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Entangled Photons
from Quantum Dots
Andreas Muller et al
Physicists supported by the PFC at the Joint Quantum
Institute have developed a new source of “entangled”
photons – fundamental units of light whose properties are
so intertwined that if the condition of one is measured, the
condition of the other is instantaneously known, even if
the photons are thousands of miles apart when the first
one is measured.
Ordinarily, quantum dots (microscopic formations of
semiconductor material) cannot form entangled pairs of
indistinguishable photons because the processes involved
in emission produce photons of slightly different
wavelengths. The researchers found, however, that by
beaming a carefully tuned laser at the dot, the wavelength
difference was suppressed and the emitted photons were
entangled.
The technique may someday enable more compact and
convenient sources of entangled photon pairs than
presently available for quantum information applications
such as the distribution of "quantum keys" for encrypting
sensitive messages.
When an electron and a “hole”
(the absence of an electron, which behaves
like a positively charged particle) combine in a dot, they emit a
photon. Pairs of electrons and holes produce pairs of photons,
usually with different energy levels. By applying a tuning laser
to the dot, the scientists remove the difference. Because it is
impossible to tell which electron-hole pair created which
photon, the photons are entangled.
"Creating polarization-entangled photon pairs from a
semiconductor quantum dot using the optical stark effect," A.
Muller, W.F. Fang, J. Lawall and G.S. Solomon, Phys. Rev. Lett., 103,
217402, (2009)