Number 654, September 17, 2003
by Phillip F. Schewe, Ben Stein, and James Riordon
A Single-Atom Laser
A single-atom laser, a device employing a single trapped atom to resonantly
emit light back and forth between two reflective mirrors, has been created
by Jeffrey Kimble at Caltech. Although single-atom lasers have been
demonstrated before (See
PNU #204), Kimble's is the first to use a single atom nearly at
rest, and not a parade of atoms in a dilute beam entering a reflective
cavity one at a time. The singleness of the source means that the number
of photons emitted by the laser over a certain time interval is, while
not exactly predictable (which would be outlawed by Heisenberg's uncertainty
principle), much less jittery than emission from multi-atom lasers.
The emission is weak by laser standards---only about 100,000 photons
per second---but this quiet, more controllable form of photons should
aid future quantum information schemes. (McKeever et al., Nature,
18 September 2003.)
The Laser Interferometry Space Antenna (LISA)
The Laser Interferometry Space Antenna (LISA) is not due for launch
until 2012 but tests of components are of course going forward now.
LISA will search for gravity waves passing the sun's vicinity by watching
how the distance between two test masses changes. A gravity wave can
be thought of as a traveling disturbance in spacetime itself; such a
wave would temporarily shorten and then lengthen the path between the
test masses. In this case the masses would be 5 million km apart, an
interval that would be monitored every instant by the interference of
laser beams traveling back and forth between the masses. Actually three
pairs of test masses would be mounted on three far-flung satellites,
spread out in space in an equilateral triangle where each leg is 5 million
km long, with all three craft in independent orbit around the sun (see
LISA websites and http://lisa.jpl.nasa.gov/).
While the spaceborne LISA would look for waves with very low frequencies
(.001-.1 Hz), the earthbound detector LIGO would search for gravity
waves in a higher frequency range (100-1000 Hz).
As an interim step toward deploying LISA, the European Space Agency
(ESA) plans to launch in 2007 its Pathfinder mission, a craft serving
as a miniature version of LISA, two free-floating test masses 35 cm
apart (small thruster rockets will be used to reposition the spacecraft
so its sides do not come in contact with the masses), will be tried
out. The test, watching that the masses move along in parallel trajectories,
is not unlike the famous (or apocryphal) experiment conducted by Galileo
Galilei to affirm that two objects, one light and one heavy, would fall
at the same rate from the Leaning Tower of Pisa. And to perform the
test in 2007 some terrestrial tests have now been carried out in 2003.
Basically, scientists at the Universita' di Trento (Italy) are attempting
to understand all the possible forces, in addition to gravity, that
could influence the motion of the test mass. In an ideal experiment,
the test mass (2 kg or, in units of weight, about 20 newtons) would
be hung from a thin wire and surrounded by all the apparatus that will
accompany it into space, including the motion sensor needed to reorient
the spacecraft, and all extraneous forces on the mass, down to a precision
of a femto-newton (10-15 newtons) would have to be accounted
for if the desired levels of precision needed for LISA were to be achieved.
Such precision is not possible with ground-based detectors, so the experimenters
used not the full test mass, but a hollow facsimile. At this early stage
in understanding, the Trento physicists found a satisfactorily "quiet"
force environment, but there are still a fact of 10 away from the precision
needed for Pathfinder and a factor of 100 away from the precision needed
for LISA. (Carbone et al., Physical
Review Letters, upcoming article; contact Stefano
Vitale, 39-0461-881568 )
The Fraction of Physics Graduate Students
The fraction of physics graduate students coming to the US from abroad
has declined since the 2000/2001 academic year, reversing a steady climb
that had been in effect since 1970. The fraction of non-US first year
grad students grew from about 20% in 1970 to a peak of 55% in 2001.
However, in the past two years the fraction has eased back to less than
50%, a new report shows. Two-thirds of the PhD-granting physics departments
in the US say that at least some of their admitted students from abroad
have been unable to attend owing to visa problems. Students from China
and from the Middle East seem to have had the most trouble entering
the US. ("Physics Students from Abroad in the Post-9/11 Era,"
report prepared by the Statistical Research Center at the American Institute
of Physics; contact Patrick J. Mulvey;
a copy of the report can be obtained at http://www.aip.org/statistics/trends/undtrends.htm)
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