Quantum Repeaters

Quantum Repeaters

A quantum repeater is a network node that extends the range of quantum entanglement by combining entanglement swapping with quantum memory.

The previous lesson ended with grim arithmetic: 1,000 km of fiber attenuates light by a factor of 10²⁰, so even a ten-gigahertz single-photon source delivers roughly one photon every three centuries. Repeaters attack that exponential head-on. Divide the link into segments short enough that each succeeds often, then stitch the segments together. Done right, the cost of adding distance changes from exponential to roughly polynomial, which is the entire reason a global quantum internet is considered possible at all.

Why Classical Repeaters Don't Work

Classical fiber amplifiers copy a signal and re-amplify it. But quantum mechanics forbids copying (no-cloning theorem). You need a completely different approach.

Look closely and the classical strategy fails twice over. An amplifier is, at heart, a copying machine, and no-cloning forbids copying unknown quantum states. The obvious fallback, measure the photon and resend what you saw, is worse: measurement collapses exactly the superposition and entanglement the network exists to deliver. A repeater must therefore extend entanglement without ever reading the quantum data passing through it.

The Quantum Repeater Protocol

Step 1: Divide the long-distance link into shorter segments A–Relay1 (100 km), Relay1–Relay2 (100 km), Relay2–B (100 km)

Step 2: Generate entanglement on each segment simultaneously A–Relay1 share a Bell pair; Relay1–Relay2 share a Bell pair; Relay2–B share a Bell pair

Step 3: Entanglement swapping at each relay Relay1 performs a Bell measurement on its two qubits (one from A, one from Relay2). This projects A and Relay2 into an entangled state, "teleporting" the entanglement.

Entanglement swapping is teleportation applied to half of a Bell pair, and you already built it yourself earlier in this track. The relay's Bell measurement consumes both of its qubits and broadcasts two classical bits telling the endpoints which Bell state they now share, so they can apply Pauli corrections. The relay never learns, and never touches, any message data: there is nothing to eavesdrop on at the relay.