A quick flash of laser light aimed at the well generates pairs of electrons and positively charged «holes» in the middle layer. (sciencemag.org)
The problem here: single electrons are highly reactive — they try to form pairs of electrons as quickly as possible. (sciencedaily.com)
The laws of quantum mechanics allow pairs of electrons to «tunnel» through the barrier only very occasionally — helping the team to isolate and control the pairs as they slowly break out. (scientificamerican.com)