Excitons trapped and cooled so effectively that they condensed and cohered to form a giant matter wave will enable scientists to study their physical properties, which exist only fleetingly yet offer promising applications in solar energy and ultrafast computing. Although whole atoms have been seen to do this when confined in a trap and cooled, this is the first time that researchers have seen subatomic particles form coherent matter waves in a trap. “The realization of the exciton condensate in a trap opens the opportunity to study this interesting state,” said Leonid Butov, a professor of physics at the University of California, San Diego. “Traps allow control of the condensate, providing a new way to study fundamental properties of light and matter.” Excitons are composite particles composed of an electron and a “hole” left by a missing electron in a semiconductor. These coupled pairs, which are created by light, exist in nature and often play a critical role in photosynthesis, for example. As excitons cool to a fraction of a degree above absolute zero, they condense at the bottom of an electrostatic trap and spontaneously form coherent matter waves. Creating indirect excitons, with electrons and holes in separate layers of a semiconductor, allowed them to persist long enough to cool into this state. (Image: Butov group/UCSD) In a quantum mechanical view, excitons have a dual nature of both particle and wave. Waves are usually unsynchronized, but when particles are cooled enough to condense, their waves synchronize and combine to form a giant matter wave, a state that others have observed in atoms. Excitons can be created easily by shining light on a semiconductor, but for excitons to condense, they must be chilled before they recombine. To achieve this, the UCSD scientists separated the electrons far enough from their holes so that excitons could last long enough to be cooled into a condensate. They then created coupled quantum wells that separate electrons from holes in layers of alloys made of gallium, arsenic and aluminum. Next, they set an electrostatic trap made by a diamond-shaped electrode and chilled their special semiconducting material in an optical dilution refrigerator to as cold as 50 mK, just a fraction of a degree above absolute zero. By focusing a laser on the surface of the material, excitons were created and began to accumulate at the botton of the trap as they cooled. The entire cloud of excitons cohered below 1 K to form a single matter wave, a signature of a state called a Bose-Einstein condensate. Scientists can study to properties of light and matter in a new way by varying the size and depth of the trap to alter the coherent exciton state. The study appeared in Nano Letters. For more information, visit: www.ucsd.edu