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
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
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.
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
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 (firstname.lastname@example.org) 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
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
the UCSD Web site
Also see the UCSD press release