The title may seem a little confusing because how can empty space have color? At the nanoscale where quantum mechanics reigns, empty space really can have color, provided it is the space between two pieces of metal. Researchers at the University of the Basque Country and the University of Cambridge have found a quantum effect that not only create color at such a gap, but allows it to be changed by altering the gap.
The experiment the researchers performed involved placing two nanoscale spheres of gold a single nanometer apart, or less, and shining a light at the gap. As the light strikes the electrons of the spheres, it causes them to move, and their proximity causes a plasmonic effect to produce a red glow. When the spheres are moved closer together, the glow intensifies, until the spheres are just 0.35 nm apart. At this point quantum tunneling may occur, so the electrons will jump from one sphere to another, which results in the glow turning blue.
To model this result, the researchers had to combine the quantum and classical world to create new theories. Now armed with this information, it should be possible to use it to better study how light and matter interact at the sub-nanometer scale, as well as developing new kinds of optoelectronic technologies.