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 straight through and reflects V by 90° into that same output line, so the combined beam still carries both quantum states, just sharing one output port.
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 port 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.
- Polarization dials (top right) show each channel's actual axis (solid arrow) against its ideal axis (dashed) — the angle between them is θ.
- Extinction-ratio strip chart (bottom right) plots the last 20 s of the live ER readout so a slider change is visible as a trend, not just a number.
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.