Two teams in Germany have built transistors that control light at the single-photon level.
Transistors that switch light instead of electrical current can enable ultra-fast computing. But making optical transistors with 'gain' — when one photon affects many others to drive further switches — has been tricky because photons do not interact with each other.
To overcome this problem, a team at the Max Planck Institute of Quantum Optics in Garching and a separate group at the University of Stuttgart passed a single photon through a cloud of ultracold rubidium atoms. The photon converted one atom into a type of large, excited particle called a Rydberg atom, which blocked the next photon from passing through.
In the Stuttgart team's transistor, one photon diverted another 10, whereas in the Max Planck device, a photon controlled a further 20.