Coherent Exciton Matter

Coherent exciton matter has been reported by a University of California, San Diego group. They say their cold swarm of excitons acts like a Bose-Einstein condensate (BEC) but might also be some new kind of quantum condensate in which many particles act as if they were a single entity.

Excitons are tiny paired artificial objects adrift in a semiconductor and consist of an electron excited from its home orbital plus a vacancy left behind. The negatively charged electron and the positively charged hole are bound to each other and will usually, after a nanosecond or so, recombine, an event which knocks the electron back into its home band and releases a tiny parcel of light. This is how light emitting diodes (LEDs) produce their illumination.

In the UCSD experiment, the excitons live much longer than usual -- long enough to study as if there were a species of atom -- since the pair partners are held somewhat apart in nano-size structures (quantum wells) in the body of the semiconductor sample. The excitons can move about in the plane of the quantum well semiconductor structure and, in a 20-micron-wide ring, constitute a sort of gas which can be cooled down to ultralow temperatures.

When chilled below 5 degrees Kelvin, this gas began to show signs that it had spontaneously condensed into a kind of quantum coherent state. Just as atomic BECs (in which atoms are cooled to the point where their matter waves overlap and become, in effect, a single coherent quantum system) are imaged by releasing the atoms from their confining magnetic fields, so in this case the exciton condensate reveals itself by telltale photons. The photons, coming from the annihilation of electrons with their hole partners, leave the ring area, enter an optical device and, in the form of waves (which are proxies for the original excitons), produce a high-contrast interference pattern, suggesting the coherent nature of the exciton gas. Furthermore, the contrast of the interference fringes provides the coherence length -- about 2 microns (see images at the UCSD Web site).

San Diego researcher Leonid Butov (lvbutov@physics.ucsd.edu) believes that controversy has surrounded previous reports of exciton condensates and believes the new results are particularly clear in displaying an interference pattern and in demonstrating quantum coherence. Like others who study coherent matter, Butov predicts that practical quantum computing devices will follow from this line of research.

Yang et al., Physical Review Letters, 3 November 2006
Contact Leonid Butov
University of California, San Diego
lvbutov@physics.ucsd.edu
Images at the UCSD Web site
Also see the UCSD press release