The Repeater Network Map

Mission 7: Building the Quantum Internet. Part of the Quantum Courier Service, a free course from the Oxford University Quantum Information Society.

Last drill of the course, recruit. Oxford to King’s Cross, a long run of thirsty fibre, and a rack of repeaters at your disposal. Drag the "repeaters" slider up from zero and watch the secret key rate climb off the floor, hop by hop, as each added station shortens the gaps the photons have to survive.

Leave the span wide with no repeaters and the link is exponentially dead: most photons die red in the fibre long before they reach the far end, because no amount of gold-plated hardware fixes a gap that’s too wide. Add repeaters and each one performs an entanglement swap, sacrificing its local pair to fuse the link one hop further along the chain, all the way to the other side.

And before you celebrate: drag the "memory lifetime" slider down and watch the gain from those extra repeaters evaporate. More segments means more waiting, and a pair that decoheres before its neighbour arrives was never worth having. Find the spacing where the secret key rate actually holds, then measure it against that faint dashed direct link, the very thing your repeaters are meant to beat.