Polarization Beam-Splitter Photon Combiner

Input Polarization

Channel A misalignment (from H axis) 0°

Channel B misalignment (from V axis) 0°

PBS Hardware

Crosstalk / extinction imperfection ε 1%

Photon emission rate (per stream) 15/s

Live Readouts

Extinction ratio

— dB

Combined throughput

— /s

Channel fidelity (preserved state)

—%

How it works

A polarizing beam-splitter (PBS) combiner merges two photon streams carrying orthogonal polarizations — nominally horizontal (H) on channel A and vertical (V) on channel B — into a single spatial output mode without destroying either polarization state. An ideal PBS transmits H and reflects V with 100% fidelity, so the combined beam still carries both quantum states, just sharing one fibre or free-space path.

Real photons rarely arrive perfectly aligned, and no physical PBS has infinite extinction. For a channel misaligned by angle θ from its intended axis, Malus's law gives the fraction that lands in the correct output port, with a crosstalk floor ε from the PBS coating itself:

T(θ, ε) = (1 − ε)·cos²θ + ε·sin²θ

Extinction ratio (dB) = 10·log10( T / (1 − T) )
Combined throughput  = rate·T_A + rate·T_B
Channel fidelity     = (T_A + T_B) / 2

Channel A / B misalignment sliders — rotate each input polarization away from its ideal axis; watch particles peel off into the reject ports instead of joining the combined output beam.
Crosstalk ε — models a real, non-ideal PBS coating: even at perfect 0° alignment some light always leaks the wrong way.
Emission rate — how many photons per second each input stream emits, which sets how busy the combined output looks.

Real-world relevance: PBS combiners like this sit at the heart of polarization-encoded quantum key distribution (BB84 with polarization qubits) and dense polarization-division multiplexing in classical optical links — the extinction ratio directly limits the quantum bit error rate an eavesdropper-detection scheme can tolerate.

Channel A (H-polarized) Channel B (V-polarized) Combined output port

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