Device fabrication being an imperfect art, single photons from solid-state systems such as quantum dots are not identical; a collection of dots designed to emit at nominally the same frequency will actually produce a range of frequencies. That phenomenon, called inhomogeneous broadening, is an obstacle to experimentalists who need a stream of identical photons—say, in applications requiring entanglement. One way to overcome the obstacle is to tune the solid-state devices themselves; applying appropriate strains, for example, can change the internal structure of the devices so they all give off photons of the same frequency. Now Kartik Srinivasan of NIST, his postdoc Serkan Ates , and other collaborators have demonstrated an alternative approach: Let the devices emit as they will, but use nonlinear optics to convert the frequencies of the resulting photons. When a source photon and light from a pump laser interact in a nonlinear crystal, the result could be a photon whose frequency is the sum of source and pump frequencies. As schematically indicated in the figure, with carefully tuned pump-laser frequencies ω p1 and ω p2, Srinivasan and colleagues produced photons with the same frequency ω c from input photons of different frequencies ω s1 and ω s2. The research team established that the output photons were indeed identical by observing their telltale interference. In the future, such frequency conversions may enable photons to communicate between the nodes of a quantum internet. (S. Ates et al., Phys. Rev. Lett. , in press.)—Steven K. Blau
