PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 619 January 3, 2003   by Phillip F. Schewe, Ben Stein, and James
Riordon

X-RATED INTERFEROMETRY. The appearance of an x-ray interference pattern in
a Fabry-Perot interferometer has been achieved, for the first time, by a
group of physicists at the University of Hamburg (Yuri Shvyd'ko,
yuri.shvydko{at}desy.de, 49-40-8998-2200). This might lead to a new
generation of x-ray optical devices, such as high-resolution x-ray spectral
filters, or x-ray clocks, and, more important still, a new way of
calibrating length measurements at the atomic scale. X-rays are a potent
type of electromagnetic radiation, with a much higher energy and smaller
wavelength than visible light. But because x-rays are so potent and because
they see various materials as having essentially the same indices of
refraction, x-rays are much harder to reflect at a surface.  Indeed, x-ray
telescopes in orbit use only grazing-incidence (reflected through an angle
of a milliradian or less) mirrors to focus x-rays on a detector.
In the last few years, though, the scientists in Hamburg have succeeded in
reflecting x-ray light directly backwards with special sapphire (Al2O3)
mirrors; the price for this high-angle reflectivity (other than the
difficulty of preparing faultless crystalline mirrors) is that the
reflection occurs only for an extremely narrow spectral range (x-ray color),
precluding the mirrors' use in telescopes, where x-radiation over a broad
range is important. In the Hamburg device, an x-ray version of a Fabry-Perot
Interferometer (FPI), the reflecting waves will resonate if the cavity
between two exquisitely polished mirrors is a multiple of the radiation
half-wavelength. Light entering the cavity  bounces back and forth between
the mirrors producing multiple sub-waves emerging from the cavity. Their
interference shows up as a modulation in the radiation that exits the
cavity, both on time and wavelength scales.  The Fabry-Perot interference
pattern provides a  means of measuring of the x-ray wavelength, and this
provides an opportunity for creating a new, higher-precision, length
standard. Currently the most accurate way to measure x-ray wavelength is to
produce a Bragg scattering pattern by sending x rays into a silicon crystal,
whose lattice spacing (the distance between atoms) is known with an
uncertainty of about one part in 6 x 10^-8.
There is, however, a nuclear process related to the Mossbauer effect which
produces x-rays (better known as Mossbauer radiation) with an
extraordinarily narrow spectral line. The most familiar is the Mossbauer
radiation originating from the decay of the first excited state of 57-Fe
nuclei. The radiation wavelength of about 0.086 nm is perfectly suited for
atomic scale measurements. Its stability, about one part in 10^-15, is
comparable to the best cesium fountain clocks. If Mossbauer x rays could be
used to calibrate an FPI device capable of operating in both x-ray and
visible ranges, then this could facilitate a stable, reproducible,
wavelength (and hence length) standard far better than is possible (about
one part in 3 x 10^-11) with, say, helium-neon lasers.
An important step toward this goal has now been attained in the experiments
of the Hamburg group conducted at synchrotron radiation facilities including
the Advanced Photon Source at Argonne (near Chicago) and HASYLAB at DESY
(near Hamburg).  The x-rays, from the synchrotron-radiation sources, were
chosen to be as similar to Mossbauer rays as possible. For the first time,
interference patterns in a Fabry-Perot interferometer have been observed for
x-rays.  From the attenuation time of the multiple sub-waves emerging from
the cavity, the spectral sharpness of the Fabry-Perot interference fringes
was estimated to be less than a micro-electron-volt. This is more than 100
times better than the best x-ray crystal monochromators can do. (Shvyd'ko et
al., upcoming article in  Physical Review Letters; accompanying figure will
be posted on Jan 6 at www.aip.org/mgr/png ; see also related PRL article, 17
July 2002; http://focus.aps.org/story/v6/st2 )

FEASIBLE CHAOTIC ENCRYPTION.   Encryption schemes that hide messages in
chaotic signals have attracted attention in recent years as a means to
transmit information securely (Update 170, 361), but most work has been
either theoretical or strictly limited to laboratory experiments. Now a
group of researchers in Beijing have managed to demonstrate chaotically
encrypted, two-way voice transmission through the Beijing Normal University
computer network. With a 32-bit encryption structure, a 750 MHz personal
computer can encode information at speeds comparable to the widely
recognized Advanced Encryption Standard, and support voice communication at
typical telephone speeds and quality. While no encryption technique is
absolutely impenetrable, the researchers (Hu Gang, Beijing Normal
University, hugang{at}sun.ihep.ac.cn, 86-10-62208420) explain that their
communication scheme is reasonably secure (it would take an intruder armed
with a personal computer more than a million times the lifetime of the
universe to break the code) as well as being feasible in realistic,
commercial settings. (S. Wang et al., Physical Review E, December 2002.)

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