update.528 (fwd)

[Jim Choate]

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Date: Thu, 1 Mar 2001 13:35:36 -0500 (EST)
From: AIP listserver <physnews at aip.org>
To: physnews-mailing at aip.org
Subject: update.528

PHYSICS NEWS UPDATE                         
The American Institute of Physics Bulletin of Physics News
Number 528  March 1, 2001   by Phillip F. Schewe, Ben Stein, and
James Riordon

SWITCHABLE X-RAY STORAGE.  Recently we reported on
two experiments at Harvard (Update 521) in which a light signal
enters a volume of gas where, under the action of an additional
control laser beam, it is greatly slowed; in fact, as the signal slows
down to zero the light energy is coherently converted into a
collective excitation of the spins of the
atoms in the gas.  Later the light signal can be resurrected and sent
on its way.  It has come to our attention that something like this,
only in a nuclear system, was reported in an article some four years
ago (Shvyd'ko et al., Physical Review Letters, 7 October 1996).  In
the earlier experiment, conducted at HASYLAB (DESY) in
Hamburg (contact Yuri Shvyd'ko, University of Hamburg, 49-40-
8998-2200, yuri.shvydko at desy.de), the incoming light consists of
x rays, and the receiving medium is an ensemble of iron-57 nuclei,
ensconced in a FeBO3 crystal.  All 10^18 iron nuclei in the 20-
micron-thick sample interact collectively in a single "nuclear
exciton" state with each new incoming x-ray photon. As APS
Editor-in-Chief Martin Blume (631-591-4000) points out, such a
"solid-state nuclear physics" effect is well known also under the
name of coherent nuclear resonant scattering.  By abrupt switching
of a weak (60 Gauss) external magnetic field, the nuclear exciton
can be manipulated in such a way that its decay is prohibited.  In
this way the x rays are stored, and can be released on demand by
reversing the magnetic switching.  The released light signal is
coherent with the incoming one. The coherence of quantum
information transfer is perfectly preserved.  Put differently, one
can say that in the Harvard experiments the atoms, behaving like
tiny electric dipoles, converted the incoming light into a coherent
excitation of atomic spins (a polariton), whereas in the HASYLAB
experiment the iron nuclei, behaving like tiny magnetic dipoles,
converted the incoming x rays into a coherent nuclear excitation
spread over the whole crystal (a nuclear exciton).  As in the
Harvard experiments, the pent-up electromagnetic energy stored in
the system can be released (and sent moving in its original
direction) by the flip of a switch, in this case by adjusting the
external magnetic field  direction.

A SHARP GAMMA-RAY HOLOGRAM, with atomic-scale
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                                           George Zebrowski

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