He was referring to so-called photon up-conversion nanomaterials, named for their ability to convert low-energy, invisible photons (including infrared light) into higher-energy, visible bits of light. The key to this function is in the nanomaterials' composition, says Chris Murray, a materials scientist at the University of Pennsylvania who was not involved in the work. "They are made of a group of elements called rare earth metals," he says. "One of their fascinating properties is they have very long-lived excitation states."