A team of physicists has exploited quantum quirkiness to reflect atoms off a silicon chip and fashion a hologram for atoms. Their technique provides a new way to control atom beams, which nanotechnology researchers hope to use to fashion nanometer-sized devices.
The task would appear simple: reflect an atom beam off a smooth solid surface. But even the smoothest surface appears rough to an atom; the ultracold atoms used in beams are invariably attracted to a surface through the Van der Waals force, a subtle interaction in which the atoms electrically polarize each other. Atoms in the beam accelerate toward the surface, which shortens the wavelengths of the quantum waves describing them. By the time the atoms crash-land, the ripples in their quantum waves are as tightly spaced as the inevitable imperfections in the surface. That means that the surface looks rough from the atoms' perspective, and the atoms scatter in all directions, instead of reflecting like light from a mirror.
Ironically, the solution to this problem is to cover the surface with ridges, say physicists Fujio Shimizu of the University of Electro-Communications in Tokyo, and Jun-ichi Fujita of the NEC Fundamental Research Laboratories in Tsukuba, Japan. The ridges--just tens of nanometers wide and separated by tens of micrometers--cause the atoms in the beam to sense the surface while they are still moving slowly and their wavelengths are long. So despite the ridges, the surface looks smooth to the atoms. When the atoms first encounter the Van der Waals tug of the surface, some are reflected, much as light is partially reflected when it passes from air into water. Shimizu and Fujita used such "quantum reflection" to create an atom hologram, they report in the 25 March issue of Physical Review Letters. The researchers covered a silicon wafer with ridged patches arranged so that reflected neon atoms would conspire to spell the word "SURFACE".
The technique might provide a tool for controlling beams of all kinds of atoms in nanotechnology, says Daniel Kleppner, a physicist at the Massachusetts Institute of Technology in Cambridge. But the real value of the hologram may be in demonstrating the fundamental principles of quantum reflection, he says: "Whether or not practical applications follow, the work represents a most interesting advance in atom optics."
Abstract for Shimizu and Fujita's paper, Physical Review Letters