Quantum Optical Network: Fiber Loss & WDM

Fiber Link

Fiber length 40 km

Attenuation α 0.20 dB/km

Multiplexing (WDM)

Wavelength channels 4

Pair rate / channel 6 pairs/s

Live Stats

Pairs launched

Coincidences

Single-span T(L/2)

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End-to-end η(L)

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Aggregate key rate

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vs. 1 channel

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Each dot is one photon of an entangled pair generated at the central SPDC source and sent down its own fiber span toward Alice (left) or Bob (right). A pair only counts as a coincidence if both photons survive their span — loss is sampled continuously along the flight using the real fiber attenuation law, not a fixed coin-flip at launch.

Physics notes

Standard telecom fiber loss (dB convention):

T(L) = 10^(-αL/10)

α ≈ 0.20 dB/km near 1550 nm (the WDM C-band) is close to the practical floor for silica fiber. With the source at the midpoint, each photon of a pair crosses L/2, so the joint (coincidence) survival probability is:

η(L) = T(L/2)² = 10^(-αL/10)

which is exactly the direct-transmission decay curve — no repeaters here, just raw fiber physics. Wavelength-division multiplexing (WDM) puts N independent entangled-photon channels, each on its own wavelength, onto the same fiber core. Loss is per-channel independent, so the aggregate secure-key rate scales linearly:

R_agg(L) = N · R₀ · η(L)

This sim's quantum-networks sibling models multi-hop repeater relays and entanglement swapping instead — no wavelength multiplexing, no continuous-distance attenuation curve.
Real deployments (e.g. Toshiba/BT metro QKD trials) multiplex 8+ channels on one strand alongside classical DWDM traffic.