Entangled photons stay entangled, even after being converted to plasmons, according to the findings of researchers at Leiden University in the Netherlands. They used glass/gold arrays that, at holes in the gold interface (right), convert photons into surface plasmons, which are optically excited charge density waves. Plasmons on either side of the gold film couple to each other through the holes and are reradiated as photons at the far side. The researchers sent entangled photons through two arrays and measured the remaining entanglement with PerkinElmer single-photon-counting modules. As they reported in the July 18 issue of Nature, the fact that the photons stayed entangled leads them to believe that surface plasmons demonstrate a quantum nature on a macroscopic scale, which could be a valuable tool for developing many quantum technologies.