Hi EARL.

04-Jan-03 11:01:00, EARL TRUSS wrote to PHYSICS ECHO READERS


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

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

 ET> FEASIBLE CHAOTIC ENCRYPTION.   Encryption schemes that hide
 ET> messages in chaotic signals have attracted attention in recent
 ET> years as a means to transmit information securely (Update 170,
 ET> 361), but most work has been either theoretical or strictly
 ET> limited to laboratory experiments. Now a group of researchers in
 ET> Beijing have managed to demonstrate chaotically encrypted, two-way
 ET> voice transmission through the Beijing Normal University computer
 ET> network. With a 32-bit encryption structure, a 750 MHz personal

Dunno what the big noise is about 32 bit encryption's not eaxctly bleeding
edge.

 -=> Bye <=-

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