When two gold nanoparticle spheres are pushed sufficiently close together, the gap between them acquires a red colour, as the gold’s electrons interact with white light.

When pushed even closer together – less than 0.35 nanometres apart – the electrons can switch from one particle to the other and produce a change in colour, from red to blue, in the space between the particles.

Physicists have observed this phenomenon – known as the quantum regime – for the first time, confirming electrons can change from one nanoparticle to another when they are close enough, to reduce accumulated charge.