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| Physics Update
- March 2000 |
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| Phase-coherent
amplification of matter
waves has been demonstrated by two independent research groups.
Unlike photons in an optical laser, the number of atoms is conserved
in an atom laser, so researchers must rely on a reservoir of
atoms to amplify the initial atomic beam. Both groups used similar
techniques, starting with a Bose–Einstein condensate (of sodium
atoms at MIT, and of rubidium atoms at the University of Tokyo).
First, lasers were used to isolate a small fraction of atoms
from the BEC to act as a “seed.” Then, another laser was used
to set up the so-called superradiant Rayleigh scattering condition,
under which the seed population grew in a demonstrably phase-coherent
way. These atom-wave amplifiers are the first truly active elements
for atom-optics research; earlier elements such as mirrors,
polarizers, and beam splitters have all been passive. Active
amplification could offer improvements in, for example, atom-wave
gyroscopes and lithography. (S. Inouye et al., Nature
402, 641, 1999; M. Kozuma et al., Science
286, 2309, 1999.) |
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| High
proton polarization has been achieved at 77 K and
with modest 0.3-tesla magnetic fields. Researchers at Kyoto
University in Japan used naphthalene doped with pentacene (an
aromatic organic molecule chain), knowing that pentacene is
diamagnetic in its ground state and paramagnetic in its optically
excited triplet state. First, the physicists used a laser beam
to optically polarize the electrons in the pentacene. Next,
they used carefully tuned microwaves to transfer the polarization
to protons in both the pentacene and the naphthalene. The technique,
called microwave-induced optical nuclear polarization (MIONP),
gave them 32% polarization, and they are working to extend it
to room temperature. For comparison, proton targets for particle
physics experiments can reach polarizations of up to about 70%,
but both higher fields (2–5 T) and lower liquid-helium temperatures
(0.3 K) are needed. And magnetic resonance imaging involves
proton polarization levels of only 0.0003% (still good enough
for spotting tumors) at room temperature in a typical magnetic
field of 1 T. The Kyoto researchers suspect that their approach
will be useful in both of those arenas, as well as in quantum
computing and neutron scattering. The polarized protons would
be portable, having a relaxation time of more than 3 hours at
almost zero magnetic field. (M. Iinuma et al., Phys.
Rev. Lett. 84, 171, 2000.) |
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| Multiple-ionization
mechanisms in an intense laser beam have been investigated
in two multi-institutional experiments in Germany. At the focus
of a strong laser field, an electron in an atom is subjected
to an electric field almost as strong as that from the nucleus.
Under such conditions, a surprisingly large number of atoms
become multiply ionized—losing two (or more) electrons at the
same time. It is generally accepted that the two electrons are
not ejected independently, but rather are correlated. There
has been debate, however, over the applicability of three possible
mechanisms: shake-off, in which the second electron is removed
by the altered potential field after the first electron is emitted;
collective multielectron tunneling; and rescattering, in which
an electron is freed by the strong field and then, about half
of an optical cycle later, accelerated back toward the parent
ion, knocking out the second electron. The new experiments used
cold target recoil-ion momentum spectroscopy (COLTRIMS), one
group on helium and the other on neon atoms, to determine the
three-dimensional momentum distributions of the resulting ions.
Both groups found a two-peak distribution along the direction
of the laser polarization, interpreted by the neon experimenters
as clear evidence for the rescattering model. The helium collaboration
prefers to make correlated measurements of both the electrons
and the ions before ruling a mechanism in or out. (Th. Weber
et al., Phys. Rev. Lett. 84, 443, 2000.
R. Moshammer et al., Phys. Rev. Lett. 84,
447, 2000.) |
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 Solitary,
wandering black holes have been detected through
gravitational microlensing—whereby the gravitational field of
an object between us and a distant star distorts the star’s
light. The Massive Compact Halo Object (MACHO) collaboration
regularly views millions of stars in the direction of the dense
central bulge of our galaxy, hoping to occasionally observe
a star brightening through such a lensing effect. The brightening
can last from two days to several years. Careful analysis of
the longest-lasting events by the MACHO and Microlensing Planet
Search (MPS) collaborations led to the conclusion that, for
two of them, the intervening objects were about six times as
massive as the Sun. As announced by David Bennett of the University
of Notre Dame at the January meeting of the American Astronomical
Society, lone-wolf black holes, unaccompanied by bright accretion
disks or rapidly orbiting stars or gas, seem to be the only
reasonable candidates for the gravitational interlopers. Shown
here is a Hubble Space Telescope image of one of those events.
The identification of the lensed star allowed them to pin down
the lens mass to within a factor of two. However, Bennett and
his colleague Sun Rhie point out that if an optical interferometer
(at either the Keck telescope or the Very Large Telescope) were
operating, multiple images of the lensed star could be resolved,
constraining the mass to 10%. |
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©
2000 American Institute of Physics
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