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Single photons will play an important role in quantum communication. This will require quantum repeaters, in much the same way that optical amplifiers are required for the transmission of digital data through optical fibers. However, these quantum repeaters will work only if all the single photons that they receive have the exact same wavelength. A team of researchers at universities in Switzerland, Germany, and France reported on 8 September in Nature Communications that they’ve developed quantum dots that can produce streams of photons with identical wavelengths. 

To create the photons, the researchers used self-assembled indium gallium arsenide (InGaAs) quantum dots embedded in gallium arsenide. Each quantum dot, although it consists of about a hundred thousand atoms, traps a single electron that can occupy two energy levels. By illuminating the quantum dot with laser light, the electron is moved into the higher energy state. When the electron drops down to its lower energy level, it emits a single photon whose wavelength is determined by the difference in the energy of the two levels. “In many ways it behaves like a single atom, and this is why it is often called an 'artificial atom,’” says Andreas Kuhlmann a post-doctoral researcher at the University of Basel who was the paper’s lead author. “But because it is inside a semiconductor it is quite robust, and that is, of course, nice if you want at one point to develop a product.”

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