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 A
million-volt field-emission transmission electron microscope
(FE-TEM) has been built by a team led by Akira Tonomura at Hitachi’s
Advanced Research Laboratory in collaboration with the Japan
Science and Technology Corp. In an FE-TEM, electrons are drawn
out of a cathode and accelerated by a high voltage. The one
million volts produced a beam of electrons that was four times
brighter than the best previous FE-TEM (300 kV) and 1000 times
brighter than conventional thermionic-emission TEMs. The device
is a marvel of engineering. The 1 MV must be stable to within
half a volt, and the electron source must be held steady to
within 0.5 nm. As shown here, the new device can image rows
of atoms only half an angstrom apart (thus rivaling scanning
tunneling microscopes) and can even take pictures fast enough—60
per second—to make movies of fine gold particles changing their
shapes. In addition, due to their higher energies, the electrons
can penetrate deeper into a sample than was previously possible,
and thus provide three-dimensional information. Hitachi’s Takeshi
Kawasaki says that the microscope will be useful for observing
certain dynamic properties of condensed matter systems—for example,
the movement of vortices in high-temperature superconductors.
(T. Kawasaki et al., Appl. Phys. Lett. 76, 1342,
2000.) |
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| The
far side of the Sun can be monitored for activity, in principle,
with two new techniques. Both methods use instruments aboard
the SOHO satellite, a joint mission of the European Space Agency
and NASA. One method, dubbed helioseismic holography, uses the
distortion introduced into sound waves—forever rumbling through
the body of the Sun—when they reflect from magnetically active
regions around sunspots. The wave fields are observed by SOHO
on the near side of the Sun, but some of the distortions can
be traced back to sources on the far side. The other method
makes use of hydrogen’s Lyman-alpha emission, which is stronger
from active regions than from elsewhere on the Sun. SOHO routinely
makes all-sky maps of interplanetary hydrogen’s Lya.
With careful processing, those maps can show where an active
region’s “beacon” of Lya excites
more than the usual amount of complementary emission from interplanetary
space, and researchers can watch as the beacon moves across
the sky with the Sun’s rotation. Both groups of researchers
are part of a large effort to predict “space weather”; they
see their efforts as leading to an extra week or two of advance
warning, for astronauts and satellite operators, of large Earth-bound
disturbances coming from the Sun. (C. Lindsey, D. C. Braun,
Science 287, 1799, 2000. J.-L. Bertaux et al.,
Geophys. Res. Lett. 27, 1331, 2000.) |
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| Hard
x rays stored in a crystal resonator. Cavities for optical
wavelengths of light are commonplace, but not so for x rays,
particularly the high-energy (hard) ones. Now, however, physicists
have built and demonstrated such a cavity at the European Synchrotron
Radiation Facility (ESRF) in Grenoble, France. The “mirrors”
were a pair of vertical thin slabs, each 70% reflective, cut
from a single crystal of silicon (ensuring the perfect alignment
of rows of atoms in both walls) and placed 150 mm apart. The
researchers, led by ESRF’s Klaus-Dieter Liss, sent 10–10 s pulses
of 15.8 keV x rays into the resonator, and saw as many as 14
back-and-forth reflections between the crystals due to multiple
Bragg scattering. The new resonator can serve as a narrow-band
filter, because the x rays that leak through the crystals on
each pass have an ever-narrowing range of energies. Other x-ray
optics applications are expected. (K.-D. Liss et al., Nature
404, 371, 2000.) |
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Previous
Physics Updates:
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| Quantum
key distribution using entangled photons has been demonstrated
by three independent research groups. For any coded message
to be useful, a key must be available for decoding it. Classically,
however, it is always possible for an eavesdropper to both intercept
the key and avoid being detected. Using quantum entanglement,
though, a completely secure key can be generated and distributed.
Any eavesdropper’s attempt to intercept the quantum key will
alter the contents in a detectable way, enabling the users to
discard the compromised parts of the data. To generate the key,
a nonlinear crystal splits a single photon into a pair of entangled
photons. The sender (Alice) and the receiver (Bob) each get
one of the photons. Alice and Bob each have a detector for measuring
their photon’s properties, such as polarization or time of arrival.
With the right combination of detector settings on each end,
Alice and Bob will get the exact same value of the property.
After receiving a string of entangled photons, Alice and Bob
discuss which detector settings they used, rather than the actual
readings they obtained, and they discard readings made with
incorrect settings. At that point, Alice and Bob have their
secure key. The three groups of researchers—based at the University
of Vienna, Los Alamos National Laboratory, and the University
of Geneva—all succeeded in distributing their keys while avoiding
interception. In these experiments, the three groups used relatively
slow key generation rates, but improvements are anticipated.
(T. Jennewein et al., Phys. Rev. Lett. 84,
4729, 2000. D. S. Naik et al., Phys. Rev. Lett.
84, 4733, 2000. W. Tittel et al., Phys. Rev.
Lett. 84, 4737, 2000.) |
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©
2000 American Institute of Physics
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