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)