Similar to how light fibres transformed optics, physicists hope guided atom beams will revolutionize atom optics. Two groups have developed an atomic analogue of a beam splitter, which until now has only been used to separate atoms moving in free space. By guiding atoms along a surface, greater control has been achieved by D. Cassettari et al. (Phys. Rev. Lett. 85, 5483–5487; 2000) and D. Müller et al. (http://xxx.lanl.gov/abs/physics/0003091).
Modern atom interferometers use beam splitters to split up matter waves and recombine them to produce sensitive interference patterns, used in precision measurements of gravity and the rotation of the Earth. But such free-space beam splitters can only separate beams by small angles thus limiting the scope of the experiments.
In their study, Cassettari et al. guided cold lithium atoms along a Y-shaped current-carrying wire patterned on the surface of a semiconductor chip. Fluorescence images taken of the wire show the beam splitter in action (see ). When current was passed through only one of the output wires, the atoms take the left or right fork (left two images). By varying the current in each arm, the splitting fraction of the atoms can be changed continuously (50:50 in right images). The angle between the two beams is 15°.
In similar work, Müller et al. split a beam of cold rubidium atoms along two current-carrying wires on a glass substrate. Neither of these studies split a coherent matter wave — in which all the atoms are in step with one another. This has been shown for atoms in free space, but needs to be done with guided atoms if they are to be used in precision atom interferometers. One solution might be to use a coherent atom source, such as a Bose–Einstein condensate.