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; writers can get the text at Physics
In the earlier
experiment, conducted at HASYLAB (DESY) in Hamburg (contact Yuri Shvyd'ko,
University of Hamburg, 49-40-8998-2200, firstname.lastname@example.org),
the incoming light consists of x rays, and the receiving medium is an
ensemble of iron-57 nuclei, ensconced in a FeBO3 crystal.
All 1018 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.
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.