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Number 528, March 1, 2001 by Phil Schewe, James Riordon, and Ben Stein

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; writers can get the text at Physics News Select).

In the earlier experiment, conducted at HASYLAB (DESY) in Hamburg (contact Yuri Shvyd'ko, University of Hamburg, 49-40-8998-2200, yuri.shvydko@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 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.

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

A sharp gamma-ray hologram, with atomic-scale resolution, has been achieved for the first time by physicists in Krakow at the University of Mining and Metallurgy. It is useful to compare x-ray with gamma holograms. In an x-ray hologram (Update 262) a beam of x rays strikes atoms and promotes electrons into excited states. The atoms return to their ground states by emitting fluorescence x rays some of which reach a detector unscattered and some of which scatter from surrounding atoms. The interference of the scattered ("object") and unscattered ("reference") x rays forms a hologram which provides an atomic-scale image of the atoms in their crystalline matrix.

In the gamma approach the incoming gamma rays excite not atoms (iron-57 atoms) but their nuclei. Any particular nucleus in the sample can be excited by either an unscattered gamma (acting as the "reference wave") or a previously scattered gamma (acting as the"object wave "). The excited nucleus de-excites by emitting electrons (conversion electrons) which are then detected. One problem plaguing previous attempts at gamma holography, that of "twin images," a sort of double vision suffered by the image reconstruction process, has now been overcome, resulting in 3D images of the local crystal structure to be rendered with half-angstrom spatial resolution (see figure at Physics News Graphics).

Pawel Korecki, now at the DESY lab in Hamburg (49-408-908-2602, pawel.korecki@desy.de), and his colleagues believe that soon gamma holography will map not only structure but also the local magnetic environment as well. (Korecki et al., Physical Review Letters, 19 February 2001; text at Physics News Select)

Correction

In Update 527, the presence of iridium (not indium) in sediments at the KT boundary is what implicated asteroid impacts as the likely killer of the dinosaurs.