Different photons in, indistinguishable photons out

Different photons in, indistinguishable photons out. 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...