Number 806, December 20 , 2006
by Phil Schewe, Ben Stein and Davide Castelvecchi
Guided Atom Laser
A cloud of atoms distilled into the form of a
Bose Einstein condensate (BEC) acts likes a single coherent thing.
Furthermore, the BEC acts like a wave. It can and has been
extracted from the trap structure in which the condensate was made
and allowed to propagate just like a laser beam, except that the
waves in this case consist not of electromagnetic radiation but
atoms. In previous atom lasers the atoms, subject to the force of
gravity, accelerated; this has the effect of decreasing the
wavelength of the atom waves.
Now, for the first time, physicists of
Alain Aspect's atom optics group (www.atomoptic.fr) from the
Institut d'Optique Graduate School, Palaiseau (south of Paris), have
been able to extract atoms from a BEC trap on a quasicontinuous
basis while simultaneously sending them down a horizontal optical
guide with an unprecedented level of control over direction,
intensity, and wavelength, the latter being kept constant during the
propagation.
William Guerin (william.guerin@institutoptique.fr), a
researcher on the team led by Vincent Josse and Philippe Bouyer,
says that the advantage of this quasicontinuous guided atom laser
beam over the previously realized pulsed guided beams (when the BEC
is extracted all at once) is its much narrower velocity spread. In
the Palaiseau atom laser, the atoms are extracted by converting some
of them from a magnetic state to a nonmagnetic state. After this,
the confining magnetic fields of the trap no longer influence the
atoms and the atom waves emerge with a typical velocity of 10 mm/sec
and a velocity spread of a few microns/sec, a factor of 1000 sharper
than for pulsed laser operation (see figure at
http://www.aip.org/png/2006/273.htm).
The atom laser in the Paris
device is driven forward by a beam of light in a very directional
and efficient process; no atoms are lost during extraction or
transport across a 1 mm guide. This new atom laser opens promising
prospects for applications in atom interferometry or more
fundamental studies of matter wave propagation. (Guerin et al.,
Physical Review Letters, 17 November 2006
Diamond-Quality Properties
Much of what we know about how materials
behave under extreme pressures and temperatures (millions of
atmospheres and thousands of Kelvin) is learned using diamond anvil
cells. In these tiny enclosures, material can be squeezed between
the flat, hard, transparent faces of two gem quality diamonds. Because
diamond is transparent over much of the electromagnetic spectrum, many types of radiation, such
as laser beams or light emitted from or scattered by the sample
(light containing valuable spectroscopic information) can enter and
exit through the diamond windows.
However, the diamond itself can
introduce subtle optical distortions, and some physicists believe
experimenters need to take a closer look at two important
parameters: dispersion (the optical property that gives diamonds
their "fire"; appearance) and absorbance. Both of these parameters
are crucial for spectro-radiometry (the determination of the
temperature by spectroscopic methods) of samples contained in the
diamond anvil cell.
Laura Robin Benedetti and Daniel Farber of the
Livermore National Lab and Nicolas Guigot of the European
Synchrotron Radiation Facility believe that by not taking into
account the effects of dispersion and absorbance, experimenters can
introduce errors in measured temperatures (typically in the
1500-4000 K range) of as much as several hundred Kelvin.
However,
Benedetti (925-424-5466, benedetti3@llnl.gov) says that their new
work presents ways of compensating for the distortions introduced by
the optical properties of the diamond windows. It's appropriate that
this new look at diamonds appears in the Journal of Applied Physics
(JAP), which this year marks its diamond anniversary (http://jap.aip.org/jap/top.jsp).
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