Quantum Repeaters

Quantum Repeaters

The fundamental challenge of long-distance quantum networking is fiber loss: at 0.2 dB/km, photon transmission probability drops exponentially with distance. At 500 km, no photons arrive. Classical networks solve this with optical amplifiers that copy the signal. Quantum networks cannot amplify, but they can use a smarter approach: quantum repeaters based on entanglement swapping.

The Basic Idea

Instead of transmitting a qubit directly from A to Z over 1000 km (impossible), divide the path into segments:

A ←→ B ←→ C ←→ D ←→ ... ←→ Z

Each adjacent pair shares an entangled Bell pair (short-range entanglement distribution is feasible). Then, through a sequence of Bell state measurements at each intermediate node, the entanglement is "swapped" to create a Bell pair between A and Z: without any qubit traveling the full distance.

This is entanglement swapping: a Bell state measurement on one qubit from each of two Bell pairs creates a new Bell pair between the remaining qubits, destroying the intermediate pairs.

Entanglement Swapping Protocol

Say node B holds one qubit from the A-B pair and one qubit from the B-C pair. B performs a Bell state measurement (BSM) on its two qubits. The result:

After swapping across n nodes, A and Z share a Bell pair. The entanglement has been "extended" without any physical qubit traveling the full distance.

Quantum Memory: The Bottleneck

Entanglement swapping requires synchronization: the BSM at B can only happen after both A-B and B-C entanglement attempts have succeeded. Since photon arrival is probabilistic, B must store the A-side qubit in quantum memory while waiting for the B-C entanglement to succeed.