Quantum Wavelength-Division Multiplexer

Active channels

Input channels

Output fidelity

Why wavelength separation matters

A quantum wavelength-division multiplexer couples several single-photon channels into one shared fiber by giving each its own wavelength band. Because the bands don't overlap, each channel's quantum state — the point it occupies on its own Bloch sphere — rides through the shared fiber undisturbed and is separated back out at the far end.

Push two channels' wavelengths close together and their bands start to overlap. A wavelength-selective demultiplexer can no longer cleanly tell them apart, so each output collects a little of its neighbor's state — the outgoing Bloch vector shrinks toward the centre of the sphere (a mixed state) and the fidelity readout drops. That's cross-talk: information leaking between channels that were meant to stay independent.

λ sliders — set each channel's wavelength inside the fiber's transmission band.
State buttons — prepare each channel's own independent qubit state (|0⟩, |1⟩ and equatorial superpositions).
Fidelity — how closely the state recovered at the output matches what was sent; 100% only when every pair of active channels is well-separated.

Multiplexer / demultiplexer Shared fiber (all channels) Cross-talk (bands overlap)

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