Quantum Repeater Network: Entanglement Swapping

Network Topology

Repeater stations 2

Link length (fiber, km) 25

Bell-state swap fidelity 0.95

Idle. Press "Distribute entanglement" to run the protocol.

End-to-End Link

Total distance

75 km

Per-link fidelity

0.79

End-to-end fidelity

0.60

Chain success prob.

81%

How it works

Direct transmission of a single photon through optical fiber decays exponentially with distance, so entanglement carried on one photon dies off after a few tens of kilometers. A quantum repeater chain fixes this: entanglement is generated only over short elementary links, then a Bell-state measurement at each repeater station performs entanglement swapping — it stitches two short-range entangled pairs into one longer-range pair without ever transmitting the original photon further.

Elementary link fidelity (Werner-state model):
  F_link = 0.5 + 0.5 · exp(-L / L_att),   L_att ≈ 22 km

Visibility:      V = 2F - 1
After n swaps (n+1 links):
  V_total = V_link^(n+1) · F_swap^n
  F_total = 0.5 + 0.5 · V_total

Chain success probability (independent swaps):
  P = p_swap^n

Repeater stations — more stations mean shorter elementary links (higher per-link fidelity) but more imperfect swap operations multiplying the noise in.
Link length — the fiber distance between adjacent nodes; total range = length × (repeaters + 1).
Swap fidelity — how close each repeater's Bell-state measurement is to ideal; F=1 is a perfect swap.
Distribute entanglement — animates photon pairs racing out from each elementary link to the two nearest repeater stations, then each repeater performing its swap in turn, extending one Bell pair all the way to both end nodes.

This is the same architecture proposed for a real quantum internet: rather than fighting fiber attenuation with more power (impossible for single photons, since amplification destroys the quantum state), repeaters keep every hop short enough to be reliable and let swapping do the distance.

End node Repeater station Entangled link (photon pair)

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