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Making light stop
1 January 2004
M.D. Lukin and A.S. Zibrov (both of the P.N. Lebedev Physics
Institute, RAS) and their Harvard colleague M.Bajcsy succeeded in
stopping a light pulse in a medium in an experiment they
performed at Harvard University. In previous light stopping
experiments, photons in the signal pulse had been transformed
into spin excitations in a resonantly absorbing medium in which
another control laser beam was used to achieve the effect of
electromagnetically induced transparency. When the control beam
was adiabatically turned off, the signal light pulse continued to
exist in the form of spin excitations, and when the control beam
was turned back on, the pulse transformed into photons again (see
Physics Uspekhi 44 217 (2001)) for
more details). In a new experiment by D.M. Lukin and colleagues,
the stopped pulse could exist in exactly the form of photons, not
spin excitations. This was achieved by using counterpropagating
control beams which produced an interference pattern in rubidium
vapor. First, as before, the light pulse was transferred into a
spin excitation by adiabatically turning off one of the control
beams. After that, however, when both control beams were
simultaneously turned off, the light pulse recreated from spin
excitations turned out to be localized between the standing
wave's neighboring nodes. Due to the Bragg reflection taking
place in the (periodic) structure, the light pulse did not move
as a whole. Turning off one of the control beams made the pulse
move again.
Source: Nature 426 638 (2003)
Magnetism of carbon
1 January 2004
In recent years a number of organic substances have been
discovered which, while they do not contain metal atoms, possess
magnetic properties at low temperatures. Recently Japanese
scientists announced that they had discovered magnetic properties
in non-crystalline carbon reduced from hydrogen compounds. This
result have caused doubts, however: many researchers believed
metal impurities might have been present in the substance. Now P.
Esquinazi and his colleagues performed an experiment which showed
that ultra-pure graphite irradiated with an accelerator proton
beam is weakly ferromagnetic even at room temperature. It is
believed that the protons introduced into the crystal lattice of
carbon modify the electron bonds in a certain way - leading to
ferromagnetism. A complete theoretical explanation of the
observed effect is still lacking.
Source: Phys. Rev. Lett. 91 227201 (2003)
A pulsar in a close pair
1 January 2004
Astronomers using the 64-m radio telescope in Australia have
discovered a pulsar, PSR J0737-3039, which is in a binary system
with another neutron star. This neutron star pair has a record
short orbital period of just 2.4 hr, so that relativistic effects
in its orbital motion are much more pronounced than in the Hulse-
Taylor pulsar PSR B1913-16. In particular, the astronomers
discovered a shift in the perihelion of the pair's orbit -
similar to the perihelion shift of Mercury - which is due to
the spacetime curvature and has a value of 16.88o per year. Due
to the emission of gravitational waves, the neutron stars will
merge in about 85 million years, and the effect of a decreasing
orbit will already be noticeable in 15 months. There is a danger,
however, that due to the relativistic precession, the pulsar will
become invisible in the nearest future. The masses of the pulsar
and its companion neutron star are 1.24 and 1.35 solar masses,
respectively. The pulsar PSR J0737-3039 is relatively close to
earth - only 600 parsecs away - but its radio luminosity is very
low. Weak pulsars like PSR J0737-3039 are difficult to detect, so
such close pairs of neutron stars may be abundant in the Galaxy.
Earlier calculations of the neutron star merger rate relied on
five known neutron star pairs. The detection of the pulsar J0737-
3039 enabled the conclusion that the merger rate is nearly an
order of magnitude higher than previously believed. The merger of
neutron stars is accompanied by a powerful burst of gravitational
waves. According to optimistic estimates, the gravitational wave
detectors due to start operating will be able to detect one burst
each year or two years. The study of the pulsar PSR J0737-3039
and the possible detection of gravitational wave bursts in the
future provide good opportunities for testing the predictions of
Einstein's general theory of relativity.
Source: Nature 426 531 (2003)
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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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