Quantum Repeater
The unbuilt device long-distance quantum networks are waiting for, stitching short entangled hops into long ones because quantum signals can't be amplified.
Photons carrying quantum states die exponentially in optical fiber: after a couple hundred kilometers, almost nothing survives. Classical networks fix this with amplifiers, but quantum signals can't be amplified, since the no-cloning theorem forbids copying an unknown quantum state. The quantum repeater is the workaround, a chain of stations that create entanglement over short hops, store it in quantum memories, and stitch the hops together via entanglement swapping.
The catch: a true quantum repeater needs long-lived quantum memory, high-fidelity photon-matter interfaces, and fast entanglement swapping, all in one box, all at telecom compatibility. No one has fielded a practical one yet, which is why today's 'quantum networks,' including the 300-km-class QKD links making headlines, rely on trusted intermediate nodes instead, a security compromise wearing a networking costume.
Whoever builds a real repeater builds the quantum internet's backbone.