619 (2004)). A spin-off of the Colorado experiment is a limit on the way in which Newton's force of gravitational attraction at small distances can be modified (the Yukawa potential being an example). This limitation is less restrictive than, but serves as an independent confirmation of, those obtained in other experiments.
Source: Phys. Rev. A72 033610 (2005)
Curvilinear motion of a soliton
1 October 2005
A computer simulation experiment by N.N.Rozanov, S.V.Fedorov, and
A.N.Shatsev of the Research Institute of Laser Physics in St.Petersburg has revealed the possibility that a dissipative soliton can follow a curvilinear path in its motion. The condition for this is asymmetry in the distribution of the field which forms the soliton and the energy flows into and out of it. The system studied was that of several vortex solitons described by nonlinear equations in the Ginzburg-Landau form. A detailed study was made of a pair of strongly coupled solitons interacting with other solitons. It was found that the complex of solitons not only rotated but also its center of mass moved along a curvilinear path. It is believed that using laser radiation in an active optical medium may be a way to reproduce these findings in a real-life experiment.
Source: Phys. Rev. Lett. 95 053903 (2005)
1 October 2005
An MIT team led by A.Rogers has detected radio emission from deuterium in space for the first time. Deuterium is difficult to observe due to its small amount and because its spectral lines are close to those of hydrogen. Detecting deuterium is of great interest for the theory of nucleosynthesis in the early Universe because its amount is a sensitive indicator of physical processes taking place during nucleosynthesis. In particular, by knowing the amount of deuterium, the average density of baryonic matter can be calculated - thus providing a more accurate estimate for the amount of dark matter (or unseen mass) in the Universe. Further data accumulation and analysis will allow more accurate determination of these quantities.
The fastest pulsar
1 October 2005
Measurements with the VLBA radio telescope have yielded the parallax and angular speed of the pulsar B1508+55, allowing the model-independent calculation of its distance (2.4 kpc) and proper motion speed (1100km/s). The pulsar B1508+55 is thus the fastest pulsar know and will necessarily leave the Galaxy in the future. Measurements, in the frequency range 1.4-1.7MHz, were made in eight series at 3-month intervals. From the rate of decrease of its pulse period, the age of the pulsar is found to be 2.34x106years. The present distance of the pulsar from the disk of the Galaxy is about 2kpc, but given the direction of its motion it is concluded that the pulsar was born in a disk near an OB star association in the constellation Cygnus. How the pulsar has been accelerated to 1100km/s is not yet known. Of several mechanisms proposed to explain high pulsar speeds, the most likely one is an asymmetric supernova explosion, but even this mechanism needs improvement when it comes to speeds in excess of 1000km/s.
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.