Polarization Quantum Demultiplexer

Photon Source

Input polarization angle θ 30°

Emission rate 2.0 /s

Routing Statistics

|ψ⟩ = cos θ|H⟩ + sin θ|V⟩

P(H) theory = 75%P(V) theory = 25%

H-channel count

V-channel count

Measured P(H)

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Photons emitted

How it works

A polarization-based quantum demultiplexer routes photons to different output channels based on their polarization state, using a polarizing beamsplitter (PBS). Each incoming photon is prepared in the superposition

|ψ⟩ = cos θ |H⟩ + sin θ |V⟩

where θ is the angle between the photon's polarization and the PBS transmission axis. The PBS is a projective measurement in the {|H⟩, |V⟩} basis: it does not "look" at the photon's polarization directly — it forces a binary outcome, and the Born rule fixes the probability of each outcome:

P(H, transmitted)  = |⟨H|ψ⟩|² = cos²θ
P(V, reflected)    = |⟨V|ψ⟩|² = sin²θ

This is exactly Malus's law applied one photon at a time: classically cos²θ describes the intensity of light through a polarizer, but for a single photon it is the probability that the photon is found in the transmitted channel — each individual photon takes only one path, and only the statistics over many photons reproduce the classical intensity curve.

θ slider — rotates the input polarization relative to the PBS axis, changing the split ratio between the H (transmitted) and V (reflected) output channels.
Emission rate — how many photons per second leave the source; higher rates build up statistics faster.
Pause / Reset — freeze the stream or clear the running channel counts to restart the statistics from zero.

Real-world relevance: exactly this device — a polarizing beamsplitter used as a router — is the polarization-encoding quantum demultiplexer used in quantum key distribution (QKD) receivers and in quantum networks to send different qubit channels down separate fibers or to separate detectors.

Photon H channel (transmitted) V channel (reflected)

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