Jennifer Ouellette, for NOVA:

The Glasgow researchers used a special liquid crystal mask to impose a pattern on one of two photons in a pair. Because light can act like both a particle and a wave—the famous wave-particle duality—the researchers could use the mask to reshape the wavefront of that photon, so instead of spreading out like an ocean wave traveling to the shore, it was focused onto a point. That change in shape corresponded to a slight decrease in speed. To the researchers’ surprise, the light continued to travel at the slightly slower speed even after leaving the confines of the mask.