TRILOBITE MOLECULES. New research predicts the existence of a giant two-atom molecule with an electron cloud resembling a trilobite, the ancient, hard-shelled creature which lived in the Earth's seas over 300 million years ago (see figure at www.aip.org/physnews/graphics). Made of two rubidium atoms spaced very far apart, the trilobite molecule could conceivably materialize in a Bose-Einstein condensate (BEC). This is because a BEC's ultracold, dense environment favors the creation of exotic species in addition to the condensate itself. The trilobite molecule has many remarkable properties in addition to its shape, according to the collaboration that predicts its existence (Chris Greene, University of Colorado and JILA, 303-492-4770, chris.greene@colorado.edu). For starters, it would be huge for something consisting of just two atoms: the cores of the Rb atoms are separated by anywhere between 50 nm and 5 microns. Rubidium molecules in BECs have been formed before (Update 471), but they have been much smaller (only 2-4 nm). The researchers believe the trilobite molecule can be created by manipulating a rubidium BEC with laser pulses or external electromagnetic fields. One of the rubidium atoms in the pair must first be converted into a Rydberg atom, which contains an electron in a very high orbit. Ultra-long-range molecules would then form from a weak attraction between the Rydberg atom's outermost electron and another Rb atom. Some of these molecules would have no permanent separation of electric charge, but ones with the trilobite-shaped electron cloud could possess a large permanent electric dipole moment. With dipole moments roughly 1,000 times larger than typical polar diatomic molecules, these would be the first-ever polar molecules made up of two atoms of the same element and isotope. While extremely fragile, their large dipole moments suggest that trilobite molecules could be accelerated, transported, cooled and decelerated using much smaller electric fields than those required for any other molecule. (Greene, Dickinson, and Sadeghpour, Physical Review Letters, 18 Sept 2000; Select Article.)

DIRECT PHOTONS ARE SEEN IN HEAVY-ION COLLISIONS. In high energy heavy-ion collisions heated nuclear matter, both the original protons and neutrons (or maybe even their constituent quarks) as well as additional particles created out of the excess energy, can be thought of as a hot gas. High energy gamma photons have been observed, as expected, from the decay of particles exiting the hot nuclear gas. But gammas are also expected to be emitted at a lower rate as a kind of thermal glow from the interactions of the particles in the gas. Such "direct photons" have now been seen for the first time in an experiment conducted at CERN, where lead ions smashed into a stationary lead target (contact Terry Awes, Oak Ridge Natl.Lab, 865-574- 4587, awes@mail.phy.ornl.gov). Some theorists believe that direct photons, like the suppression of psi mesons or an enhancement in the production of strange mesons, might constitute evidence for the production of quark gluon plasma. (Aggarwal et al., Physical Review Letters, 2 Oct; Select Articles.)

CONNECTING THE WAVE AND PARTICLE ASPECTS OF LIGHT by detecting a photon and then measuring the fluctuations of a closely associated electromagnetic field has been experimentally achieved for the first time. In most experiments, researchers focus upon either light's particle aspects (by counting photons, for instance) or wave aspects (by measuring an interference between electromagnetic fields, to cite a simple example). Now, researchers at SUNY-Stony Brook and the University of Oregon (Luis Orozco, Stony Brook, 631-632-8138, lorozco@notes.cc.sunysb.edu) have demonstrated an experimental setup, which they call a "Wave-Particle Correlator," for determining the relationships between both aspects of the light that comes from a single physical process. The "light source" in their experiment consists of a beam of rubidium atoms passing in between a highly reflecting pair of mirrors (a "cavity QED system"). In their setup, a laser aims light into the cavity through one of its mirrors. Acting as a sort of "artificial molecule," the cavity absorbs the light and re-emits it. A photon occasionally escapes through an output mirror, only to be detected as a particle by a photodiode. The photon detection sets up a subsequent measurement of wavelike properties, as the cavity occasionally gets rid of a second photon to relax to a stable state. The resulting electromagnetic field gets mixed with another known electromagnetic wave to produce an interference pattern. The pattern emerges only after averaging over many such "conditional" measurements triggered by photodiode detections. It reveals that the electromagnetic field inside the cavity after the first photon's departure contains, in effect, a tenth of a photon, since a second photon is only emitted about 10 percent of the time. Measuring such wave-particle correlations might bring about new microscopy techniques. (Foster et al., Physical Review Letters, 9 October 2000; Select Article).