A Rydberg-atom photon source

A laser or other conventional light source can be attenuated to produce pulses with an average of one photon each, but the number of photons per pulse will follow a Poisson distribution, with some pulses containing no photons, some containing one, and some containing two or more. An important measure of a single photon source’s quality is the degree of suppression of two-photon pulses relative to the Poisson distribution. Now Yaroslav Dudin and Alex Kuzmich (Georgia Tech) have shown that they can reduce the number of two-photon events by a record-breaking factor of 25, with a photon source based on so-called Rydberg interactions. An atom in a highly excited (or Rydberg) state can prevent another nearby atom from being similarly excited by shifting it out of resonance with the exciting laser. (See Physics Today, February 2009, page 15 .) Dudin and Kuzmich show that in a cold gas of some 500 rubidium atoms, held in a trap 15 µm wide, one atom in a Rydberg state can influence all the others. As a result, they can reliably excite exactly one atom in the gas—so when they convert the atomic excitation back into light, they get exactly one photon. Their system could also serve as a photon–photon quantum gate, using Rydberg excitations to force interactions between normally non-interacting photons. (Y. O. Dudin, A. Kuzmich, Science 336, 887, 2012 .)—Johanna Miller